CVE-2026-34240

High

Published: 31 March 2026

Published

31 March 2026

Modified

06 April 2026

KEV Added

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Patch

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CVSS Score 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

EPSS Score 0.0001 1.9th percentile

Risk Priority 15 60% EPSS · 20% KEV · 20% CVSS

Summary

CVE-2026-34240 is a high-severity Improper Verification of Cryptographic Signature (CWE-347) vulnerability in Appsup-Dart Jose. Its CVSS base score is 7.5 (High).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 1.9th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

The strongest mitigations our analysis identified are NIST 800-53 CM-6 (Configuration Settings) and SI-10 (Information Input Validation).

Threat & Defense at a Glance

What attackers do: exploitation maps to Exploit Public-Facing Application (T1190) and 1 other technique. What defenders deploy: see the NIST 800-53 controls recommended below.

Threat & Defense Details

Mitigating Controls (NIST 800-53 r5)AI

SI-2 Flaw Remediation good match

prevent

Flaw remediation directly mitigates CVE-2026-34240 by applying the patch in JOSE version 0.3.5+1, which prevents the library from treating untrusted header-provided JWKs as verification candidates.

SI-10 Information Input Validation good match

prevent

Information input validation enforces rejection of JOSE tokens containing header JWKs unless they match the trusted key store, directly countering the forgery mechanism.

CM-6 Configuration Settings good match

prevent

Configuration settings secure the JOSE library or application to disable trust in header-provided JWKs, implementing the recommended workaround to block forged tokens.

MITRE ATT&CK Enterprise TechniquesAI

T1190 Exploit Public-Facing Application Initial Access

Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.

attack.mitre.org →

T1606 Forge Web Credentials Credential Access

Adversaries may forge credential materials that can be used to gain access to web applications or Internet services.

attack.mitre.org →

Why these techniques?

Vulnerability allows forging valid JWT/JWS tokens via untrusted JWK in header, directly enabling T1606 Forge Web Credentials; exploitable remotely in public-facing apps using the library via T1190.

Confidence: HIGH · MITRE ATT&CK Enterprise v18.1

NVD Description

JOSE is a Javascript Object Signing and Encryption (JOSE) library. Prior to version 0.3.5+1, a vulnerability in jose could allow an unauthenticated, remote attacker to forge valid JWS/JWT tokens by using a key embedded in the JOSE header (jwk). The…

vulnerability exists because key selection could treat header-provided jwk as a verification candidate even when that key was not present in the trusted key store. Since JOSE headers are untrusted input, an attacker could exploit this by creating a token payload, embedding an attacker-controlled public key in the header, and signing with the matching private key. Applications using affected versions for token verification are impacted. This issue has been patched in version 0.3.5+1. A workaround for this issue involves rejecting tokens where header jwk is present unless that jwk matches a key already present in the application's trusted key store.

Deeper analysisAI

CVE-2026-34240 is a vulnerability in the JOSE library, a JavaScript Object Signing and Encryption (JOSE) implementation, affecting versions prior to 0.3.5+1. The flaw enables forging of valid JWS/JWT tokens by embedding a JSON Web Key (JWK) in the untrusted JOSE header. This happens because the library's key selection treats the header-provided JWK as a valid verification candidate, even if it is absent from the application's trusted key store. Applications relying on affected versions for token verification are impacted, with the issue mapped to CWE-347 and assigned a CVSS v3.1 base score of 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N).

An unauthenticated remote attacker can exploit this vulnerability by crafting a token payload, embedding an attacker-controlled public key in the JOSE header as a JWK, and signing the token with the corresponding private key. When verified, the library accepts the header-provided key for signature validation, allowing the forged token to pass as authentic. This bypasses reliance on pre-configured trusted keys, potentially granting unauthorized access or privilege escalation in token-based authentication systems.

The vulnerability has been addressed in JOSE version 0.3.5+1. A recommended workaround is to reject any tokens containing a header JWK unless it exactly matches a key already present in the application's trusted key store. Additional details are available in the GitHub security advisory at https://github.com/appsup-dart/jose/security/advisories/GHSA-vm9r-h74p-hg97 and the patching commit at https://github.com/appsup-dart/jose/commit/b07799aac1f56a9a21483feac026272aab30cc5d.