CVE-2026-87018

undici vulnerable to Denial of Service via TLS 1.2 certificate-changing renegotiation

Workarounds

Restrict outbound connections to TLS 1.3, for example by setting connect: { minVersion: 'TLSv1.3', maxVersion: 'TLSv1.3' } on the client or agent, so that renegotiation cannot occur, when the target endpoints support TLS 1.3.

undici crashes the client process when a TLS 1.2 server renegotiates after the connection is established and presents a different, CA-trusted certificate whose hostname does not match the requested host. Node.js re-runs hostname verification on the renegotiated handshake and destroys the socket with ERR_TLS_CERT_ALTNAME_INVALID. undici's established socket error handler asserted that this error code could never occur, so the peer-controlled error became an uncaught AssertionError that terminates the Node.js process. Both the HTTP/1.1 and HTTP/2 paths are affected.

A malicious or compromised HTTPS endpoint reached through undici or the global fetch() can crash the client with a single server-initiated renegotiation, taking down all in-flight requests and background work in the process. Only TLS 1.2 is affected, because TLS 1.3 does not support renegotiation. Exploitation does not require a CA compromise: the attacker controls the endpoint and can present two ordinary valid certificates, the first matching the requested URL and the second intentionally not matching it.

CVE-2026-87018 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 (6.29.1, 7.30.1, 8.12.2); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

Upgrade to undici 6.29.1, 7.30.1, or 8.12.2.

1. What is CVE-2026-87018? CVE-2026-87018 is a high-severity security vulnerability in undici (npm), affecting versions >= 4.2.1, < 6.29.1. It is fixed in 6.29.1, 7.30.1, 8.12.2.
2. How severe is CVE-2026-87018? CVE-2026-87018 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.
3. Which versions of undici are affected by CVE-2026-87018? undici (npm) versions >= 4.2.1, < 6.29.1 is affected.
4. Is there a fix for CVE-2026-87018? Yes. CVE-2026-87018 is fixed in 6.29.1, 7.30.1, 8.12.2. Upgrade to this version or later.
5. Is CVE-2026-87018 exploitable, and should I be worried? Whether CVE-2026-87018 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
6. What actually determines whether CVE-2026-87018 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.
7. How do I fix CVE-2026-87018?
   • Upgrade undici to 6.29.1 or later
   • Upgrade undici to 7.30.1 or later
   • Upgrade undici to 8.12.2 or later

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