Medium [CVE-2026-47778] Information disclosure via malicious certificate with NUL byte in DNS Subject Alternative Name (SAN)

Envoy is an open source edge and service proxy designed for cloud-native applications. Prior to 1.35.11, 1.36.7, 1.37.3, and 1.38.1, a structural flaw was identified in DefaultCertValidator::verifySubjectAltName where the extracted DNS SAN string is cast to a C-style string using.c_str() before being passed to the Utility::dnsNameMatch() algorithm.

If the attacker serves a certificate with a dNSName SAN containing an embedded NUL byte, the helper Utility::generalNameAsString captures the complete string including the NUL. However, when.c_str() evaluates it, implicit conversion to absl::string_view inside dnsNameMatch relies on strlen(), prematurely truncating the evaluation context.

Envoy evaluates trucated string against the exact required config_san match and returns true, thereby successfully validating the string with the Nul byte for an upstream routing. This vulnerability is fixed in 1.35.11, 1.36.7, 1.37.3, and 1.38.1.

A remote attacker could exploit a structural flaw in the DefaultCertValidator::verifySubjectAltName function by presenting a specially crafted certificate. This certificate would contain a NUL byte within its DNS Subject Alternative Name (SAN) field.

Due to incorrect string handling, Envoy would prematurely truncate the DNS SAN string during validation, allowing the malicious certificate to be accepted for upstream routing.