VulnerabilityPublished 13 Aug 2026Updated 29 Oct 2026

CVE-2026-27962: mcp-atlassian · Authlib JWS JWK Header Injection: Signature Verification Bypass

Affects mcp-atlassianSource GCVE Database

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Not known to be exploited · CVSS 9.1 Critical : upgrade to 0.21.0 as a priority.

Data as of 30 Oct 2026, 08:31 UTC

CVSS 3.1 base score

9.1Critical

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

Patch: Available – fixed in 0.21.0

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

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: High
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: High: There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.
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 29 Oct 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 mcp-atlassian to 0.21.0.

Patches and advisories

Affected software

Product	Affected	Fixed in
Homebrewmcp-atlassian`pkg:brew/mcp-atlassian`Other threats	Affected >=0.11.11 <0.21.0	Fixed in 0.21.0

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 in authlib version 0.21.0 that fixes this vulnerability. Users should upgrade to version 0.21.0 or later. Until patched, avoid passing key=None to JWS deserialization functions and ensure that key resolver callables never return None for unknown 'kid' values without proper handling. Review application code to prevent fallback to the 'jwk' header as a verification key. Follow vendor advisories for detailed remediation instructions.

Description

Authlib's JWS implementation contains a JWK header injection vulnerability that allows unauthenticated attackers to forge JWT tokens that pass signature verification. When the deserialization function is called with key=None or a key resolver returns None, the library uses the attacker-controlled 'jwk' header as the verification key, violating RFC 7515. This enables attackers to bypass authentication and authorization by crafting tokens with arbitrary claims and embedding their own public key in the header.

Impact

This vulnerability allows an unauthenticated attacker to bypass authentication and authorization controls by forging JWT tokens that the server accepts as valid. The attacker can impersonate any user or assume any privilege encoded in the JWT claims without possessing legitimate credentials or server-side keys. This can lead to unauthorized access to protected resources and elevated privileges within affected applications.

Summary

The vulnerability exists in authlib's JWS deserialization methods when key=None is passed or when a key resolver callable returns None for unknown 'kid' values. The library then extracts the 'jwk' field from the JWT header, which is attacker-controlled, and uses it as the verification key. This behavior violates RFC 7515 §4.1.3 and §5.2, which state that the verification key must come from the application context, not the token itself. An attacker can generate their own keypair, sign a token with their private key, embed the public key in the 'jwk' header, and have the server accept the forged token as valid, bypassing authentication and authorization. The vulnerability was confirmed in authlib version 1.6.6 and affects versions >=0.11.11 <0.21.0. A fix is available in version 0.21.0.

References

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