CVE-2026-26308 Envoy has an RBAC Header Validation Bypass via Multi-Value Header Concatenation

What is CVE-2026-26308?

CVE-2026-26308 is a high-severity authentication bypass vulnerability in Envoyproxy Envoy, published on 10 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

Envoy is a high-performance edge/middle/service proxy. Prior to 1.37.1, 1.36.5, 1.35.8, and 1.34.13, the Envoy RBAC (Role-Based Access Control) filter contains a logic vulnerability in how it validates HTTP headers when multiple values are present for the same header name. Instead of validating each header value individually, Envoy concatenates all values into a single comma-separated string. This behavior allows attackers to bypass RBAC policies—specifically "Deny" rules—by sending duplicate headers, effectively obscuring the malicious value from exact-match mechanisms. This vulnerability is fixed in 1.37.1, 1.36.5, 1.35.8, and 1.34.13.

Weakness: CWE-863

Which versions are affected by CVE-2026-26308?

9 open source packages ship an affected version, including github.com/envoyproxy/envoy and envoy. It affects 1 product.

Products include Envoyproxy Envoy.

How do I fix CVE-2026-26308?

Upgrade github.com/envoyproxy/envoy to a fixed version for your release line: 1.34.13, 1.35.9, 1.36.5 or 1.37.1.

Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.

Is CVE-2026-26308 being exploited?

There is no known exploitation of CVE-2026-26308 in the wild and no public exploit code.

Other vulnerabilities in Envoyproxy Envoy

• CVE-2022-29226 Trivial authentication bypass in Envoy Critical 10.0
• CVE-2019-18801 Memory corruption in n/a Critical 9.8
• CVE-2019-18802 Security feature bypass in n/a Critical 9.8
• CVE-2020-35470 Information disclosure in n/a High 8.8
• CVE-2024-23324 Envoy ext auth can be bypassed when Proxy protocol filter sets invalid UTF-8 metadata High 8.6

All Envoyproxy Envoy vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as BIT-envoy-2026-26308, GHSA-ghc4-35x6-crw5.