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CVE-2025-59420

Authlib broken access control

7.5 HIGHPublic exploitEPSS 0.26%
  1. Published22 Sept 2025

Description

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.

CVSS 3.1 breakdown

Attack vectorNetwork
Attack complexityLow
Privileges requiredNone
User interactionNone
ScopeUnchanged
ConfidentialityNone
IntegrityHigh
AvailabilityNone
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Affected products

Authlib< 1.6.4

Remediation

Fixed in
Authlib≥ 1.6.4

References

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Frequently asked

Is CVE-2025-59420 being actively exploited?

CVE-2025-59420 is not in the CISA KEV catalog. A public exploit reference exists, so treat it as exploitable. Its EPSS exploit probability is 0.26%.

How severe is CVE-2025-59420?

CVE-2025-59420 has a CVSS 3.1 base score of 7.5, rated high. EPSS estimates a 0.26% probability of exploitation.

Which products are affected by CVE-2025-59420?

Affected products include Authlib.

How do I fix CVE-2025-59420?

Upgrade affected software to a fixed version, for example Authlib 1.6.4 or later, then confirm no affected versions remain in your inventory.

How Secuno handles this

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Data from the NVD, CISA KEV catalog and FIRST EPSS. CVSS 3.1 base score shown where available.