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The Bug
The tokenPath() function in internal/oauth/oauth.go:301-317 used strings.HasPrefix to verify paths stayed within tokensDir, but
didn't detect symlinks. An attacker who could create a symlink inside tokensDir (e.g., tokensDir/evil.json -> /etc/passwd) could
cause token data to be written outside the tokens directory.
The Fix
Added symlink detection using os.Lstat() and filepath.EvalSymlinks():
// Check if path is a symlink that could escape tokensDir
if info, err := os.Lstat(cleanPath); err == nil && info.Mode()&os.ModeSymlink != 0 {
// Path exists and is a symlink - resolve it and verify it stays within tokensDir
resolved, err := filepath.EvalSymlinks(cleanPath)
if err != nil || !isPathWithinDir(resolved, cleanTokensDir) {
// Symlink resolution failed or escapes tokensDir - use hash-based fallback
return filepath.Join(m.tokensDir, fmt.Sprintf("%x.json", sha256.Sum256([]byte(email))))
}
}
Also added a helper function isPathWithinDir() that properly resolves symlinks in the base directory before comparing paths.
Changes Made
- internal/oauth/oauth.go: Added symlink detection in tokenPath() and new isPathWithinDir() helper (13 lines added)
- internal/oauth/oauth_test.go: Added TestTokenPath_SymlinkEscape test case (47 lines added)
- Fix incomplete path prefix check: Replace strings.HasPrefix with hasPathPrefix() that properly handles directory separators, preventing attacks where tokensDir-evil would pass the check for tokensDir - Add hasPathPrefix() helper for safe directory prefix checking without symlink resolution, keeping isPathWithinDir() for resolved symlink checks - Document TOCTOU limitation in symlink check with explanation of the low practical risk due to required attacker capabilities - Improve test to verify exact hash-based fallback path format Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
wesm
approved these changes
Feb 5, 2026
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Fix for a security attack
The tokenPath() function in internal/oauth/oauth.go:301-317 used strings.HasPrefix to verify paths stayed within tokensDir, but didn't detect symlinks. An attacker who could create a symlink inside tokensDir (e.g., tokensDir/evil.json -> /etc/passwd) could cause token data to be written outside the tokens directory.