VulnerabilityPublished 30 Sep 2026Updated 27 Oct 2026

CVE-2026-102265: PyJWT · Uncaught RecursionError in jwt.decode() on deeply nested token header

Affects hermes-agentSource GCVE Database

Authoritative record:CVE-2026-102265· Medium 5.3

Not known to be exploited · CVSS 5.3 Medium : upgrade to 0.22.4 when convenient.

Data as of 27 Oct 2026, 21:01 UTC

CVSS 3.1 base score

5.3Medium

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

Patch: Available – fixed in 0.22.4

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

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

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: Low: Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the impacted component are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 28 Sep 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: 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 26 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.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, 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 hermes-agent to 0.22.4.

Affected software

Product	Affected	Fixed in
Homebrewhermes-agent`pkg:brew/hermes-agent`Other threats	Affected >=0.21.6 <0.22.4 >=2026.7.1 <=2026.9.24	Fixed in 0.22.4

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

AI-suggested

Upgrade PyJWT to version 2.14.0 or later, which includes a fix that catches RecursionError during JWT header parsing and raises DecodeError instead. This prevents unhandled exceptions and ensures that malformed tokens result in proper authentication errors. Until upgrading, consider implementing additional input validation or limiting token size and nesting depth at the application level if feasible.

Description

PyJWT versions prior to 2.14.0 contain a vulnerability where deeply nested JSON arrays in the JWT header cause a RecursionError during decoding. This error is not caught by the library's usual exception handling, resulting in an unhandled exception that leads to HTTP 500 server errors and full tracebacks logged. The issue allows an unauthenticated attacker to cause a denial of service on JWT validation endpoints by sending crafted tokens with excessively nested headers. The flaw is fixed in PyJWT 2.14.0 by catching RecursionError and converting it to a DecodeError.

Impact

An unauthenticated attacker can send a crafted JWT with a deeply nested header to any endpoint that validates tokens using PyJWT's jwt.decode() method. This triggers a RecursionError during JSON parsing of the header, causing the server to respond with HTTP 500 errors and log full tracebacks. This results in a denial of service on the authentication path without crashing the server or exposing sensitive data. The attack is cheap, repeatable, and bypasses normal exception handling that expects PyJWTError exceptions.

Summary

PyJWT's _load() method decodes the JWT header using json.loads() wrapped in a try-except block that only catches ValueError. However, when the header contains deeply nested JSON arrays exceeding Python's recursion limit (~65,000 levels), json.loads() raises a RecursionError, which is not caught and escapes the PyJWTError exception hierarchy. This leads to unhandled exceptions during jwt.decode() calls, causing HTTP 500 errors and tracebacks in logs. The vulnerability affects all versions that only translate ValueError in _load(), confirmed in version 2.13.0. The fix, committed and released in PyJWT 2.14.0, extends exception handling to include RecursionError, converting it to DecodeError and preventing unhandled exceptions.

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 27 Oct 2026, 21:01 UTC

Technical details

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