diff --git a/packages/app-lib/src/api/curseforge.rs b/packages/app-lib/src/api/curseforge.rs
index 4e8e333..bd9e147 100644
--- a/packages/app-lib/src/api/curseforge.rs
+++ b/packages/app-lib/src/api/curseforge.rs
@@ -5197,19 +5197,9 @@ fn modpack_zip_error(error: zip::result::ZipError) -> crate::Error {
 }
 
 fn safe_archive_relative_path(value: &str) -> crate::Result<String> {
-    let path = Path::new(value);
-    if value.is_empty()
-        || path.is_absolute()
-        || path
-            .components()
-            .any(|component| !matches!(component, Component::Normal(_)))
-    {
-        return Err(ErrorKind::InputError(
-            "CurseForge modpack contains an invalid override path".to_string(),
-        )
-        .into());
-    }
-    Ok(path.to_string_lossy().replace('\\', "/"))
+    let normalized = value.replace('\\', "/");
+    crate::util::io::safe_relative_join(Path::new(""), &normalized)?;
+    Ok(normalized)
 }
 
 pub(crate) fn loader_family(loader_id: &str) -> &str {
@@ -10933,6 +10923,21 @@ mod tests {
         );
         assert!(safe_archive_relative_path("../options.txt").is_err());
         assert!(safe_archive_relative_path("/options.txt").is_err());
+        for path in [
+            r"..\options.txt",
+            "config/.. /outside",
+            "config/file:stream",
+            "C:/outside",
+            "C:outside",
+            "config/NUL",
+            "config/file.",
+        ] {
+            assert!(safe_archive_relative_path(path).is_err(), "{path}");
+        }
+        assert_eq!(
+            safe_archive_relative_path(r"config\example.toml").unwrap(),
+            "config/example.toml"
+        );
     }
 
     #[test]
diff --git a/packages/app-lib/src/api/pack/archive_util.rs b/packages/app-lib/src/api/pack/archive_util.rs
index 254a11b..5c74296 100644
--- a/packages/app-lib/src/api/pack/archive_util.rs
+++ b/packages/app-lib/src/api/pack/archive_util.rs
@@ -1,6 +1,6 @@
 //! Shared helpers for extracting content from local modpack archives.
 
-use std::path::{Component, Path, PathBuf};
+use std::path::{Path, PathBuf};
 
 use super::detect::decode_zip_entry_name;
 use crate::util::io;
@@ -13,22 +13,6 @@ fn archive_error(error: zip::result::ZipError) -> crate::Error {
         .into()
 }
 
-pub(crate) fn safe_relative_path(value: &str) -> crate::Result<String> {
-    let path = Path::new(value);
-    if value.is_empty()
-        || path.is_absolute()
-        || path
-            .components()
-            .any(|component| !matches!(component, Component::Normal(_)))
-    {
-        return Err(crate::ErrorKind::InputError(
-            "Modpack archive contains an invalid file path".to_string(),
-        )
-        .into());
-    }
-    Ok(path.to_string_lossy().replace('\\', "/"))
-}
-
 /// Extracts every file under `prefix` in the archive into `target_dir`,
 /// preserving the directory structure below the prefix. Returns the number of
 /// files written.
@@ -114,7 +98,8 @@ fn extract_archive_subdir_sync(
             )
             .into());
         }
-        let target = target_dir.join(safe_relative_path(relative)?);
+        let target =
+            io::safe_relative_join(target_dir, &relative.replace('\\', "/"))?;
         if let Some(parent) = target.parent() {
             std::fs::create_dir_all(parent)
                 .map_err(|error| io::IOError::with_path(error, parent))?;
@@ -252,6 +237,87 @@ pub(crate) async fn create_import_scratch_dir(
 #[cfg(test)]
 mod tests {
     use super::*;
+    use std::io::Write;
+
+    #[test]
+    fn unsafe_override_paths_fail_without_writing_outside_target() {
+        let root = tempfile::tempdir().unwrap();
+        let archive_path = root.path().join("pack.zip");
+        let target_dir = root.path().join("server");
+        std::fs::create_dir(&target_dir).unwrap();
+        let outside = root.path().join("outside.txt");
+        std::fs::write(&outside, b"unchanged").unwrap();
+        for prefix in ["overrides/", "server/overrides/"] {
+            for relative in [
+                "../outside.txt",
+                r"..\outside.txt",
+                "config/../../outside.txt",
+                "/outside.txt",
+                "C:/outside.txt",
+                "C:outside.txt",
+                r"\\server\share\outside.txt",
+                "config/.. /outside.txt",
+                "config/mod.jar:payload",
+                "config/NUL",
+                "config/secret.",
+            ] {
+                let mut archive = zip::ZipWriter::new(
+                    std::fs::File::create(&archive_path).unwrap(),
+                );
+                archive
+                    .start_file(
+                        format!("{prefix}{relative}"),
+                        zip::write::SimpleFileOptions::default(),
+                    )
+                    .unwrap();
+                archive.write_all(b"malicious").unwrap();
+                archive.finish().unwrap();
+                assert!(
+                    extract_archive_subdir_sync(
+                        &archive_path,
+                        prefix,
+                        &target_dir,
+                        None,
+                    )
+                    .is_err(),
+                    "{prefix}{relative}"
+                );
+                assert_eq!(std::fs::read(&outside).unwrap(), b"unchanged");
+                assert_eq!(std::fs::read_dir(&target_dir).unwrap().count(), 0);
+            }
+        }
+    }
+
+    #[test]
+    fn override_paths_with_backslashes_are_normalized() {
+        let root = tempfile::tempdir().unwrap();
+        let archive_path = root.path().join("pack.zip");
+        let target_dir = root.path().join("server");
+        let mut archive =
+            zip::ZipWriter::new(std::fs::File::create(&archive_path).unwrap());
+        archive
+            .start_file(
+                r"overrides/config\example.toml",
+                zip::write::SimpleFileOptions::default(),
+            )
+            .unwrap();
+        archive.write_all(b"valid").unwrap();
+        archive.finish().unwrap();
+        assert_eq!(
+            extract_archive_subdir_sync(
+                &archive_path,
+                "overrides/",
+                &target_dir,
+                None,
+            )
+            .unwrap(),
+            1
+        );
+        assert_eq!(
+            std::fs::read(target_dir.join("config/example.toml")).unwrap(),
+            b"valid"
+        );
+    }
 
