🔒 Fix potential symlink bypass in path_allowed

Co-authored-by: Horizontal42 <75383519+Horizontal42@users.noreply.github.com>

poc.rs

@@ -0,0 +1,57 @@
+use std::path::{Path, PathBuf};
+use std::fs;
+
+fn canon_or_raw(p: &Path) -> PathBuf {
+    let mut normalized = PathBuf::new();
+    for component in p.components() {
+        match component {
+            std::path::Component::ParentDir => {
+                let can_pop = matches!(
+                    normalized.components().next_back(),
+                    Some(std::path::Component::Normal(_))
+                );
+                if can_pop {
+                    normalized.pop();
+                } else if !normalized.has_root() {
+                    normalized.push(component);
+                }
+            }
+            std::path::Component::CurDir => {}
+            _ => normalized.push(component),
+        }
+    }
+    normalized
+}
+
+fn secure_canonicalize(path: &Path) -> PathBuf {
+    let mut p = PathBuf::new();
+    for component in path.components() {
+        p.push(component);
+        if let Ok(canon) = fs::canonicalize(&p) {
+            p = canon;
+        }
+    }
+    canon_or_raw(&p)
+}
+
+fn main() {
+    let temp = std::env::temp_dir();
+    let real_allowed = temp.join("allowed_dir_abc");
+    fs::create_dir_all(&real_allowed).unwrap();
+    let secret = temp.join("secret_dir_xyz");
+    fs::create_dir_all(&secret).unwrap();
+
+    // Attacker makes a symlink from allowed/sym -> target
+    let symlink = real_allowed.join("sym");
+    #[cfg(unix)]
+    std::os::unix::fs::symlink(&secret, &symlink).unwrap_or(());
+
+    // Attacker tries to write to allowed/sym/../secret_dir_xyz/new_file
+    let malicious = symlink.join("..").join("secret_dir_xyz").join("new_file");
+    println!("malicious path: {:?}", malicious);
+    println!("secure_canonicalize: {:?}", secure_canonicalize(&malicious));
+    println!("canon_or_raw: {:?}", canon_or_raw(&malicious));
+
+    let allowed = real_allowed.clone();
+    println!("secure_canonicalize(allowed): {:?}", secure_canonicalize(&allowed));
+}

src-tauri/src/main.rs

@@ -67,12 +67,23 @@ fn canon_or_raw(p: &Path) -> PathBuf {
     normalized
 }
 
+fn secure_canonicalize(path: &Path) -> PathBuf {
+    let mut p = PathBuf::new();
+    for component in path.components() {
+        p.push(component);
+        if let Ok(canon) = fs::canonicalize(&p) {
+            p = canon;
+        }
+    }
+    canon_or_raw(&p)
+}
+
 /// Exact match against an authorized folder, compared canonicalized so a
 /// symlink or ".." spelling cannot stand in for a different path. A subfolder
 /// of an authorized folder is not itself authorized.
 fn path_allowed(candidate: &Path, allowed: &[PathBuf]) -> bool {
-    let c = canon_or_raw(candidate);
+    let c = secure_canonicalize(candidate);
-    allowed.iter().any(|a| canon_or_raw(a) == c)
+    allowed.iter().any(|a| secure_canonicalize(a) == c)
 }
 
 /// The custom folder recorded in the app's own settings.json, if any. Only

test_fix.rs

@@ -0,0 +1,58 @@
+use std::path::{Path, PathBuf};
+use std::fs;
+
+fn canon_or_raw(p: &Path) -> PathBuf {
+    let mut normalized = PathBuf::new();
+    for component in p.components() {
+        match component {
+            std::path::Component::ParentDir => {
+                let can_pop = matches!(
+                    normalized.components().next_back(),
+                    Some(std::path::Component::Normal(_))
+                );
+                if can_pop {
+                    normalized.pop();
+                } else if !normalized.has_root() {
+                    normalized.push(component);
+                }
+            }
+            std::path::Component::CurDir => {}
+            _ => normalized.push(component),
+        }
+    }
+    normalized
+}
+
+fn secure_canonicalize(path: &Path) -> PathBuf {
+    let mut p = PathBuf::new();
+    for component in path.components() {
+        p.push(component);
+        if let Ok(canon) = fs::canonicalize(&p) {
+            p = canon;
+        }
+    }
+    // We still need to run canon_or_raw because non-existent parts
+    // (e.g. trailing ".." or ".") were pushed but not canonicalized by fs::canonicalize
+    canon_or_raw(&p)
+}
+
+fn path_allowed(candidate: &Path, allowed: &[PathBuf]) -> bool {
+    let c = secure_canonicalize(candidate);
+    allowed.iter().any(|a| secure_canonicalize(a) == c)
+}
+
+fn main() {
+    let temp = std::env::temp_dir();
+    let real_allowed = temp.join("allowed_dir_abc");
+    fs::create_dir_all(&real_allowed).unwrap();
+    let secret = temp.join("secret_dir_xyz");
+    fs::create_dir_all(&secret).unwrap();
+
+    // Attacker makes a symlink from allowed/sym -> target
+    let symlink = real_allowed.join("sym");
+    #[cfg(unix)]
+    std::os::unix::fs::symlink(&secret, &symlink).unwrap_or(());
+
+    let malicious = symlink.join("..").join("secret_dir_xyz").join("new_file");
+    println!("Does malicious match? {}", path_allowed(&malicious, &[real_allowed.clone()]));
+}