CVE-2026-47778: Envoy: Embedded NUL in TLS DNS SAN Truncation in the Default TLS Certificate Validator. (Auth Bypass)

Severity: medium (CVSS 4.4 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-06-26 · updated 2026-10-25
Weakness: CWE-158

Description

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.

Affected products

envoyproxy envoy: before 1.35.13; from 1.36.0 before 1.36.9; from 1.37.0 before 1.37.5; from 1.38.0 before 1.38.3 — fixed in 1.35.13, 1.36.9, 1.37.5, 1.38.3

How to fix

Upgrade to a fixed version: envoyproxy envoy 1.35.13, 1.36.9, 1.37.5, 1.38.3. Check the vendor advisory for mitigations.

References

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