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Vulnerabilities

Vulnerability · CVE-2025-59420

Authlib is a Python library which builds OAuth and OpenID Connect servers.

Published Sep 22, 2025 · catalog updated Oct 19, 2026 · Authlib is a Python

Last checked Oct 26, 2026. A connected source was read within the last 7 days.

Check if you run it

No confirmed exploitation is on record for CVE-2025-59420. EPSS is 0.26%. Act only if a listed version of Authlib is a Python is installed.

Gaps on this record

  • The stored product name is a package slug. The name on this page is taken from the title (Authlib is a Python).
  • No version range is stored. A customer cannot check an installed version against this record.
  • No vendor fix or required action is stored. Compare the linked advisory before you change production.

Same product and reporting

Severity
High7.5
Known exploitation
Not in CISA/VulnCheck KEV
EPSS
0.26%
Remediation
Advisory link
Affected products
cpe authlib/authlib

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.

Are you affected?

Enter the version you run. The result uses only the ranges stored on this record.

The check returns affected, not affected, or incomplete. It does not guess a version that is missing from the record.

  • cpe authlib/authlibversion unspecified

How to fix CVE-2025-59420

  • No stored fix

    No vendor fix text is stored. Use the linked advisory before you change production.

  • Version check

    No version range is stored, so an installed version cannot be checked on this page.

  • Advisory

    Open the cve_org record and follow its fix.

  • Official advisories

What to do

Three questions, each answered from this record only. Confirm the vendor advisory before you patch.

Should you patch?

No confirmed exploitation is on record for CVE-2025-59420. EPSS is 0.26%. Act only if a listed version of Authlib is a Python is installed.

Which versions are affected?

Affected version ranges are not on this record. Compare the installed version with the linked vendor advisory before you change production.

What should an analyst verify?

CWE-345: Insufficient Verification of Data Authenticity; CWE-863: Incorrect Authorization. EPSS 0.26%. There is no CISA KEV listing. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.

Code

The project this record applies to. Exploit listings stay in References.

Timeline

Detection, updates, and resolution for this record.

  1. Sep 22, 2025 · NIST NVD

    Detected

    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

  2. Oct 19, 2026 · NIST NVD

    Updated

    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

Sources

  • CVE.orgVulnerability recordCVE-2025-59420

    Official CVE identifier

    cve.org
  • FIRST EPSSExploitation probabilityCVE-2025-59420

    30-day exploitation probability 0.26%

    first.org
  • NIST NVDVulnerability recordCVE-2025-59420

    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 toke

    nvd.nist.gov

References

Freshness

Last verified in this catalog Oct 26, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.

  • FIRST EPSSOct 23, 2026
  • CVE.orgOct 21, 2026
  • NIST NVDOct 19, 2026