How severe is CVE-2025-59420?

CVE-2025-59420 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity high, and availability none.

Is CVE-2025-59420 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (16th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2025-59420?

CVE-2025-59420 affects Authlib. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2025-59420?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

Does CVE-2025-59420 have an EU (EUVD) identifier?

Yes. CVE-2025-59420 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2025-30790.

When was CVE-2025-59420 published?

CVE-2025-59420 was published on 2025-09-22 and last updated on 2026-06-17.

CVE-2025-59420

CVE-2025-59420 is a high-severity vulnerability in Authlib with a CVSS 3.x base score of 7.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-345.

Key facts

Severity: High (CVSS 3.x base score 7.5)
EPSS exploit prediction: 0% (16th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2025-30790
Weakness: CWE-345
Affected product: Authlib
Published: 2025-09-22
Last modified: 2026-06-17

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.

Frequently asked questions

What is CVE-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 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.
How severe is CVE-2025-59420?: CVE-2025-59420 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity high, and availability none.
Is CVE-2025-59420 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (16th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2025-59420?: CVE-2025-59420 affects Authlib. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2025-59420?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
Does CVE-2025-59420 have an EU (EUVD) identifier?: Yes. CVE-2025-59420 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2025-30790.
When was CVE-2025-59420 published?: CVE-2025-59420 was published on 2025-09-22 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:authlib:authlib:*:*:*:*:*:*:*:*

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