VulnerabilityPublished 13 Aug 2026Updated 3 Nov 2026

CVE-2026-44681: Fastmcp · Authlib OIDC Implicit/Hybrid Authorization Vulnerable to Open Redirect

Affects fastmcpSource GCVE Database

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Not known to be exploited · CVSS 6.1 Medium : upgrade to 3.2.4_2 when convenient.

Data as of 3 Nov 2026, 22:58 UTC

CVSS 3.1 base score

6.1Medium

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

EPSS: 0.2% chance of exploitation in 30 days – higher than 16% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 3.2.4_2

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

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: Required
Scope: Changed
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: 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: Required: Successful exploitation of this vulnerability requires a user to take some action before the vulnerability can be exploited.
Scope: Changed: An exploited vulnerability can affect resources beyond the security scope managed by the security authority of the vulnerable component. In this case, the vulnerable component and the impacted component are different and managed by different security authorities.
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 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 3 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 fastmcp to 3.2.4_2.

Affected software

Product	Affected	Fixed in
Homebrewfastmcp`pkg:brew/fastmcp`Other threats	Affected >=2.12.3 <3.2.4_2	Fixed in 3.2.4_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 3.2.4_2 that fixes this vulnerability. Deployments should upgrade to this fixed version or later to remediate the issue. Until patched, avoid enabling the OpenIDImplicitGrant and OpenIDHybridGrant flows if possible. The vendor advisory confirms the fix in 3.2.4_2. No other mitigations are specified or required.

Description

Authlib's OpenIDImplicitGrant and OpenIDHybridGrant authorization endpoints contain an unauthenticated open redirect vulnerability triggered by authorization requests missing the 'openid' scope. This causes the authorization server to issue an HTTP 302 redirect to an attacker-chosen URL without validating the redirect URI. The flaw affects servers supporting OIDC Implicit or Hybrid flows using Authlib versions >=2.12.3 and <3.2.4_2. No user authentication or valid client ID is required to exploit this. The vulnerability enables phishing attacks leveraging the trusted authorization server domain in the URL bar but does not leak tokens or codes directly.

Impact

The vulnerability enables open redirect attacks from a trusted authorization server domain, which can be leveraged for phishing and credential harvesting by making malicious URLs appear legitimate. The authorization server's domain reputation can be abused to bypass domain allow-lists. Although no tokens or authorization codes are leaked, the flaw facilitates phishing and downstream confused-deputy attacks. It also violates OAuth 2.0 and OIDC security specifications requiring redirect URI validation. The severity is medium due to the phishing leverage and protocol violation but no direct token compromise.

Summary

The vulnerability exists in Authlib's OpenIDImplicitGrant and OpenIDHybridGrant classes where the presence of the 'openid' scope is checked before validating the redirect URI. If the 'openid' scope is missing, an InvalidScopeError is raised with the raw, attacker-controlled redirect_uri, which is then used to issue an HTTP 302 redirect without validation. This allows an attacker to craft an authorization request that causes the authorization server to redirect victims to an arbitrary URL. The issue is a variant of a previously fixed open redirect flaw but was missed in these OIDC grant types. Exploitation requires no authentication or valid client ID and affects any deployment using these grant types in Authlib versions >=2.12.3 and <3.2.4_2. The flaw violates OAuth 2.0 security best practices by redirecting to unvalidated URIs even in error responses.

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 3 Nov 2026, 21:34 UTC

Technical details

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