CVE-2026-119283VulnerabilityPublished 31 Oct 2026

fastmcp · joserfc: Unbounded JWE General JSON `recipients` Array Leads to CPU Exhaustion (DoS) via Repeated RSA Private-Key

Affects fastmcpSource GCVE Database

Compare

Authoritative record:CVE-2026-119283· High 7.5

Not known to be exploited · CVSS 7.5 High : upgrade to 4.0.0 soon.

Data as of 31 Oct 2026, 16:20 UTC

CVSS 3.1 base score

7.5High

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	7.5

Patch: Available – fixed in 4.0.0

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 29 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 30 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 4.0.0.

Affected software

Product	Affected	Fixed in
Homebrewfastmcp`pkg:brew/fastmcp`Other threats	Affected >=3.2.4_2 <4.0.0	Fixed in 4.0.0

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 joserfc version 4.0.0 that addresses this issue by implementing appropriate limits on the recipients array. Users should upgrade to version 4.0.0 or later. The existing verify_all_recipients=False option does not mitigate the CPU exhaustion risk and should not be relied upon as a defense.

Description

The joserfc library's decrypt_json() function for JWE General JSON Serialization accepts an unbounded recipients array, performing a costly RSA private-key decryption for each recipient without limit. This allows an unauthenticated attacker to cause CPU exhaustion (Denial of Service) by sending a single JWE JSON with many recipients. The library caps sizes of other JWE elements but does not limit recipient count. The documented mitigation option verify_all_recipients=False does not reduce CPU usage, as all recipients are decrypted before this flag is checked. This vulnerability is tracked as CVE-2026-119283 with a CVSS 3.1 score of 7.5 (High). A fix is available in joserfc version 4.0.0.

Impact

An unauthenticated network attacker can cause a denial of service by forcing the victim to perform a large number of RSA private-key decryptions, exhausting CPU resources. This leads to service degradation or outage without requiring authentication or special privileges. The attack is highly asymmetric, with a small increase in request size causing a proportional increase in CPU load.

Summary

The joserfc library's public API decrypt_json() parses JWE General JSON Serialization objects and processes the recipients array without any upper bound on its length. For each recipient, it performs a full RSA private-key decryption operation (decrypt_cek), which is computationally expensive. An attacker can exploit this by crafting a JWE JSON with a large recipients array, causing the victim server to perform an attacker-chosen number of RSA decryptions from a single request, leading to CPU exhaustion (CWE-400). The library enforces size limits on other JWE components but lacks any limit on recipients count. The verify_all_recipients option only controls exception raising and does not reduce the number of decryptions performed. This vulnerability was confirmed by source code inspection and runtime testing. It affects joserfc versions >=3.2.4_2 and <4.0.0 and is fixed in version 4.0.0.

References

Community reviews

Loading reviews…

Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 31 Oct 2026, 16:20 UTC

Technical details

References

Community reviews

Related threats