rustls webpki improper certificate validation of wildcard name constraints

Executive brief

A vulnerability was found in the webpki library, which is used to verify digital certificates for secure internet connections. The library incorrectly handled certain restrictions on website names, potentially allowing a certificate to be used for a domain it was not intended for. Exploiting this requires a certificate to be misissued by a trusted authority, making the practical risk to most operations low.

Technical details

The webpki library (rustls-webpki) failed to properly enforce permitted subtree name constraints when a certificate asserted a wildcard name (e.g., *.example.com). This logic error could allow a certificate to be valid for a name that should have been rejected by the constraint (e.g., a constraint for accept.example.com incorrectly permitting a wildcard that covers reject.example.com). This is a certificate validation vulnerability (CWE-295) that is only reachable after signature verification. Exploitation requires a CA to misissue a certificate that conflicts with its name constraints. The issue is fixed in versions 0.103.12 and 0.104.0-alpha.6.

Affected products

• rustls rustls-webpki >= 0.101.0, < 0.103.12; >= 0.104.0-alpha.1, < 0.104.0-alpha.6

Timeline

• 2026-04-15: disclosed: Initial disclosure by ctz
• 2026-04-16: advisory: GitHub Advisory published

References

• https://api.github.com/users/1seal
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