CVE-2026-114616VulnerabilityPublished 29 Oct 2026

Mcp atlassian: Authlib accepts an empty HMAC key, allowing JWS/JWT signature forgery (HS256/HS384/HS512)

Affects mcp-atlassianSource GCVE Database

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Authoritative record:CVE-2026-114616· High 7.4

Not known to be exploited · CVSS 7.4 High : upgrade to 0.23.1_2 soon.

Data as of 30 Oct 2026, 08:32 UTC

CVSS 3.1 base score

7.4High

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

Thin evidence

One scorer or none

CVSS scores by source
CNA (GitHub_M)	CVSS 3.1	7.4

Patch: Available – fixed in 0.23.1_2

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

Attack Vector: Network
Attack Complexity: High
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: High: A successful attack depends on conditions beyond the attacker's control. That is, a successful attack cannot be accomplished at will, but requires the attacker to invest in some measurable amount of effort in preparation or execution against the vulnerable component before a successful attack can be expected. For example, a successful attack may require an attacker to: gather knowledge about the environment in which the vulnerable target/component exists; prepare the target environment to improve exploit reliability; or inject themselves into the logical network path between the target and the resource requested by the victim in order to read and/or modify network communications (e.g., a man in the middle attack).
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 22 Oct 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 0.23.1_2.

Affected software

Product	Affected	Fixed in
Homebrewmcp-atlassian`pkg:brew/mcp-atlassian`Other threats	Affected >=0.11.11 <0.23.1_2	Fixed in 0.23.1_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 in Authlib version 0.23.1_2 that rejects empty HMAC keys. Users should upgrade to this fixed version or later. Until upgraded, ensure that the HMAC verification key is never empty by validating configuration sources and key resolvers to prevent empty keys from being used.

Description

Authlib versions up to 1.7.2 accept an empty HMAC key for JWS/JWT HS256/HS384/HS512 signature verification, allowing attackers to forge tokens that pass signature checks. This occurs because empty keys produce deterministic digests that anyone can reproduce, enabling token forgery and authentication bypass in OAuth 2.0 and OIDC implementations using Authlib. The vulnerability is confirmed in the v1.7.2 release and remains in the current main branch. A fix is available in version 0.23.1_2.

Impact

An attacker can forge JWT tokens with arbitrary claims that pass signature verification, leading to authentication bypass and unauthorized access in OAuth 2.0 resource servers and OIDC relying parties using vulnerable Authlib versions. This compromises confidentiality and integrity of authentication tokens, potentially allowing privilege escalation or impersonation.

Summary

Authlib's symmetric (oct) key handling does not reject empty keys (empty bytes or empty strings) as valid HMAC signing and verification keys. The key validation only checks type and known unsafe prefixes, but empty keys bypass these checks. Consequently, HMAC operations with an empty key produce deterministic, publicly reproducible digests, allowing attackers to forge arbitrary tokens that pass signature verification under HS256, HS384, and HS512 algorithms. This flaw is distinct from other recent Authlib JWT verification issues and affects the code in authlib/jose/rfc7518/oct_key.py and authlib/jose/rfc7518/jws_algs.py. The vulnerability is realistic when the verification key is sourced from environment variables, configuration, or key resolvers that may yield empty values. The issue is fixed in version 0.23.1_2, and similar protections exist in other JOSE libraries.

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Affected software

Description

AI analysisMachine-generated, updated 30 Oct 2026, 08:32 UTC

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