@cloudflare/workers-oauth-provider, PKCE Bypass via Downgrade Attack, CVE-2025-XXXX (Moderate)

How the CVE Works

The vulnerability in @cloudflare/workers-oauth-provider allows an attacker to bypass Proof Key for Code Exchange (PKCE) protection via a downgrade attack. PKCE is a security mechanism in OAuth 2.0 (and required in OAuth 2.1) designed to prevent authorization code interception attacks. The flaw occurs when an attacker manipulates the OAuth flow to skip PKCE validation, effectively downgrading the security to a weaker OAuth 2.0 implementation. By omitting or tampering with the `code_challenge` parameter, the attacker forces the server to process the authorization request without PKCE verification, leaving the system vulnerable to code injection or token theft.

DailyCVE Form

Platform: Cloudflare Workers
Version: < 0.0.5
Vulnerability: PKCE bypass
Severity: Moderate
Date: May 1, 2025

What Undercode Say:

Exploitation:

Intercept OAuth Flow – Capture the authorization request before PKCE validation.
Strip PKCE Parameters – Remove `code_challenge` and code_challenge_method.
Force Legacy OAuth – Send the modified request to exploit missing server-side checks.

Protection:

1. Update Immediately – Upgrade to `@cloudflare/[email protected]`.

2. Enforce PKCE – Reject requests without `code_challenge`.

Log Tampering Attempts – Monitor for missing PKCE parameters.

Commands:

npm update @cloudflare/workers-oauth-provider --save

Code Fix (Server-Side Validation):

if (!req.query.code_challenge) {
throw new Error("PKCE required: code_challenge missing");
}

Analytics:

Attack Vector: Network-based manipulation.
Exploit Complexity: Low (requires MITM or API tampering).
Mitigation Difficulty: Easy (patch available).

Detection:

grep -r "code_challenge" /path/to/oauth/configs