CVE-2026-84961 undici vulnerable to TLS certificate validation bypass via dropped connect options in BalancedPool
Attackers can bypass custom TLS certificate validation in undici's BalancedPool by exploiting a JSON serialization bug that drops function-based security checks.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-84961?
CVE-2026-84961 is a high-severity vulnerability in Nodejs Undici, published on 4 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
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 functions, any function-valued TLS option, such as a caller-supplied checkServerIdentity callback or a custom connector inside the connect option, is silently discarded before it reaches the TLS layer. As a result a peer whose certificate the application's custom checkServerIdentity was written to reject, but which still passes Node's default hostname and chain checks, is accepted when reached through BalancedPool. The Client, Pool, and Agent dispatchers are not affected because they extract the connect and tls options before cloning. This affects undici versions from 7.24.1 up to 7.29.1 and from 8.0.0 up to 8.10.2, and only when the application supplies a function-valued connect or tls option to BalancedPool. Users should upgrade to undici 7.29.1 or 8.10.2.
Weakness: CWE-295
Which versions are affected by CVE-2026-84961?
4 open source packages ship an affected version, including undici. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
undici npm | From 7.24.1 | 7.29.1 |
undici npm | From 7.24.1 | 7.29.1 |
undici npm | From 8.0.0 | 8.10.2 |
undici npm | From 8.0.0 | 8.10.2 |
Products include Nodejs Undici.
How do I fix CVE-2026-84961?
Upgrade undici to a fixed version for your release line: 7.29.1 or 8.10.2.
Is CVE-2026-84961 being exploited?
There is no known exploitation of CVE-2026-84961 in the wild and no public exploit code.
Other vulnerabilities in Nodejs Undici
- CVE-2026-109855 undici WebSocket client vulnerable to Denial of Service via failed HTTP/2 handshake
- CVE-2026-109869 undici vulnerable to Denial of Service via unhandled error on excessive content-encodings
- CVE-2026-87018 undici vulnerable to Denial of Service via TLS 1.2 certificate-changing renegotiation
- CVE-2026-19534 undici vulnerable to Denial of Service via unrequested WebSocket subprotocol
- CVE-2026-12151 undici WebSocket client vulnerable to denial of service via fragment count bypass
Sources
Also known as CLEANSTART-2026-NT71796, CLEANSTART-2026-XJ20290, CLEANSTART-2026-XX88554, DEBIAN-CVE-2026-84961, ECHO-e485-5b9d-e4bc, GHSA-w293-vg96-wgc3 and 15 more.