VulnerabilityPublished 13 Aug 2026Updated 3 Nov 2026

CVE-2025-62706: Authlib · JWE zip=DEF decompression bomb enables DoS

Affects fastmcpSource GCVE Database

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Not known to be exploited · CVSS 6.5 Medium : upgrade to 2.12.4_1 when convenient.

Data as of 3 Nov 2026, 22:58 UTC

CVSS 3.1 base score

6.5Medium

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

EPSS: 0.4% chance of exploitation in 30 days – higher than 39% 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:L/UI:N/S:U/C:N/I:N/A:H

Attack Vector: Network
Attack Complexity: Low
Privileges Required: Low
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: 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: 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 fix is available in Authlib version 2.12.4_1 that adds a maximum output size limit to DEFLATE decompression to prevent unbounded expansion. Until upgrading, applications should reject or strip JWE tokens with 'zip=DEF' compression at the application boundary. Additional mitigations include enforcing strict maximum token sizes, failing fast on oversized inputs, and applying rate limiting to reduce impact of repeated attacks.

Description

Authlib's JWE implementation with 'zip=DEF' (DEFLATE) decompression performs unbounded decompression without limits, allowing a small compressed ciphertext to expand into very large plaintext during decryption. This can cause excessive memory and CPU usage, leading to denial of service. The vulnerability affects services that decrypt JWE tokens with 'zip=DEF' where attackers can supply decryptable tokens. No confidentiality or integrity impact is reported, but availability is severely affected. A fix is available in version 2.12.4_1. Until patched, rejecting or stripping 'zip=DEF' tokens is recommended.

Impact

The vulnerability enables denial of service by allowing an attacker to cause excessive memory and CPU consumption during JWE token decryption when 'zip=DEF' compression is used. This can exhaust system resources and disrupt availability of services relying on Authlib for JWE decryption. There is no direct impact on confidentiality or integrity of data. The severity is medium (CVSS 6.5) when attackers require some privilege to produce decryptable tokens, and high (CVSS 7.5) if arbitrary attackers can submit such tokens.

Summary

Authlib's JOSE library for JSON Web Encryption (JWE) supports compression with 'zip=DEF' using DEFLATE. The decompression method calls zlib.decompress without a maximum output size, enabling an attacker to craft a small compressed ciphertext that decompresses into a very large plaintext, causing memory and CPU exhaustion during decryption. This unbounded decompression is triggered when the JWE protected header includes 'zip=DEF'. The vulnerability allows denial of service by resource exhaustion. The issue contrasts with other implementations like 'joserfc' that enforce output size limits. A proof of concept demonstrates a 4KB ciphertext inflating to 50MB plaintext, spiking resource usage. The vulnerability is tracked as CVE-2025-62706 with a CVSS 3.1 base score of 6.5 (medium) under typical attacker privileges, rising to 7.5 (high) if attackers can submit decryptable tokens without privileges. A patch is available in Authlib version 2.12.4_1 that adds decompression output limits.

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What to do

Affected software

Description

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

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