CVE-2026-26308: Github.com/envoyproxy/envoy Envoy Has RBAC Header Validation Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-ghc4-35x6-crw5",
  "modified" : "2026-04-13T17:29:34Z",
  "published" : "2026-03-10T18:30:42Z",
  "aliases" : [ "CVE-2026-26308" ],
  "summary" : "Envoy has RBAC Header Validation Bypass via Multi-Value Header Concatenation",
  "details" : "## 1. Summary\nThe 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.\n\n## 2. Attack Scenario\nConsider an environment where an administrator wants to block external access to internal resources using a specific header flag.\n\n### Configuration\nThe Envoy proxy is configured with a **Deny** rule to reject requests containing the header `internal: true`.\n* **Rule Type:** Exact Match\n* **Target:** `internal` header must not equal `true`.\n\n### The Bypass Logic\n1.  **Standard Request (Blocked):**\n    * **Input:** `internal: true`\n    * **Envoy Processing:** Sees string `\"true\"`.\n    * **Result:** Match found. **Request Denied.**\n\n2.  **Exploit Request (Bypassed):**\n    * **Input:**\n        ```http\n        internal: true\n        internal: true\n        ```\n    * **Envoy Processing:** Concatenates values into `\"true,true\"`.\n    * **Matcher Evaluation:** Does `\"true,true\"` equal `\"true\"`? **No.**\n    * **Result:** The Deny rule fails to trigger. **Request Allowed.**\n\n## 3. Implications\n* **RBAC Bypass:** Remote attackers can bypass configured access controls.\n* **Unauthorized Access:** Sensitive internal resources or administrative endpoints protected by header-based Deny rules become accessible.\n* **Risk:** High, particularly for deployments relying on \"Exact Match\" strategies for security blocking.\n\n## 4. Reproduction Steps\nTo verify this vulnerability:\n\n1.  **Deploy Envoy:** Configure an instance with an RBAC **Deny** rule that performs an **exact match** on a specific header (e.g., `internal: true`).\n2.  **Baseline Test:** Send a request containing the header `internal: true`.\n    * *Observation:* Envoy blocks this request (HTTP 403).\n3.  **Exploit Test:** Send a second request containing the same header twice:\n    ```http\n    GET /restricted-resource HTTP/1.1\n    Host: example.com\n    internal: true\n    internal: true\n    ```\n    * *Observation:* Envoy allows the request, granting access to the resource.\n\n## 6. Recommendations\n**Fix Header Validation Logic:**\nModify 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.\n\n** Examine the DENY role to use a Regex style fix.\n\n**Credit:** Dor Konis",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/envoyproxy/envoy"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.37.0"
      }, {
        "fixed" : "1.37.1"
      } ]
    } ],
    "versions" : [ "1.37.0" ]
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/envoyproxy/envoy"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.36.0"
      }, {
        "fixed" : "1.36.5"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 1.36.4"
    }
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/envoyproxy/envoy"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.35.0"
      }, {
        "fixed" : "1.35.9"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 1.35.8"
    }
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/envoyproxy/envoy"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.34.13"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 1.34.12"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/envoyproxy/envoy/security/advisories/GHSA-ghc4-35x6-crw5"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-26308"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/envoyproxy/envoy/commit/b6ba0b2294b98484fb0ed8556897d1073cc27867"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/envoyproxy/envoy"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-20", "CWE-863" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-10T18:30:42Z",
    "nvd_published_at" : "2026-03-10T20:16:35Z"
  }
}