CVE-2026-114615VulnerabilityPublished 29 Oct 2026

Mcp atlassian: Authlib incomplete algorithm-confusion fix permits HMAC forgery with prefixed public keys

Affects mcp-atlassianSource GCVE Database

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Mentioned vulnerabilities

CVE-2024-37568

Authoritative record:CVE-2026-114615· 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 fixed version of Authlib is available in 0.23.1_2. Users should upgrade to this version or later to fully address the vulnerability. The vendor advisory confirms the fix in 0.23.1_2. No additional mitigations are required if the upgrade is applied. Avoid using the documented `jwt` object with mixed symmetric and asymmetric algorithms without specifying allowed algorithms explicitly.

Description

A vulnerability in the Authlib library (CVE-2026-114615) allows an attacker to forge HMAC signatures by exploiting an incomplete fix for algorithm confusion. The issue arises because the check preventing asymmetric public keys from being used as HMAC secrets only inspects the start of the key bytes. If the PEM public key or X.509 certificate includes a leading byte such as a newline, space, or UTF-8 BOM before the '-----BEGIN' marker, the check is bypassed, allowing an attacker with only the public key to create a valid HS256 token with arbitrary claims. This flaw affects versions >=0.11.11 and <0.23.1_2 of the mcp-atlassian Authlib package and has been fixed in version 0.23.1_2.

Impact

An attacker who has access only to the issuer's public key can forge valid HS256 JWT tokens with arbitrary claims, bypassing authentication and authorization controls that rely on JWT validation. This compromises the integrity and authenticity of tokens, potentially allowing unauthorized access or privilege escalation. The vulnerability affects deployments using the documented `jwt` object in Authlib versions >=0.11.11 and <0.23.1_2. There is no known exploitation in the wild as of the published date.

Summary

The vulnerability in Authlib's `OctKey.import_key()` function stems from a guard that prevents asymmetric public keys from being used as HMAC secrets by checking if the raw key bytes start with certain PEM or SSH key headers. This check uses `bytes.startswith()` anchored at byte 0. However, the asymmetric key loader (`load_pem_public_key` / `load_pem_x509_certificate`) scans forward to find the PEM header, ignoring leading bytes such as newlines, spaces, indentation, UTF-8 BOMs, or comment lines. As a result, keys with one leading byte before the PEM header bypass the guard and are treated as HMAC secrets. An attacker possessing only the issuer's public key can then forge HS256 JWT tokens with arbitrary claims that the `jwt.decode()` method accepts as authentic. This is an incomplete fix of CVE-2024-37568, reintroduced in Authlib 1.8.0 and the latest main branch. The vulnerability requires no secret or user interaction and affects deployments using the documented `jwt` object with the vulnerable versions. The issue is fixed in Authlib version 0.23.1_2.

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Affected software

Description

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

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