CVE-2025-59420

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

References

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

• 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.

The application does not correctly enforce access controls, allowing a principal to access resources or operations beyond their granted permissions. Typical impact: unauthorized data access or execution of privileged operations.

CVE-2025-59420 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.6.4); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2025-59420? CVE-2025-59420 is a high-severity incorrect authorization vulnerability in authlib (pip), affecting versions < 1.6.4. It is fixed in 1.6.4. The application does not correctly enforce access controls, allowing a principal to access resources or operations beyond their granted permissions.
2. How severe is CVE-2025-59420? CVE-2025-59420 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of authlib are affected by CVE-2025-59420? authlib (pip) versions < 1.6.4 is affected.
4. Is there a fix for CVE-2025-59420? Yes. CVE-2025-59420 is fixed in 1.6.4. Upgrade to this version or later.
5. Is CVE-2025-59420 exploitable, and should I be worried? Whether CVE-2025-59420 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2025-59420 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2025-59420? Upgrade authlib to 1.6.4 or later.