VulnerabilityPublished 30 Sep 2026Updated 27 Oct 2026

CVE-2026-102266: PyJWK accepts empty HMAC keys, bypassing PyJWT's empty-key validation

Affects hermes-agentSource GCVE Database

Authoritative record:CVE-2026-102266· High 7.4

Not known to be exploited · CVSS 7.4 High : upgrade to 0.22.4 soon.

Data as of 27 Oct 2026, 21:01 UTC

CVSS 3.1 base score

7.4High

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

Patch: Available – fixed in 0.22.4

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

Attack Vector: Network
Attack Complexity: High
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: High
Integrity: High
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: High: A successful attack depends on conditions beyond the attacker's control. That is, a successful attack cannot be accomplished at will, but requires the attacker to invest in some measurable amount of effort in preparation or execution against the vulnerable component before a successful attack can be expected. For example, a successful attack may require an attacker to: gather knowledge about the environment in which the vulnerable target/component exists; prepare the target environment to improve exploit reliability; or inject themselves into the logical network path between the target and the resource requested by the victim in order to read and/or modify network communications (e.g., a man in the middle attack).
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: High: There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.
Integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to 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 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

A fixed version of PyJWT is available in 0.22.4 that corrects the empty-key validation inconsistency. Users should upgrade to PyJWT 0.22.4 or later. Additionally, ensure that no empty HMAC keys are configured in JWK or JWKS sources. If upgrading is not immediately possible, avoid using empty symmetric keys and validate key material before use. No other specific mitigations are indicated.

Description

PyJWT versions prior to 0.22.4 contain a vulnerability where empty HMAC keys supplied via oct JWKs are accepted during HS256 token verification, bypassing the library's empty-key validation. This allows an attacker to forge valid HS256 JWTs if the application uses an empty symmetric JWK key, potentially leading to unauthorized token acceptance.

Impact

An attacker can forge valid HS256 JWTs with arbitrary claims if the application verifies tokens using an empty symmetric oct JWK key. This leads to authentication bypass, allowing unauthorized access or privilege escalation. The vulnerability does not require control over the JWK source or signing oracle, only that the application uses an empty HMAC key via PyJWK. The CVSS score of 7.4 (high) reflects the network attack vector, no privileges required, and high confidentiality and integrity impact.

Summary

The vulnerability in PyJWT 2.13.0 and affected versions arises because empty HMAC keys are rejected when supplied as raw keys but accepted when supplied as symmetric oct JWKs. An oct JWK with an empty Base64URL key decodes to an empty byte string, which PyJWT's PyJWK verification path uses directly without invoking the empty-key validation in HMACAlgorithm.prepare_key. Consequently, an attacker can compute valid HS256 signatures using the zero-length key and create attacker-controlled tokens accepted as authenticated by the service. This issue requires the application to have an empty HMAC JWK configured, often due to misconfiguration or missing secret values. The flaw was confirmed in PyJWT 2.13.0 and fixed in version 0.22.4.

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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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