undici vulnerable to TLS certificate validation bypass via dropped connect options in BalancedPool
### Impact 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. ### Patches 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. ### Workarounds Use `Client`, `Pool`, or `Agent` instead of `BalancedPool` for connections that rely on a custom `checkServerIdentity` or connector, until upgraded.
describes (1)
Related CVEs.
| ID | severity | exploitable | ||
|---|---|---|---|---|
| CVE-2026-84961US | critical9.1 | — | ||
undici's BalancedPool constructor passes its entire options object through an internal deep-clone that serializes and reparses the value as JSON. Because JSON cannot represent func… | ||||
references
External citations — patches, advisories, analysis.
- https://cna.openjsf.org/security-advisories.htmlWEB
- https://github.com/nodejs/undiciPACKAGE
- https://github.com/nodejs/undici/commit/8f5868fbdbc8f1146dfc1bcd1f2d117790141390WEB
- https://github.com/nodejs/undici/commit/f690157d728508652fef14673630c71515123e96WEB
- https://github.com/nodejs/undici/releases/tag/v7.29.1WEB
- https://github.com/nodejs/undici/releases/tag/v8.10.2WEB
- https://github.com/nodejs/undici/security/advisories/GHSA-w293-vg96-wgc3WEB
- https://nvd.nist.gov/vuln/detail/CVE-2026-84961ADVISORY
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