     struct CancelOnRead {
         cancellation: CancellationToken,
diff --git a/packages/app-lib/src/api/pack/install_from.rs b/packages/app-lib/src/api/pack/install_from.rs
index 53c812b..245ca05 100644
--- a/packages/app-lib/src/api/pack/install_from.rs
+++ b/packages/app-lib/src/api/pack/install_from.rs
@@ -20,7 +20,7 @@ use serde::{Deserialize, Serialize};
 use std::collections::HashMap;
 use std::future::Future;
 
-use std::path::{Component, Path, PathBuf};
+use std::path::{Path, PathBuf};
 use std::pin::Pin;
 
 use zip::ZipArchive;
@@ -320,23 +320,13 @@ pub(crate) async fn generate_pack_from_version_id_with_reporter(
                 "Specified version has no files".to_string(),
             )
         })?;
-    let file_name = Path::new(&pack_file.filename);
-    if file_name.components().count() != 1
-        || !matches!(file_name.components().next(), Some(Component::Normal(_)))
-    {
-        return Err(crate::ErrorKind::InputError(
-            "Modrinth returned an invalid modpack file name".to_string(),
-        )
-        .into());
-    }
     let hash = pack_file.hashes.get("sha1");
-    let pack_path = state
-        .directories
-        .caches_dir()
-        .join("modpacks")
-        .join(&project_id)
-        .join(&version_id)
-        .join(file_name);
+    let pack_path = modpack_cache_path(
+        &state.directories.caches_dir().join("modpacks"),
+        &project_id,
+        &version_id,
+        &pack_file.filename,
+    )?;
 
     let details = InstallPhaseDetails::Modpack {
         project_id: Some(project_id.clone()),
@@ -517,8 +507,70 @@ pub(crate) async fn generate_pack_from_version_id_with_reporter(
     })
 }
 
-#[tracing::instrument]
+fn modpack_cache_path(
+    root: &Path,
+    project_id: &str,
+    version_id: &str,
+    filename: &str,
+) -> crate::Result<PathBuf> {
+    let mut path = root.to_path_buf();
+    for segment in [project_id, version_id, filename] {
+        if segment.contains('/') {
+            return Err(crate::ErrorKind::InputError(
+                "Invalid modpack cache path component".to_string(),
+            )
+            .into());
+        }
+        path = crate::util::io::safe_relative_join(&path, segment)?;
+    }
+    Ok(path)
+}
 
