PYSEC-2026-1200

Summary

Authlib’s JWS verification accepts tokens that declare unknown critical header parameters (crit), violating RFC 7515 “must‑understand” semantics. An attacker can craft a signed token with a critical header (for example, bork or cnf) that strict verifiers reject but Authlib accepts. In mixed‑language fleets, this enables split‑brain verification and can lead to policy bypass, replay, or privilege escalation.

Affected Component and Versions

• Library: Authlib (JWS verification)
• API: authlib.jose.JsonWebSignature.deserialize_compact(...)
• Version tested: 1.6.3
• Configuration: Default; no allowlist or special handling for crit

Details

RFC 7515 (JWS) §4.1.11 defines crit as a “must‑understand” list: recipients MUST understand and enforce every header parameter listed in crit, otherwise they MUST reject the token. Security‑sensitive semantics such as token binding (e.g., cnf from RFC 7800) are often conveyed via crit.

Observed behavior with Authlib 1.6.3:

• When a compact JWS contains a protected header with crit: ["cnf"] and a cnf object, or crit: ["bork"] with an unknown parameter, Authlib verifies the signature and returns the payload without rejecting the token or enforcing semantics of the critical parameter.
• By contrast, Java Nimbus JOSE+JWT (9.37.x) and Node jose v5 both reject such tokens by default when crit lists unknown names.

Impact in heterogeneous fleets:

• A strict ingress/gateway (Nimbus/Node) rejects a token, but a lenient Python microservice (Authlib) accepts the same token. This split‑brain acceptance bypasses intended security policies and can enable replay or privilege escalation if crit carries binding or policy information.

Proof of Concept (PoC)

This repository provides a multi‑runtime PoC demonstrating the issue across Python (Authlib), Node (jose v5), and Java (Nimbus).

Prerequisites

• Python 3.8+
• Node.js 18+
• Java 11+ with Maven

Setup

Enter the directory authlib-crit-bypass-poc & run following commands.

make setup
make tokens

Tokens minted

• tokens/unknown_crit.jwt with protected header: { "alg": "HS256", "crit": ["bork"], "bork": "x" }
• tokens/cnf_header.jwt with protected header: { "alg": "HS256", "crit": ["cnf"], "cnf": {"jkt": "thumb-42"} }

Reproduction

Run the cross‑runtime demo:

make  demo

Expected output for each token (strict verifiers reject; Authlib accepts):

For tokens/unknown_crit.jwt:

Strict(Nimbus): REJECTED (unknown critical header: bork)
Strict(Node jose): REJECTED (unrecognized crit)
Lenient(Authlib): ACCEPTED -> payload={'sub': '123', 'role': 'user'}

For tokens/cnf_header.jwt:

Strict(Nimbus): REJECTED (unknown critical header: cnf)
Strict(Node jose): REJECTED (unrecognized crit)
Lenient(Authlib): ACCEPTED -> payload={'sub': '123', 'role': 'user'}

Environment notes:

• Authlib version used: 1.6.3 (from PyPI)
• Node jose version: ^5
• Nimbus JOSE+JWT version: 9.37.x
• HS256 secret is 32 bytes to satisfy strict verifiers: 0123456789abcdef0123456789abcdef

Impact

• Class: Violation of JWS crit “must‑understand” semantics; specification non‑compliance leading to authentication/authorization policy bypass.
• Who is impacted: Any service that relies on crit to carry mandatory security semantics (e.g., token binding via cnf) or operates in a heterogeneous fleet with strict verifiers elsewhere.
• Consequences: Split‑brain acceptance (gateway rejects while a backend accepts), replay, or privilege escalation if critical semantics are ignored.

References

• RFC 7515: JSON Web Signature (JWS), §4.1.11 crit
• RFC 7800: Proof‑of‑Possession Key Semantics for JWTs (cnf)