Executive Summary

CVE-2026-47778 is a vulnerability in Envoy Proxy's default TLS certificate validator that allows an attacker to bypass host authentication by exploiting an embedded NUL byte in the DNS Subject Alternative Name (SAN) of a TLS certificate.

The flaw occurs in the DefaultCertValidator::verifySubjectAltName function, where the extracted DNS SAN string is incorrectly cast to a C-style string using .c_str() before validation. This causes the string to be prematurely truncated at the NUL byte, allowing an attacker to craft a certificate like example.com\0.attacker.com that Envoy incorrectly validates as matching example.com.

This vulnerability only affects DNS SANs and requires Envoy to be configured with auto_sni and auto_san_validation for upstream connections.

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Impact Analysis

An attacker can exploit this vulnerability to perform a Man-In-The-Middle (MITM) attack by obtaining a malicious certificate containing an embedded NUL byte in the DNS SAN.

If the Certificate Authority (CA) in the trust chain does not reject embedded NUL characters, the attacker can intercept and access sensitive data during transit, leading to a loss of data confidentiality.

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Detection Guidance

This vulnerability involves an attacker serving a TLS certificate with a DNS Subject Alternative Name (SAN) containing an embedded NUL byte. Detection involves inspecting TLS certificates used by Envoy for embedded NUL bytes in the DNS SAN fields.

You can detect potentially malicious certificates by extracting and examining the DNS SAN entries for embedded NUL characters. For example, using OpenSSL commands to inspect certificates:

• openssl s_client -connect <host>:443 -showcerts | openssl x509 -noout -text
• Check the output for DNS SAN entries containing unexpected or suspicious characters, especially embedded NUL bytes which may appear truncated or cause parsing issues.

Additionally, monitoring Envoy logs for unusual upstream routing behavior or unexpected certificate validation successes when strict SAN validation is expected may help identify exploitation attempts.

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Mitigation Strategies

The primary mitigation is to upgrade Envoy to a fixed version where this vulnerability is patched. The patched versions are 1.35.13, 1.36.9, 1.37.5, and 1.38.3.

If upgrading immediately is not possible, implement runtime checks to explicitly reject TLS certificates containing embedded NUL bytes in the DNS SAN fields.

Additionally, ensure that your Certificate Authority (CA) rejects certificates with embedded NUL characters in SANs to prevent issuance of malicious certificates.

Avoid relying on the vulnerable `.c_str()` cast in the certificate validation code by applying patches or configuration changes that remove this unsafe string handling.

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