CVE-2026-26308
Envoy has an RBAC Header Validation Bypass via Multi-Value Header Concatenation
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.
How to fix CVE-2026-26308
To remediate CVE-2026-26308, upgrade the affected package to a fixed version below.
- —upgrade to 1.34.13 or later
- —upgrade to 1.37.1 or later
Is CVE-2026-26308 being exploited?
Low — EPSS is 0.0%, meaning exploitation activity has not been observed at scale.
Affected packages (2)
- from 0, < 1.34.13, >= 1.35.0, < 1.35.9, >= 1.36.0, < 1.36.5, >= 1.37.0, < 1.37.1
- >= 1.37.0, < 1.37.1
CVSS scores
| Source | Version | Severity | Vector |
|---|---|---|---|
| osv | CVSS 3.1 | HIGH7.5 | CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:L/A:N |