+#[cfg(test)]
+mod cache_path_tests {
+    use super::modpack_cache_path;
+    use std::path::Path;
+
+    #[test]
+    fn cache_components_cannot_escape_their_directories() {
+        let root = Path::new("modpacks");
+        assert_eq!(
+            modpack_cache_path(root, "project", "version", "pack.mrpack")
+                .unwrap(),
+            root.join("project/version/pack.mrpack")
+        );
+        for malicious in [
+            "../outside",
+            "nested/name",
+            r"..\outside",
+            "C:/outside",
+            "C:outside",
+            r"\\server\share",
+            ".",
+            "",
+            "NUL",
+            "pack.mrpack:stream",
+            ".. ",
+        ] {
+            for position in 0..3 {
+                let mut components = ["project", "version", "pack.mrpack"];
+                components[position] = malicious;
+                assert!(
+                    modpack_cache_path(
+                        root,
+                        components[0],
+                        components[1],
+                        components[2]
+                    )
+                    .is_err(),
+                    "{position}: {malicious:?}"
+                );
+            }
+        }
+    }
+}
+
+#[tracing::instrument]
 pub async fn generate_pack_from_file(
     path: PathBuf,
     instance_id: String,
diff --git a/packages/app-lib/src/api/pack/install_mrpack.rs b/packages/app-lib/src/api/pack/install_mrpack.rs
index 7de173b..d5127a5 100644
--- a/packages/app-lib/src/api/pack/install_mrpack.rs
+++ b/packages/app-lib/src/api/pack/install_mrpack.rs
@@ -891,6 +891,12 @@ pub(crate) async fn install_zipped_mrpack_files_with_reporter(
         num_files,
         content_total_bytes,
     };
+    for file in &pack.files {
+        io::join_within_root(
+            &content_context.instance_full_path,
+            &content_context.resolve_install_path(file),
+        )?;
+    }
     let skipped_missing_content_paths = Arc::new(
         reporter
             .current_state()
@@ -1039,8 +1045,14 @@ pub(crate) async fn install_zipped_mrpack_files_with_reporter(
                 let project_size = project.file_size as u64;
                 let project_path =
                     content_context.resolve_install_path(&project);
-                let target_path =
-                    content_context.instance_full_path.join(&project_path);
+                let target_path = io::join_within_root(
+                    &content_context.instance_full_path,
+                    &project_path,
+                ).map_err(|error| RequiredFileFailure::new(
+                    manifest_index,
+                    &project_path,
+                    &error.to_string(),
+                ))?;
                 let context =
                     InstallErrorContext::new("download modpack content file")
                         .maybe_project_id(
@@ -1451,7 +1463,10 @@ pub(crate) async fn install_zipped_mrpack_files_with_reporter(
                     entry_name: filename.to_string(),
                     crc32: file.crc32(),
                     uncompressed_size: file.uncompressed_size(),
-                    target_path: instance_full_path.join(&relative_path),
+                    target_path: io::safe_relative_join(
+                        &instance_full_path,
+                        &relative_path,
+                    )?,
                     relative_path,
                 })
             })
@@ -1739,6 +1754,7 @@ fn override_relative_path(
                 "Invalid modpack override path: {relative}"
             ))
         })?;
+    io::safe_relative_join(Path::new(""), relative.as_str())?;
     Ok(relative.as_str().to_string())
 }
 
@@ -1885,7 +1901,11 @@ pub async fn remove_all_related_files(
     // Iterate over all Modrinth project file paths in the json, and remove them
     // (There should be few, but this removes any files the .mrpack intended as Modrinth projects but were unrecognized)
     for file in pack.files {
-        match io::remove_file(instance_full_path.join(file.path.as_str())).await
+        match io::remove_file(io::safe_relative_join(
+            &instance_full_path,
+            file.path.as_str(),
+        )?)
+        .await
         {
             Ok(_) => (),
             Err(err) if err.kind() == ErrorKind::NotFound => (),
@@ -1904,23 +1924,14 @@ pub async fn remove_all_related_files(
 
     for file in override_file_entries {
         let entry_name = file.filename().as_str().unwrap();
-        let entry_name =
-            entry_name.strip_prefix(&archive_base).unwrap_or(entry_name);
         let relative_override_file_path =
-            SafeRelativeUtf8UnixPathBuf::try_from(entry_name.to_string())?;
-        let relative_override_file_path = relative_override_file_path
-            .strip_prefix("overrides")
-            .or_else(|_| relative_override_file_path.strip_prefix("client-overrides"))
-            .map_err(|_| {
-                crate::Error::from(crate::ErrorKind::OtherError(
-                    format!("Failed to strip override prefix from override file path: {relative_override_file_path}")
-                ))
-            })?;
+            override_relative_path(entry_name, &archive_base)?;
 
         // Remove this file if a corresponding one exists in the filesystem
-        match io::remove_file(
-            instance_full_path.join(relative_override_file_path.as_str()),
-        )
+        match io::remove_file(io::safe_relative_join(
+            &instance_full_path,
+            &relative_override_file_path,
+        )?)
         .await
         {
             Ok(_) => (),
@@ -1939,6 +1950,35 @@ mod tests {
     use async_zip::{Compression, ZipEntryBuilder};
     use std::sync::Mutex as StdMutex;
 
+    #[test]
+    fn override_paths_reject_portable_traversal_and_windows_aliases() {
+        for prefix in ["overrides/", "client-overrides/"] {
+            assert_eq!(
+                override_relative_path(
+                    &format!("{prefix}config/example.toml"),
+                    ""
+                )
+                .unwrap(),
+                "config/example.toml"
+            );
+            for relative in [
+                "../outside",
+                "config/.. /outside",
+                "config/file:stream",
+                "C:/outside",
+                "C:outside",
+                "config/NUL",
+                "config/file.",
+            ] {
+                assert!(
+                    override_relative_path(&format!("{prefix}{relative}"), "")
+                        .is_err(),
+                    "{prefix}{relative}"
+                );
+            }
+        }
+    }
+
     fn override_spec(index: usize, target: &str) -> OverrideExtractionSpec {
         OverrideExtractionSpec {
             index,
diff --git a/packages/app-lib/src/api/servers/files.rs b/packages/app-lib/src/api/servers/files.rs
index 6007c65..6580a7c 100644
--- a/packages/app-lib/src/api/servers/files.rs
+++ b/packages/app-lib/src/api/servers/files.rs
@@ -12,6 +12,8 @@ use crate::{ErrorKind, Result};
 
 use super::manifest::server_path;
 
