File Browser has an access rule bypass via HasPrefix without trailing separator in path matching

Hi,

The Matches() function in rules/rules.go uses strings.HasPrefix() without a trailing directory separator when matching paths against access rules. A rule for /uploads also matches /uploads_backup/, granting or denying access to unintended directories. Verified against v2.62.2 (commit 860c19d).

Details

At rules/rules.go:29-35:

func (r *Rule) Matches(path string) bool {
    if r.Regex {
        return r.Regexp.MatchString(path)
    }
    return strings.HasPrefix(path, r.Path)
}

When a rule has Path: "/uploads", any path starting with /uploads matches, including /uploads_backup/secret.txt. The regex variant at line 31 uses proper matching, but the non-regex path uses a prefix check without ensuring the match ends at a directory boundary.

The Check() function at http/data.go:29-48 iterates all rules with last-match-wins semantics. No secondary validation exists beyond this prefix check.

PoC

Admin configures: allow rule Path: "/shared" for a restricted user.

Filesystem contains:

• /shared/ (intended to be accessible)
• /shared_private/ (intended to be restricted)

User requests /shared_private/secret.txt:

• strings.HasPrefix("/shared_private/secret.txt", "/shared") returns true
• Allow rule applies
• Access granted to the unintended directory

Impact

Authenticated users can access files in sibling directories that share a common prefix with an allowed directory, bypassing the admin's intended access configuration.

Prior art

Prior advisories GHSA-4mh3-h929-w968 (path-based access control bypass) and GHSA-9f3r-2vgw-m8xp (path traversal in copy/rename) addressed related access control issues. This HasPrefix prefix-collision is a distinct, unreported variant.

Suggested Fix

func (r *Rule) Matches(path string) bool {
    if r.Regex {
        return r.Regexp.MatchString(path)
    }
    prefix := r.Path
    if prefix != "/" && !strings.HasSuffix(prefix, "/") {
        prefix += "/"
    }
    return path == r.Path || strings.HasPrefix(path, prefix)
}

Koda Reef

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Update: Fix submitted as PR #5889.