VulnerabilityPublished 13 Aug 2026Updated 29 Oct 2026

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

Affects mcpmSource GCVE Database

Compare

Not known to be exploited · CVSS 7.5 High : upgrade to 2.8.2 soon.

Data as of 1 Nov 2026, 22:57 UTC

CVSS 3.1 base score

7.5High

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.2% chance of exploitation in 30 days – higher than 18% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 2.8.2

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: High
Availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 13 Aug 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 1 Nov 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade mcpm to 2.8.2.

Affected software

Product	Affected	Fixed in
Homebrewmcpm`pkg:brew/mcpm`Other threats	Affected >=2.1.0 <2.8.2	Fixed in 2.8.2

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

A patch is available for this vulnerability. Users should upgrade Authlib to a fixed version that correctly enforces the RFC 7515 'must-understand' semantics for critical headers. Until patched, avoid relying on Authlib 1.6.3 for JWS verification in security-sensitive contexts involving critical headers. No vendor advisory content contradicts this guidance.

Description

Authlib version 1.6.3's JWS verification improperly accepts JSON Web Signatures (JWS) tokens containing unknown critical header parameters (`crit`), violating RFC 7515 requirements. This flaw allows tokens with critical headers that strict verifiers reject to be accepted, potentially causing authorization bypass in mixed-language environments. The vulnerability affects the `authlib.jose.JsonWebSignature.deserialize_compact(...)` API with default configuration. Exploitation can lead to split-brain verification scenarios, enabling replay or privilege escalation if critical security semantics are ignored.

Impact

The vulnerability violates the JWS specification by accepting tokens with unknown critical headers, which should be rejected. This non-compliance can cause authorization policy bypass in environments where critical headers convey mandatory security semantics such as token binding (`cnf`). Services relying on Authlib 1.6.3 for JWS verification may accept malicious tokens rejected by other strict verifiers, resulting in split-brain acceptance. This can lead to replay attacks or privilege escalation if critical header semantics are ignored.

Summary

Authlib's JWS verification does not enforce the RFC 7515 'must-understand' semantics for the `crit` header parameter. Specifically, when a token includes unknown critical headers such as `bork` or security-sensitive headers like `cnf`, Authlib 1.6.3 verifies the signature and returns the payload without rejecting the token or enforcing the semantics of these critical headers. This behavior contrasts with strict verifiers like Java Nimbus JOSE+JWT and Node jose, which reject tokens with unknown critical headers. In heterogeneous deployments, this discrepancy can cause a gateway to reject a token while a backend service using Authlib accepts it, leading to potential authorization bypass, replay attacks, or privilege escalation. The vulnerability is identified as CVE-2025-59420 with a CVSS 3.1 score of 7.5 (high severity).

References

Community reviews

Loading reviews…

Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 18 Sep 2026, 01:55 UTC

Technical details

References

Community reviews

Related threats