[Security][High] Non-expiring download tokens allow persistent file access after access revocation

[bmersereau]

Severity: High

File: backend/src/lib/downloadTokens.ts:44-52
CWE: CWE-613 — Insufficient Session Expiration
OWASP: A07:2021 — Identification and Authentication Failures

Description

HMAC-signed download tokens carry no expiration field. Once issued and embedded in chat history, they are permanently valid:

export function signDownload(path: string, filename: string): string {
    const payload = JSON.stringify({ p: path, f: filename });
    // No `exp` field — token never expires

Impact

Users who later lose access to a project or document — through removal from shared_with, document deletion, or account termination — retain the ability to download files via any download URL saved from their chat history. The download route re-checks ownership at request time (providing partial mitigation), but if the signing secret is ever compromised (see #66), all tokens ever issued become immediately forgeable with no expiry to limit the window.

Fix

Add an exp field to the token payload and validate it at verify time:

export function signDownload(path: string, filename: string, ttlSeconds = 900): string {
    const exp = Math.floor(Date.now() / 1000) + ttlSeconds;
    const payload = JSON.stringify({ p: path, f: filename, exp });
    const enc = b64urlEncode(Buffer.from(payload, "utf8"));
    const sig = crypto.createHmac("sha256", getSecret()).update(enc).digest();
    return `${enc}.${b64urlEncode(sig)}`;
}

export function verifyDownload(token: string): { path: string; filename: string } | null {
    // ... existing HMAC check ...
    const parsed = JSON.parse(b64urlDecode(enc).toString("utf8"));
    if (!parsed?.p || !parsed?.f) return null;
    if (parsed.exp && Math.floor(Date.now() / 1000) > parsed.exp) return null;
    return { path: parsed.p, filename: parsed.f };
}

Consider whether download links in chat history should be refreshed on load (client fetches a new signed URL when rendering old messages).

Remediation tier: High — fix before next release.