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

1. Summary 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.

2. Attack Scenario Consider an environment where an administrator wants to block external access to internal resources using a specific header flag.

Configuration The Envoy proxy is configured with a Deny rule to reject requests containing the header internal: true. Rule Type: Exact Match Target: internal header must not equal true.

The Bypass Logic 1. Standard Request (Blocked): Input: internal: true Envoy Processing: Sees string "true". Result: Match found. Request Denied.

2. Exploit Request (Bypassed): Input: http internal: true internal: true Envoy Processing: Concatenates values into "true,true". Matcher Evaluation: Does "true,true" equal "true"? No. Result: The Deny rule fails to trigger. Request Allowed.

3. Implications RBAC Bypass: Remote attackers can bypass configured access controls. Unauthorized Access: Sensitive internal resources or administrative endpoints protected by header-based Deny rules become accessible. Risk: High, particularly for deployments relying on "Exact Match" strategies for security blocking.

4. Reproduction Steps To verify this vulnerability:

1. Deploy Envoy: Configure an instance with an RBAC Deny rule that performs an exact match on a specific header (e.g., internal: true). 2. Baseline Test: Send a request containing the header internal: true. Observation: Envoy blocks this request (HTTP 403). 3. Exploit Test: Send a second request containing the same header twice: http GET /restricted-resource HTTP/1.1 Host: example.com internal: true internal: true Observation: Envoy allows the request, granting access to the resource.

6. Recommendations Fix Header Validation Logic: Modify the RBAC filter to validate each header value instance individually. Avoid relying on the concatenated string output of getAllOfHeaderAsString() for security-critical matching unless the matcher is explicitly designed to parse comma-separated lists.

Examine the DENY role to use a Regex style fix.

Credit: Dor Konis