+pub(super) use crate::util::io::safe_relative_join;
+
 const DOWNLOAD_PROGRESS_STEP: u64 = 512 * 1024;
 
 pub async fn read_file(server_id: &str, file: &str) -> Result<String> {
@@ -124,44 +126,7 @@ pub(super) async fn download_to_dir(
 
 async fn resolve_server_file(server_id: &str, file: &str) -> Result<PathBuf> {
     let dir = server_path(server_id).await?;
-    safe_join(&dir, file)
-}
-
-fn safe_join(dir: &Path, file: &str) -> Result<PathBuf> {
-    if file.is_empty()
-        || file.contains('\\')
-        || file.starts_with('/')
-        || file
-            .split('/')
-            .any(|segment| segment == ".." || segment.is_empty())
-    {
-        return Err(ErrorKind::InputError(format!(
-            "Invalid file name: {file}"
-        ))
-        .as_error());
-    }
-    Ok(dir.join(file))
-}
-
-/// Joins a relative path that may contain `/` separators but never escapes the
-/// server directory (no `..`, empty segments, or absolute/Windows roots).
-pub(super) fn safe_relative_join(dir: &Path, rel: &str) -> Result<PathBuf> {
-    let is_windows_drive_root =
-        rel.as_bytes().get(1).is_some_and(|byte| *byte == b':')
-            && rel.as_bytes().first().is_some_and(u8::is_ascii_alphabetic);
-    if rel.is_empty()
-        || Path::new(rel).is_absolute()
-        || is_windows_drive_root
-        || rel.starts_with('/')
-        || rel.starts_with('\\')
-        || rel.split(['/', '\\']).any(|segment| {
-            segment.is_empty() || segment == ".." || segment == "."
-        })
-    {
-        return Err(ErrorKind::InputError(format!("Invalid file path: {rel}"))
-            .as_error());
-    }
-    Ok(dir.join(rel))
+    safe_relative_join(&dir, file)
 }
 
 #[cfg(test)]
@@ -169,13 +134,34 @@ mod tests {
     use super::*;
 
     #[test]
-    fn safe_join_rejects_traversal() {
+    fn safe_relative_join_rejects_unsafe_paths() {
         let dir = Path::new("/tmp/servers/a");
-        assert!(safe_join(dir, "server.properties").is_ok());
-        assert!(safe_join(dir, "../secret").is_err());
-        assert!(safe_join(dir, "/etc/passwd").is_err());
-        assert!(safe_join(dir, "a//b").is_err());
-        assert!(safe_join(dir, "").is_err());
+        for path in [
+            "../secret",
+            "mods/../../secret",
+            r"..\secret",
+            r"mods/..\../secret",
+            "/etc/passwd",
+            "C:/windows/system32",
+            r"C:\windows\system32",
+            "C:secret",
+            r"\\server\share\secret",
+            r"\secret",
+            r"\\?\C:\secret",
+            "a//b",
+            "a/./b",
+            ".",
+            "",
+            "mods/.. /secret",
+            "mods/secret.",
+            "mods/secret ",
+            "mods/mod.jar:payload",
+            "NUL/secret",
+            "mods/CON.jar",
+            "mods/mod\0.jar",
+        ] {
+            assert!(safe_relative_join(dir, path).is_err(), "{path:?}");
+        }
     }
 
     #[test]
diff --git a/packages/app-lib/src/api/servers/forge.rs b/packages/app-lib/src/api/servers/forge.rs
index 0d1f4a3..efb8b44 100644
--- a/packages/app-lib/src/api/servers/forge.rs
+++ b/packages/app-lib/src/api/servers/forge.rs
@@ -14,7 +14,7 @@ use crate::event::emit::emit_server;
 use crate::util::io::IOError;
 use crate::{ErrorKind, Result};
 
-use super::files::download_to_dir;
+use super::files::{download_to_dir, safe_relative_join};
 use super::manifest::{
     InstallState, read_manifest, server_path, write_manifest,
 };
@@ -41,6 +41,7 @@ pub async fn install_forge(
     drop(manifest);
 
     let installer_name = format!("forge-{mc_version}-{build}-installer.jar");
+    let installer_path = safe_relative_join(&dir, &installer_name)?;
     let installer_url =
         format!("{FORGE_MAVEN}/{mc_version}-{build}/{installer_name}");
 
@@ -52,7 +53,6 @@ pub async fn install_forge(
     download_to_dir(server_id, &dir, &installer_url, &installer_name, None)
         .await?;
 
-    let installer_path = dir.join(&installer_name);
     let java = java_path.clone().unwrap_or_else(|| "java".to_string());
 
     log(server_id, "Running Forge installer (this may take a while)").await?;
diff --git a/packages/app-lib/src/api/servers/lifecycle.rs b/packages/app-lib/src/api/servers/lifecycle.rs
index f259b76..45814d2 100644
--- a/packages/app-lib/src/api/servers/lifecycle.rs
+++ b/packages/app-lib/src/api/servers/lifecycle.rs
@@ -18,6 +18,7 @@ use crate::state::{clear_log_buffer, get_log_buffer, push_log_line};
 use crate::util::io::IOError;
 use crate::{ErrorKind, Result};
 
