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

Severity: high (CVSS 8.2 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-03-10 · updated 2026-06-17
Weakness: CWE-863

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.

Affected products

envoyproxy envoy: before 1.34.13; from 1.35.0 before 1.35.8; from 1.36.0 before 1.36.5; 1.37.0 — fixed in 1.34.13, 1.35.8, 1.36.5

How to fix

Upgrade to a fixed version: envoyproxy envoy 1.34.13, 1.35.8, 1.36.5. Check the vendor advisory for mitigations.

References

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