CVE-2026-26308

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:

     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:

   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

The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.

CVE-2026-26308 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.37.1, 1.36.5, 1.35.9, 1.34.13); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-26308? CVE-2026-26308 is a high-severity improper input validation vulnerability in github.com/envoyproxy/envoy (go), affecting versions = 1.37.0. It is fixed in 1.37.1, 1.36.5, 1.35.9, 1.34.13. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
2. How severe is CVE-2026-26308? CVE-2026-26308 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/envoyproxy/envoy are affected by CVE-2026-26308? github.com/envoyproxy/envoy (go) versions = 1.37.0 is affected.
4. Is there a fix for CVE-2026-26308? Yes. CVE-2026-26308 is fixed in 1.37.1, 1.36.5, 1.35.9, 1.34.13. Upgrade to this version or later.
5. Is CVE-2026-26308 exploitable, and should I be worried? Whether CVE-2026-26308 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-26308 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-26308?
   • Upgrade github.com/envoyproxy/envoy to 1.37.1 or later
   • Upgrade github.com/envoyproxy/envoy to 1.36.5 or later
   • Upgrade github.com/envoyproxy/envoy to 1.35.9 or later
   • Upgrade github.com/envoyproxy/envoy to 1.34.13 or later