+use super::files::safe_relative_join;
 use super::logs::{
     analyze_exit_reason, stream_server_output, stream_server_pty_output,
     tail_server_log_file,
@@ -131,7 +132,7 @@ async fn start_inner(
         forge_launch_args(&dir)?
     } else {
         let jar_name = resolve_jar_name(&manifest);
-        let jar_path = dir.join(&jar_name);
+        let jar_path = safe_relative_join(&dir, &jar_name)?;
         if !jar_path.exists() {
             return Err(ErrorKind::LauncherError(format!(
                 "Server jar not found: {jar_name}. Download the server files first."
@@ -275,9 +276,11 @@ async fn start_inner(
     // so these lines stop the console from looking frozen.
     let loader_first_run =
         matches!(manifest.server_type.as_str(), "fabric" | "quilt")
-            && !dir
-                .join(format!("{}-server-launch.jar", manifest.server_type))
-                .exists();
+            && !safe_relative_join(
+                &dir,
+                &format!("{}-server-launch.jar", manifest.server_type),
+            )?
+            .exists();
 
     log_server_step(
         server_id,
diff --git a/packages/app-lib/src/api/servers/manage.rs b/packages/app-lib/src/api/servers/manage.rs
index f6b517c..5890c59 100644
--- a/packages/app-lib/src/api/servers/manage.rs
+++ b/packages/app-lib/src/api/servers/manage.rs
@@ -7,6 +7,7 @@ use crate::state::remove_log_buffer;
 use crate::util::io::{self, IOError};
 use crate::{ErrorKind, Result, State};
 
+use super::files::safe_relative_join;
 use super::lifecycle::is_running;
 use super::manifest::{
     ServerInfo, ServerManifest, build_server_info, read_manifest,
@@ -72,7 +73,7 @@ pub async fn create(
     let state = State::get().await?;
     let id = Uuid::new_v4().to_string();
     let dir_name = format!("{}-{}", sanitize_folder_name(name), &id[..8]);
-    let dir = state.directories.servers_dir().join(&dir_name);
+    let dir = safe_relative_join(&state.directories.servers_dir(), &dir_name)?;
     io::create_dir_all(&dir).await?;
 
     let manifest = ServerManifest {
diff --git a/packages/app-lib/src/api/servers/manifest.rs b/packages/app-lib/src/api/servers/manifest.rs
index 8d2d464..1985d9d 100644
--- a/packages/app-lib/src/api/servers/manifest.rs
+++ b/packages/app-lib/src/api/servers/manifest.rs
@@ -118,7 +118,7 @@ pub(super) async fn server_path(server_id: &str) -> Result<PathBuf> {
         .as_error());
     }
     let state = State::get().await?;
-    let path = state.directories.server_dir(server_id);
+    let path = state.directories.server_dir(server_id)?;
     if !path.is_dir() {
         return Err(ErrorKind::InputError(format!(
             "Unknown server: {server_id}"
diff --git a/packages/app-lib/src/api/servers/modpack.rs b/packages/app-lib/src/api/servers/modpack.rs
index cf73c93..7e0d252 100644
--- a/packages/app-lib/src/api/servers/modpack.rs
+++ b/packages/app-lib/src/api/servers/modpack.rs
@@ -33,7 +33,7 @@ use crate::util::fetch::{
 use crate::util::io::IOError;
 use crate::{ErrorKind, Result};
 
-use super::files::download_to_dir;
+use super::files::{download_to_dir, safe_relative_join};
 use super::manifest::{
     InstallState, ModpackInfo, read_manifest, server_path, write_manifest,
 };
@@ -92,10 +92,52 @@ struct MrpackFile {
     env: Option<MrpackEnv>,
     #[serde(default)]
     downloads: Vec<String>,
-    #[serde(default)]
+    #[serde(default, rename = "fileSize")]
     file_size: Option<u64>,
 }
 
+impl MrpackFile {
+    fn integrity(&self) -> Result<Integrity> {
+        let sha1 = self.hashes.get("sha1").cloned();
+        let sha512 = self.hashes.get("sha512").cloned();
+        if sha1.is_none() && sha512.is_none() {
+            return Err(ErrorKind::InputError(format!(
+                "Modpack file {} is missing a SHA-1 or SHA-512 checksum",
+                self.path
+            ))
+            .as_error());
+        }
+        for (hash, length) in [(&sha1, 40), (&sha512, 128)] {
+            if let Some(hash) = hash
+                && (hash.len() != length
+                    || !hash.bytes().all(|byte| byte.is_ascii_hexdigit()))
+            {
+                return Err(ErrorKind::InputError(format!(
+                    "Modpack file {} has an invalid checksum",
+                    self.path
+                ))
+                .as_error());
+            }
+        }
+        Ok(Integrity {
+            sha1,
+            sha512,
+            size: self.file_size,
+            ..Integrity::default()
+        })
+    }
+}
+
+/// Validate the entire index before starting concurrent downloads, including
+/// entries excluded from server installation but fetched for metadata.
+fn validate_index(index: &MrpackIndex, dir: &Path) -> Result<()> {
+    for file in &index.files {
+        safe_relative_join(dir, &file.path)?;
+        file.integrity()?;
+    }
+    Ok(())
+}
+
 #[derive(Deserialize, Clone)]
 struct MrpackEnv {
     #[serde(default)]
@@ -272,7 +314,7 @@ async fn run_modpack_install(
         &state,
         mrpack_url,
         &dir.join(MRPACK_FILENAME),
-        mrpack_sha1.map(str::to_string),
+        mrpack_sha1.map(Integrity::sha1).unwrap_or_default(),
         ResourceClass::Modpack,
         AggregateProgress::new(0),
     )
@@ -282,6 +324,7 @@ async fn run_modpack_install(
     let manifest_entry = find_manifest_entry(&archive_path).await?;
     let base_folder = base_folder(&manifest_entry);
     let index = parse_index(&archive_path, &manifest_entry).await?;
+    validate_index(&index, dir)?;
 
