VulnerabilityPublished 13 Aug 2026Updated 3 Nov 2026

CVE-2025-61920: Fastmcp · Authlib is vulnerable to Denial of Service via Oversized JOSE Segments

Affects fastmcpSource GCVE Database

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Not known to be exploited · CVSS 7.5 High : upgrade to 2.12.4_1 soon.

Data as of 3 Nov 2026, 22:58 UTC

CVSS 3.1 base score

7.5High

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

EPSS: 0.6% chance of exploitation in 30 days – higher than 50% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 2.12.4_1

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

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: High

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: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

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 2.12.4_1.

Affected software

Product	Affected	Fixed in
Homebrewfastmcp`pkg:brew/fastmcp`Other threats	Affected >=2.12.3 <2.12.4_1	Fixed in 2.12.4_1

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 that enforces maximum decoded size limits of 256 KB on JWS/JWT header and signature segments in Authlib. Users should apply the patched release immediately. Additional mitigations include rejecting oversized tokens at the proxy or WAF layer and implementing rate limiting on verification endpoints. Temporary workarounds involve enforcing input size limits before passing tokens to Authlib and applying application-level throttling to reduce amplification risk.

Description

Authlib's JOSE implementation in versions up to 1.6.3 is vulnerable to a denial of service (DoS) attack via oversized JWS/JWT header or signature segments. An attacker can submit tokens with base64url-encoded segments spanning hundreds of megabytes, causing excessive CPU and memory consumption during verification. This leads to service exhaustion and denial of service. The vulnerability is rated high severity with a CVSS score of 7.5. A patch introducing 256 KB limits on header and signature segment sizes is available and should be applied immediately.

Impact

An unauthenticated network attacker can submit malicious JWS/JWT tokens with oversized header or signature segments, causing Authlib to consume excessive CPU and memory during verification. This resource exhaustion can lead to denial of service, making the service unavailable to legitimate users. The vulnerability does not impact confidentiality or integrity but results in availability disruption. The CVSS v3.1 score is 7.5 (High).

Summary

Authlib's JOSE implementation accepts unbounded JWS/JWT header and signature segments, allowing a remote unauthenticated attacker to craft tokens with extremely large base64url-encoded header or signature segments. During token verification, Authlib decodes and processes these large segments fully before rejecting them, resulting in high CPU and memory usage that can exhaust service resources and cause denial of service. Testing showed that a 500 MB header caused approximately 4 GB of memory usage and 9 seconds of CPU time before failure. The vulnerability affects Authlib versions up to 1.6.3. The upstream patch enforces a 256 KB limit on decoded header and signature segment sizes, mitigating the issue.

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Affected software

Description

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

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