# CVE-2026-47778: Envoy improper certificate validation via NUL byte truncation in DNS SAN

Source: https://junglewise.ai/threats/cve-2026-47778-envoy-improper-certificate-validation-via-nul-byte-truncation-in  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-06-26. Updated: 2026-06-26  
License: CC BY 4.0 with a link back

CVE-2026-47778 · Severity: medium · CVSS 4.4 · Published 2026-06-26

Envoy is a popular open-source tool used to manage and secure network traffic between cloud applications. A flaw in how it checks security certificates allows an attacker to impersonate a trusted service by using a specially crafted certificate containing a hidden 'null' character. If successful, an attacker could intercept sensitive data or bypass security controls intended to verify the identity of backend servers.

Technologies: [Envoy Proxy Envoy](https://junglewise.ai/threats/technologies/envoy). Vendors: [Envoy Proxy](https://junglewise.ai/threats/vendors/envoy-proxy).

## Quick answers

**Is CVE-2026-47778 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-47778 being exploited in the wild.

**How severe is CVE-2026-47778?** Medium severity, with a CVSS score of 4.4, published 26 June 2026.

**Which products and versions are affected?** Envoy Proxy Envoy < 1.35.11, 1.36.0 to < 1.36.7, 1.37.0 to < 1.37.3, 1.38.0 to < 1.38.1.

**Is there a fix for CVE-2026-47778?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-47778 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-47778?** Check whether you run Envoy Proxy Envoy at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

A vulnerability exists in Envoy's DefaultCertValidator::verifySubjectAltName due to improper handling of NUL bytes in DNS Subject Alternative Names (SANs). When extracting a DNS SAN, the string is cast to a C-style string using .c_str() before being passed to Utility::dnsNameMatch(). Because the underlying conversion to absl::string_view relies on strlen(), the string is prematurely truncated at the NUL byte. An attacker with a certificate containing a SAN like 'target.com\0.attacker.com' can successfully match against a required 'target.com' configuration. This allows an attacker who can obtain such a certificate from a trusted CA to perform Man-in-the-Middle (MITM) attacks against upstream routing. The issue is fixed by removing the .c_str() cast to ensure the full string length is preserved during comparison.

## Affected products

- Envoy Proxy Envoy < 1.35.11, 1.36.0 to < 1.36.7, 1.37.0 to < 1.37.3, 1.38.0 to < 1.38.1

## Timeline

- 2026-06-23: advisory: GitHub advisory GHSA-f8x4-rw5x-f3r7 published
- 2026-06-26: disclosed: CVE-2026-47778 published to NVD

## References

- [https://github.com/envoyproxy/envoy/security/advisories/GHSA-f8x4-rw5x-f3r7](https://github.com/envoyproxy/envoy/security/advisories/GHSA-f8x4-rw5x-f3r7)

## Related threats

- [Envoy HTTP upgrade connection poisoning](https://junglewise.ai/threats/cve-2026-73548-envoy-is-an-open-source-edge-and-service-proxy-designed-for-cloud)
- [Envoy use-after-free in HTTP OAuth2 filter](https://junglewise.ai/threats/cve-2026-48090-envoy-use-after-free-in-http-oauth2-filter)
- [Envoy denial of service via NULL pointer dereference in log formatting](https://junglewise.ai/threats/cve-2026-47220-envoy-denial-of-service-via-null-pointer-dereference-in-log)
- [Envoy Use-After-Free in ext_authz filter during stream teardown](https://junglewise.ai/threats/cve-2026-47205-envoy-use-after-free-in-ext-authz-filter-during-stream-teardown)
- [Envoy HTTP/3 to HTTP/1 request smuggling in downstream translation](https://junglewise.ai/threats/cve-2026-48743-envoy-http-3-to-http-1-request-smuggling-in-downstream)