     let (installable_files, excluded_files): (
         Vec<&MrpackFile>,
@@ -310,19 +353,19 @@ async fn run_modpack_install(
     )
     .await?;
 
-    let files: Vec<(String, String, Option<String>, u64)> = installable_files
+    let files: Vec<(String, String, Integrity, u64)> = installable_files
         .iter()
         .filter_map(|file| {
             file.downloads.first().map(|url| {
-                (
+                Ok((
                     file.path.clone(),
                     url.clone(),
-                    file.hashes.get("sha1").cloned(),
+                    file.integrity()?,
                     file.file_size.unwrap_or(0),
-                )
+                ))
             })
         })
-        .collect();
+        .collect::<Result<_>>()?;
 
     let progress = AggregateProgress::new(total_bytes);
     let state_ref = state.clone();
@@ -336,14 +379,14 @@ async fn run_modpack_install(
         0.0,
         num_files,
         None,
-        move |(path, url, sha1, size)| {
+        move |(path, url, integrity, size)| {
             let server_id = server_id_ref.clone();
             let dir = dir_ref.clone();
             let state = state_ref.clone();
             let progress = progress.clone();
             async move {
                 log(&server_id, &format!("Downloading {path}")).await?;
-                let destination = dir.join(&path);
+                let destination = safe_relative_join(&dir, &path)?;
                 if let Some(parent) = destination.parent() {
                     crate::util::io::create_dir_all(parent).await?;
                 }
@@ -352,7 +395,7 @@ async fn run_modpack_install(
                     &state,
                     &url,
                     &destination,
-                    sha1,
+                    integrity,
                     ResourceClass::Modpack,
                     progress.clone(),
                 )
@@ -446,7 +489,7 @@ async fn download_icon(dir: &Path, icon_url: &str) -> Result<PathBuf> {
             )
         })
         .unwrap_or_else(|| "png".to_string());
-    let destination = dir.join(format!("icon.{extension}"));
+    let destination = safe_relative_join(dir, &format!("icon.{extension}"))?;
 
     let client = reqwest::Client::builder()
         .user_agent(crate::launcher_user_agent())
@@ -476,15 +519,13 @@ async fn download_with_engine(
     state: &State,
     url: &str,
     destination: &Path,
-    sha1: Option<String>,
+    integrity: Integrity,
     resource: ResourceClass,
     progress: AggregateProgress,
 ) -> Result<()> {
-    let mut request = DownloadRequest::new(url, resource);
-    if let Some(sha1) = &sha1 {
-        request = request.with_integrity(Integrity::sha1(sha1.clone()));
-    }
-    request = request.with_segmented_download(true);
+    let request = DownloadRequest::new(url, resource)
+        .with_integrity(integrity)
+        .with_segmented_download(true);
 
     let server_id = server_id.to_string();
     let progress = progress.clone();
@@ -579,7 +620,7 @@ async fn fetch_excluded_mod_ids(
     state: &State,
     excluded_files: &[&MrpackFile],
 ) -> Result<HashSet<String>> {
-    let candidates: Vec<(String, String, Option<String>)> = excluded_files
+    let candidates: Vec<(String, String, Integrity)> = excluded_files
         .iter()
         .filter_map(|file| {
             let path = file.path.replace('\\', "/");
@@ -589,14 +630,14 @@ async fn fetch_excluded_mod_ids(
                 return None;
             }
             file.downloads.first().map(|url| {
-                (
+                Ok((
                     path.rsplit('/').next().unwrap_or(&path).to_string(),
                     url.clone(),
-                    file.hashes.get("sha1").cloned(),
-                )
+                    file.integrity()?,
+                ))
             })
         })
-        .collect();
+        .collect::<Result<_>>()?;
     if candidates.is_empty() {
         return Ok(HashSet::new());
     }
@@ -611,9 +652,10 @@ async fn fetch_excluded_mod_ids(
         ErrorKind::FSError(format!("Failed to create temporary directory: {e}"))
     })?;
     let mut unavailable = HashSet::new();
-    for (filename, url, sha1) in candidates {
-        let destination = temp_dir.path().join(&filename);
-        match download_metadata_jar(state, &url, &destination, sha1).await {
+    for (filename, url, integrity) in candidates {
+        let destination = safe_relative_join(temp_dir.path(), &filename)?;
+        match download_metadata_jar(state, &url, &destination, integrity).await
+        {
             Ok(()) => match read_mod_metadata(&destination) {
                 Some(metadata) => {
                     unavailable
@@ -647,12 +689,10 @@ async fn download_metadata_jar(
     state: &State,
     url: &str,
     destination: &Path,
-    sha1: Option<String>,
+    integrity: Integrity,
 ) -> Result<()> {
-    let mut request = DownloadRequest::new(url, ResourceClass::Modpack);
-    if let Some(sha1) = &sha1 {
-        request = request.with_integrity(Integrity::sha1(sha1.clone()));
-    }
+    let request = DownloadRequest::new(url, ResourceClass::Modpack)
+        .with_integrity(integrity);
     download_to_path(
         request,
         destination,
@@ -908,6 +948,122 @@ mod tests {
     use super::*;
     use std::io::Write;
 
