VulnerabilityPublished 29 Oct 2026

CVE-2026-114619: Authlib · OpenID implicit grant checks raw scope instead of allowed scope before issuing ID tokens

Affects fastmcpSource GCVE Database

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Authoritative record:CVE-2026-114619· Medium 4.6

Not known to be exploited · CVSS 4.6 Medium : upgrade to 4.1.1 when convenient.

Data as of 30 Oct 2026, 10:16 UTC

CVSS 3.1 base score

4.6Medium

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	4.6

Patch: Available – fixed in 4.1.1

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

Attack Vector: Network
Attack Complexity: Low
Privileges Required: Low
User Interaction: Required
Scope: Unchanged
Confidentiality: Low
Integrity: Low
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: Low: The attacker is authorized with (i.e., requires) privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges may have the ability to cause an impact only to non-sensitive resources.
User Interaction: Required: Successful exploitation of this vulnerability requires a user to take some action before the vulnerability can be exploited.
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: Low: There is some loss of confidentiality. Access to some restricted information is obtained, but the attacker does not have control over what information is obtained, or the amount or kind of loss is limited. The information disclosure does not cause a direct, serious loss to the impacted component.
Integrity: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact on 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 fastmcp to 4.1.1.

Affected software

Product	Affected	Fixed in
Homebrewfastmcp`pkg:brew/fastmcp`Other threats	Affected >=2.12.3 <4.1.1	Fixed in 4.1.1

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

AI-suggested

A fix is available in Authlib version 4.1.1. Users should upgrade to this version or later to resolve the issue. No other mitigation is required as the vulnerability is addressed by correcting the scope validation logic in the OpenID implicit grant flow.

Description

Authlib's OpenID Connect implicit grant implementation improperly checks the raw requested scope instead of the filtered allowed scope before issuing ID tokens. This allows a registered OAuth client, which is not permitted the 'openid' scope but allowed other scopes like 'profile', to receive a signed ID token if the user consents. The ID token contains identity claims such as 'sub' and 'name', effectively bypassing the intended scope restriction. This issue affects versions from 2.12.3 up to but not including 4.1.1 and is fixed in version 4.1.1.

Impact

A registered OAuth client that is not authorized for the 'openid' scope can obtain a signed ID token containing user identity claims if the user logs in and consents. This bypasses the intended scope restriction, potentially exposing identity information to unauthorized clients. The access token remains properly scoped and is rejected by the UserInfo endpoint if insufficient scope is detected. The vulnerability requires the client to be registered with 'id_token' response types and the user to consent, limiting exploitation. Deployments using only the authorization code flow or where all clients with 'id_token' response types also have the 'openid' scope are not affected.

Summary

The vulnerability exists in Authlib's OpenIDImplicitGrant.validate_authorization_request() method, which incorrectly checks the raw client-supplied scope parameter rather than the filtered allowed scope. This flaw enables a client that lacks the 'openid' scope but is allowed an 'id_token' response type and other scopes (e.g., 'profile') to obtain a signed OpenID Connect ID token after user consent. The ID token includes standard claims such as 'sub' and 'name', which should only be issued if the 'openid' scope is granted. Other parts of Authlib correctly enforce scope restrictions, indicating this is a coding bug. The issue is present in versions >=2.12.3 <4.1.1 and fixed in 4.1.1.

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Exploitation evidence

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Affected software

Description

AI analysisMachine-generated, updated 30 Oct 2026, 10:16 UTC

Technical details

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