CVE-2026-84961 · GHSA-w293-vg96-wgc3

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

Risk score: 55/100
CVSS: 7.4 v3.1
Exploit chance (30 days): 0.1%
Published: 29 Sep 2026

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What to know

What it is: 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)…
Affected: undici (npm) >= 7.24.1, < 7.29.1; undici (npm) >= 8.0.0, < 8.10.2
Fix: Upgrade undici to 7.29.1 or 8.10.2 (whichever matches your release line) or later.
Exploitation: No known exploitation

Story map

Story map: the flaws, the fix, how it unfolded and what it connects to. Boxes link to the record or article.

At a glance

undici: Amber, patch this week. The undici WebSocket client throws an uncaught TypeError during the opening handshake when a server's 101 response includes a Sec-WebSocket-Protocol header that the client never requested. The throw occurs in a queueMicrotask callback with no surrounding try/catch, so it propagates as an uncaught exception and termina…

The flaws

CVE-2026-84961: Undici vulnerable to TLS certificate validation bypass via dropped connect options in BalancedPool (CVSS 7.4 · EPSS 0%)
CVE-2026-19534: Undici vulnerable to Denial of Service via unrequested WebSocket subprotocol (CVSS 7.5 · EPSS 0%)
CVE-2026-85152: Undici vulnerable to cross-origin cache poisoning via missing origin isolation in interceptors (CVSS 7.4 · EPSS 0%)
CVE-2026-84933: Undici vulnerable to cross-user cookie disclosure via Set-Cookie caching in shared caches (CVSS 6.5 · EPSS 0%)

The fix

Fix: Upgrade undici to 6.28.1 or 7.29.1 or 8.10.2 (whichever matches your release line) or later.

Who is exposed: undici (npm) >= 6.7.0, < 6.28.1; undici (npm) >= 7.0.0, < 7.29.1; undici (npm) >= 8.0.0, < 8.10.2

Where it stands

No known exploitation

Connects to

Same weakness: CWE-295 (CWE-295). 82 other warnings in 30 days

Why it is Yellow

CVSS 7.4 high
Remotely exploitable without logging in
On your watchlist: npm

Affected packages

Package	Ecosystem	Vulnerable	Fixed in
undici	npm	>= 7.24.1, < 7.29.1	7.29.1
undici	npm	>= 8.0.0, < 8.10.2	8.10.2

References

Original description

### 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.

Last activity 29 Sep 2026. Patch Forecast updates every hour.