CVE-2025-63656: 
Linux Alpine vulnerability analysis and mitigation

CVE-2025-63656 is an out-of-bounds read vulnerability in the header_cmp function of the Monkey HTTP Server's HTTP parser (mk_server/mk_http_parser.c), allowing unauthenticated remote attackers to cause a Denial of Service (DoS) by sending a crafted HTTP request. It affects Monkey HTTP Server through commit f37e984 (versions up to and including 1.8.5). CVE IDs were requested and assigned in October 2025, with public disclosure in January 2026. The vulnerability carries a CVSS v3.1 base score of 7.5 (High) (Feedly, Archer Advisory).

The root cause is an out-of-bounds read (CWE-125) in the header_cmp function within mk_server/mk_http_parser.c of Monkey commit f37e984. When processing HTTP request headers, the function reads memory beyond the intended buffer boundaries, which can be triggered by a specially crafted HTTP request. No authentication or user interaction is required; the attack is network-accessible with low complexity. This CVE is part of a broader set of nine vulnerabilities (CVE-2025-63649 through CVE-2025-63658) discovered in the same codebase and disclosed together by researcher archersec (Archer Advisory, Monkey Issue #426).

Successful exploitation causes a Denial of Service, crashing the Monkey HTTP Server process and disrupting service availability. There is no confidentiality or integrity impact — the vulnerability is limited to availability. Since no authentication is required and the attack is remotely initiated, any internet-exposed Monkey server instance running commit f37e984 or earlier (up to v1.8.5) is at risk of service disruption (Feedly, Archer Advisory).

Proof-of-concept exploit code is publicly available via the archersec security advisory and the associated GitHub issue, which includes a PoC zip archive (monkey-poc.zip) (Archer Advisory, Monkey Issue #426). There is no evidence of active in-the-wild exploitation at this time, and no threat actor attribution has been reported. The EPSS score is approximately 0.51%, indicating a low but non-negligible probability of exploitation. CVE-2025-63656 is not currently listed in the CISA Known Exploited Vulnerabilities (KEV) catalog.

1. Reconnaissance: Identify internet-facing Monkey HTTP Server instances (versions ≤ 1.8.5 or commit f37e984) using tools like Shodan or Censys by searching for the Monkey server banner.
2. Craft malicious HTTP request: Construct an HTTP request with a specially malformed or oversized header value designed to trigger the out-of-bounds read in the header_cmp function during header parsing.
3. Send the crafted request: Transmit the malformed HTTP request directly to the target server's listening port (typically TCP 80 or 443) without any authentication.
4. Trigger DoS: The header_cmp function in mk_server/mk_http_parser.c reads beyond the intended buffer boundary, causing a crash or abnormal termination of the Monkey server process, resulting in a denial of service (Archer Advisory, Monkey Issue #426).

• Network: Unusual or malformed HTTP requests with abnormal header values or lengths directed at the Monkey HTTP Server port; repeated connection attempts from a single source IP targeting the server.
• Logs: Monkey server access logs showing requests with malformed headers immediately preceding a server crash or restart; error logs indicating segmentation faults or memory access violations in mk_http_parser.c.
• Process: Unexpected termination or restart of the Monkey HTTP Server process (monkey); crash dump files generated in the server's working directory following receipt of crafted requests.

As of the disclosure date (January 2026), no official patch has been released by the Monkey project for this vulnerability — it remains unFixed in commit f37e984 and all versions through 1.8.5 (Archer Advisory). Recommended interim mitigations include: (1) placing the Monkey server behind a reverse proxy or Web Application Firewall (WAF) capable of filtering malformed HTTP headers; (2) implementing network-level access controls to restrict which clients can reach the Monkey server; (3) monitoring server logs for anomalous HTTP requests; and (4) considering migration to an actively maintained HTTP server until a patch is available. Users should monitor the Monkey project repository for any upstream fixes.

The vulnerability was reported by researcher archersec, who disclosed nine vulnerabilities in Monkey HTTP Server simultaneously in January 2026 after CVE IDs were assigned in October 2025. The issue was filed on the Monkey GitHub repository as issue #426, but the project maintainers have not publicly responded or committed a fix as of the disclosure date (Monkey Issue #426, Archer Advisory). Red Hat has tracked the CVE in their security advisory database (Red Hat CVE).