CVE-2026-47778: Envoy Proxy Certificate SAN Validation Bypass (NUL Byte)

Technical summary

The vulnerability resides in Envoy's DefaultCertValidator::verifySubjectAltName function. The flaw occurs in a two-stage string processing error: first, Utility::generalNameAsString correctly captures the full dNSName SAN including any embedded NUL bytes into an std::string; second, when this string is converted via .c_str() and passed to Utility::dnsNameMatch for validation against the configured upstream SAN requirement, the function internally converts the C-style pointer to absl::string_view. The string_view constructor calls strlen() on the pointer, which stops at the first NUL byte, effectively truncating the validation context. If the attacker's certificate contains a SAN like 'attacker.com\x00legitimate.com', only 'attacker.com' is evaluated against the config_san match. If 'attacker.com' matches a valid upstream alias, validation succeeds despite the certificate being invalid for the actual upstream domain.

Business impact

Organizations deploying Envoy as an edge proxy or service mesh control point face a certificate validation bypass that could allow traffic interception or service impersonation. An attacker capable of obtaining a certificate for attacker-controlled or compromised domains could potentially route traffic intended for legitimate upstreams through malicious endpoints, leading to data theft, session hijacking, or man-in-the-middle attacks. The impact is constrained by the requirement that the attacker must obtain or control a valid certificate, but the vulnerability effectively nullifies Envoy's SAN pinning and strict upstream verification controls.

Affected systems

Envoy proxy deployments running versions 1.35.0–1.35.10, 1.36.0–1.36.6, 1.37.0–1.37.2, and 1.38.0 are vulnerable. Organizations should verify their exact Envoy version and patch status immediately. This affects Envoy when configured with strict upstream certificate validation (particularly common in service mesh and zero-trust architectures where SAN pinning is enforced).

Exploitability

Exploitation requires two conditions: (1) the attacker must obtain a valid TLS certificate (for any domain they control or can compromise), and (2) that certificate's dNSName SAN must be crafted with an embedded NUL byte and an additional payload after it that matches a legitimate upstream SAN in the target environment's Envoy config. The attack is not trivial but is feasible for sophisticated threat actors or insiders with certificate-issuing capabilities. CVSS 4.4 (Medium) reflects the high access requirements (privileged attacker context) and conditional triggering, though the impact when successful is confidentiality-critical.

Remediation

Upgrade affected Envoy instances to patched versions: 1.35.11 or later, 1.36.7 or later, 1.37.3 or later, or 1.38.1 or later. No workaround exists short of upgrading. Organizations should prioritize patching in environments where Envoy enforces upstream certificate pinning or SAN validation as a primary security control.

Patch guidance

Apply the following patches based on your current version: upgrade from 1.35.x to 1.35.11+, from 1.36.x to 1.36.7+, from 1.37.x to 1.37.3+, or from 1.38.0 to 1.38.1+. Verify patch application by confirming the new version in Envoy's admin interface or log output. Test patched deployments in a staging environment to confirm upstream service connectivity remains stable before production rollout.

Detection guidance

Monitor for certificates presented to Envoy with SAN values containing embedded NUL bytes (0x00). Review upstream certificate logs and Envoy's certificate validation audit events for any unexpected SAN values or validation results. Intrusion detection systems can flag TLS handshakes where certificate SAN contains NUL bytes. Additionally, audit Envoy configuration files to identify which upstreams enforce SAN pinning—these are the highest-risk targets for this vulnerability.

Why prioritize this

Despite the medium CVSS score, this vulnerability warrants prompt attention because it directly undermines certificate-based authentication in service-to-service communication. In zero-trust and service mesh deployments, certificate validation is often the last line of defense; bypass of this control can lead to lateral movement and data exfiltration. Prioritize patches for Envoy instances handling sensitive inter-service or edge traffic.

Risk score, explained

CVE-2026-47778 scores 4.4 MEDIUM under CVSS 3.1 (AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:N/A:N). The score reflects network accessibility but high attack complexity and high privilege requirements (the attacker must obtain a valid certificate and knowledge of upstream SAN configuration). Confidentiality impact is high because successful exploitation enables traffic interception. Integrity and availability impact are rated none because the attack does not modify upstream responses or disrupt service. The relatively low score should not diminish urgency in environments where SAN validation is a critical control.

Frequently asked questions