CVE-2025-59420 | Authlib: JWS/JWT accepts unknown crit headers (RFC violation → possible authz bypass)

Exp

Authlib is a Python library which builds OAuth and OpenID Connect servers. Prior to version 1.6.4, 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. This issue has been patched in version 1.6.4.

Published: 2025-09-22 Last update: 2025-11-03 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2025-59420 is rated Exploit Available (50/100): CVSS High severity, with low exploitation likelihood (EPSS 0.01%). 1 public exploit reference(s) are indexed (Exploit-DB). Public exploits are available—assess exposure, apply mitigations, and prioritize patching.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Public exploit references (Exploit-DB) for CVE-2025-59420

EDB-ID Source Kind Published Link
— nvd_ref exploit_tag Exploit-DB ↗

Exploit prediction scoring system (EPSS) score for CVE-2025-59420

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2025-09-23 — 0.01% —

Full EPSS history (1 record total)

Common vulnerability scoring system (CVSS) metrics for CVE-2025-59420

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
7.5 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:H)
They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:N)
Service keeps running; no real outage angle.
 3.9 3.6 [email protected]

Weakness enumeration for CVE-2025-59420

GitHub Security Advisory for CVE-2025-59420

GHSA-9ggr-2464-2j32 · Severity: high · Ecosystem: pip — Authlib: JWS/JWT accepts unknown crit headers (RFC violation → possible authz bypass)

OS Trackers for CVE-2025-59420

vendor priority summary link
debian not yet assigned CVE-2025-59420 not yet assigned priority: Debian including 1 source packages (python-authlib), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 5. https://security-tracker.debian.org/tracker/CVE-2025-59420
redhat high — https://access.redhat.com/security/cve/CVE-2025-59420
ubuntu medium CVE-2025-59420 medium priority: Ubuntu including 1 source packages (python-authlib), 5 status rows across 5 suites (jammy, noble, plucky, questing, upstream): released 2, ignored 1, needed 1, needs-triage 1. https://ubuntu.com/security/CVE-2025-59420

Affected software / configurations for CVE-2025-59420

Vendor Product Version Raw CPE
authlib authlib < 1.6.4 cpe:2.3:a:authlib:authlib:*:*:*:*:*:*:*:*

References for CVE-2025-59420

cvelogic Threat Intelligence