Summary
Workarounds
Reject tokens where header jwk is present unless that jwk matches a key already present in the application's trusted key store.
Resources
Fix commit: fix: improved key resolution in JsonWebKeyStore
Impact
A vulnerability in jose versions up to and including 0.3.5 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.
CVE-2026-34240 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.3.5+1); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
Upgrade to 0.3.5+1 or later.
Frequently Asked Questions
- What is CVE-2026-34240? CVE-2026-34240 is a high-severity security vulnerability in jose (pub), affecting versions <= 0.3.5. It is fixed in 0.3.5+1.
- How severe is CVE-2026-34240? CVE-2026-34240 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of jose are affected by CVE-2026-34240? jose (pub) versions <= 0.3.5 is affected.
- Is there a fix for CVE-2026-34240? Yes. CVE-2026-34240 is fixed in 0.3.5+1. Upgrade to this version or later.
- Is CVE-2026-34240 exploitable, and should I be worried? Whether CVE-2026-34240 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-34240 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-34240? Upgrade
joseto 0.3.5+1 or later.