+    fn index_file(path: &str) -> serde_json::Value {
+        serde_json::json!({
+            "path": path,
+            "hashes": { "sha1": "0".repeat(40) },
+            "downloads": ["https://example.com/mod.jar"],
+            "fileSize": 10
+        })
+    }
+
+    #[tokio::test]
+    async fn malicious_index_paths_fail_before_materializing_files() {
+        let temp = tempfile::tempdir().unwrap();
+        let server = temp.path().join("server");
+        std::fs::create_dir(&server).unwrap();
+        let outside = temp.path().join("outside.txt");
+        std::fs::write(&outside, "unchanged").unwrap();
+        let archive = temp.path().join("pack.mrpack");
+        for path in [
+            "../outside.txt",
+            "mods/../../outside.txt",
+            r"..\outside.txt",
+            r"mods/..\../outside.txt",
+            "/tmp/outside.txt",
+            "C:/outside.txt",
+            r"C:\outside.txt",
+            "C:outside.txt",
+            r"\\server\share\outside.txt",
+            r"\outside.txt",
+            r"\\?\C:\outside.txt",
+            "mods//mod.jar",
+            "mods/./mod.jar",
+            "mods/.. /outside.txt",
+            "mods/mod.jar:payload",
+            "NUL/outside.txt",
+            "mods/CON.jar",
+            "",
+        ] {
+            for environment in ["required", "unsupported"] {
+                let mut malicious = index_file(path);
+                // Filtered entries must also fail before any downloads.
+                malicious["env"] = serde_json::json!({"server": environment});
+                let contents = serde_json::json!({
+                    "files": [index_file("mods/valid.jar"), malicious]
+                })
+                .to_string();
+                create_jar(&archive, &[(MRPACK_MANIFEST_ENTRY, &contents)]);
+                let index =
+                    parse_index(&archive, MRPACK_MANIFEST_ENTRY).await.unwrap();
+                assert!(validate_index(&index, &server).is_err(), "{path:?}");
+                assert_eq!(
+                    std::fs::read_to_string(&outside).unwrap(),
+                    "unchanged"
+                );
+                assert_eq!(std::fs::read_dir(&server).unwrap().count(), 0);
+            }
+        }
+    }
+
+    #[test]
+    fn index_requires_valid_supported_checksums() {
+        for hashes in [
+            serde_json::json!({}),
+            serde_json::json!({"md5": "0".repeat(32)}),
+            serde_json::json!({"sha1": ""}),
+            serde_json::json!({"sha1": "0".repeat(39)}),
+            serde_json::json!({"sha512": "g".repeat(128)}),
+            serde_json::json!({"sha1": "invalid", "sha512": "0".repeat(128)}),
+        ] {
+            let mut file = index_file("mods/mod.jar");
+            file["hashes"] = hashes;
+            let index: MrpackIndex =
+                serde_json::from_value(serde_json::json!({"files": [file]}))
+                    .unwrap();
+            assert!(validate_index(&index, Path::new("server")).is_err());
+        }
+        let mut file = index_file("mods/mod.jar");
+        file.as_object_mut().unwrap().remove("hashes");
+        let index: MrpackIndex =
+            serde_json::from_value(serde_json::json!({"files": [file]}))
+                .unwrap();
+        assert!(validate_index(&index, Path::new("server")).is_err());
+    }
+
+    #[tokio::test]
+    async fn index_checksums_verify_file_contents() {
+        use crate::util::fetch::verify_file;
+        use sha2::{Digest, Sha512};
+
+        let temp = tempfile::tempdir().unwrap();
+        let target = temp.path().join("mod.jar");
+        let contents = b"verified mod";
+        std::fs::write(&target, contents).unwrap();
+        for hashes in [
+            serde_json::json!({"sha1": sha1_smol::Sha1::from(contents).digest().to_string()}),
+            serde_json::json!({"sha512": format!("{:x}", Sha512::digest(contents))}),
+            serde_json::json!({
+                "sha1": sha1_smol::Sha1::from(contents).digest().to_string(),
+                "sha512": format!("{:x}", Sha512::digest(contents))
+            }),
+        ] {
+            let mut file = index_file("mods/mod.jar");
+            file["hashes"] = hashes;
+            file["fileSize"] = serde_json::json!(contents.len());
+            let index: MrpackIndex =
+                serde_json::from_value(serde_json::json!({"files": [file]}))
+                    .unwrap();
+            validate_index(&index, temp.path()).unwrap();
+            let integrity = index.files[0].integrity().unwrap();
+            assert_eq!(integrity.size, Some(contents.len() as u64));
+            verify_file(&target, &integrity).await.unwrap();
+            std::fs::write(&target, b"tampered mod").unwrap();
+            assert!(verify_file(&target, &integrity).await.is_err());
+            std::fs::write(&target, contents).unwrap();
+        }
+    }
+
     #[test]
     fn server_only_files_are_installable() {
         let installable = MrpackFile {
diff --git a/packages/app-lib/src/state/dirs.rs b/packages/app-lib/src/state/dirs.rs
index 4d48b76..c29bf92 100644
--- a/packages/app-lib/src/state/dirs.rs
+++ b/packages/app-lib/src/state/dirs.rs
@@ -171,8 +171,8 @@ impl DirectoryInfo {
 
