CVE-2026-84961

undici vulnerable to TLS certificate validation bypass via dropped connect options in BalancedPool

Workarounds

Use Client, Pool, or Agent instead of BalancedPool for connections that rely on a custom checkServerIdentity or connector, until upgraded.

undici's BalancedPool passes its constructor options through a JSON-based deep clone (JSON.parse(JSON.stringify(...))) before forwarding them to each per-upstream Pool. JSON cannot represent functions, so a caller-supplied connect or tls option containing a checkServerIdentity callback (or a custom connector function) is silently dropped before it reaches the TLS layer. As a result, a TLS peer whose certificate a custom checkServerIdentity was written to reject, but which passes Node's default hostname and chain checks, is silently accepted when the request is made through BalancedPool. Client, Pool, Agent, and RoundRobinPool destructure connect/tls before the clone and are not affected. Only applications that use BalancedPool with a function-valued connect/tls option (such as a custom checkServerIdentity or connector) are affected.

CVE-2026-84961 has a CVSS score of 7.4 (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 (7.29.1, 8.10.2); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

Upgrade to 7.29.1 or 8.10.2. BalancedPool now preserves the connect and tls options outside the JSON clone, so custom TLS verification callbacks are forwarded to each upstream unchanged.

1. What is CVE-2026-84961? CVE-2026-84961 is a high-severity security vulnerability in undici (npm), affecting versions >= 7.24.1, < 7.29.1. It is fixed in 7.29.1, 8.10.2.
2. How severe is CVE-2026-84961? CVE-2026-84961 has a CVSS score of 7.4 (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-2026-84961? undici (npm) versions >= 7.24.1, < 7.29.1 is affected.
4. Is there a fix for CVE-2026-84961? Yes. CVE-2026-84961 is fixed in 7.29.1, 8.10.2. Upgrade to this version or later.
5. Is CVE-2026-84961 exploitable, and should I be worried? Whether CVE-2026-84961 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-2026-84961 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-2026-84961?
   • Upgrade undici to 7.29.1 or later
   • Upgrade undici to 8.10.2 or later

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