     /// Gets the directory of a managed dedicated server by id
     #[inline]
-    pub fn server_dir(&self, server_id: &str) -> PathBuf {
-        self.servers_dir().join(server_id)
+    pub fn server_dir(&self, server_id: &str) -> crate::Result<PathBuf> {
+        crate::util::io::safe_relative_join(&self.servers_dir(), server_id)
     }
 
     #[inline]
@@ -708,6 +708,28 @@ mod resolve_game_dir_tests {
         }
     }
 
+    #[test]
+    fn server_directory_rejects_absolute_and_windows_alias_paths() {
+        let dirs = dirs();
+        assert_eq!(
+            dirs.server_dir("server-1234").unwrap(),
+            dirs.servers_dir().join("server-1234")
+        );
+        for id in [
+            "../outside",
+            "C:outside",
+            "C:/outside",
+            r"\outside",
+            r"\\server\share",
+            ".",
+            "NUL",
+            "server:stream",
+            "server.",
+        ] {
+            assert!(dirs.server_dir(id).is_err(), "{id:?}");
+        }
+    }
+
     #[test]
     fn override_used_when_set_and_absolute() {
         let dirs = dirs();
diff --git a/packages/app-lib/src/util/io.rs b/packages/app-lib/src/util/io.rs
index 6b6fa57..1ac23d0 100644
--- a/packages/app-lib/src/util/io.rs
+++ b/packages/app-lib/src/util/io.rs
@@ -60,6 +60,34 @@ pub(crate) fn io_error_with_lock_info(
     io_error_with_lock_info_for_paths(source, path, &[path])
 }
 
+/// Joins an untrusted Unix-style relative file path, rejecting traversal and
+/// Windows aliases on every platform. This is a lexical check, not a symlink
+/// sandbox.
+pub(crate) fn safe_relative_join(
+    root: &Path,
+    relative: &str,
+) -> crate::Result<PathBuf> {
+    path_util::SafeRelativeUtf8UnixPathBuf::try_from(relative.to_string())
+        .map_err(|error| {
+            crate::ErrorKind::InputError(format!(
+                "Invalid file path {relative}: {error}"
+            ))
+        })?;
+    if relative.split('/').any(|segment| {
+        segment.is_empty()
+            || segment == "."
+            || segment.ends_with(['.', ' '])
+            || segment.contains(':')
+            || segment.chars().any(char::is_control)
+    }) {
+        return Err(crate::ErrorKind::InputError(format!(
+            "Invalid file path: {relative}"
+        ))
+        .into());
+    }
+    Ok(root.join(relative))
+}
+
 /// Join a user-provided relative path onto `root`, rejecting traversal.
 pub(crate) fn join_within_root(
     root: &Path,
@@ -95,7 +123,7 @@ pub(crate) fn join_within_root(
         )
         .into());
     }
-    Ok(root.join(safe))
+    safe_relative_join(root, &safe.to_string_lossy().replace('\\', "/"))
 }
 
 pub(crate) fn io_error_with_lock_info_for_paths(
@@ -895,6 +923,39 @@ mod windows_sharing_violation_tests {
     }
 }
 
+#[cfg(test)]
+mod path_join_tests {
+    use super::*;
+
+    #[test]
+    fn join_within_root_uses_portable_path_validation() {
+        let root = Path::new("root");
+        assert_eq!(
+            join_within_root(root, r"config\example.toml").unwrap(),
+            root.join("config/example.toml")
+        );
+        assert_eq!(
+            join_within_root(root, "./config/./example.toml").unwrap(),
+            root.join("config/example.toml")
+        );
+        for relative in [
+            "../outside",
+            r"..\outside",
+            "/outside",
+            "C:/outside",
+            "C:outside",
+            r"\\server\share",
+            "config/.. /outside",
+            "config/file:stream",
+            "config/NUL",
+            "config/file.",
+            "config/file\0",
+        ] {
+            assert!(join_within_root(root, relative).is_err(), "{relative:?}");
+        }
+    }
+}
+
 #[cfg(test)]
 mod copy_dir_tests {
     use super::*;