CVE-2025-63651: 
Linux Alpine vulnerability analysis and mitigation

CVE-2025-63651 is a use-after-free vulnerability in the mk_string_char_search function within mk_core/mk_string.c of the Monkey HTTP Server (monkey-project/monkey). It allows unauthenticated remote attackers to cause a Denial of Service (DoS) by sending a crafted HTTP request. The vulnerability affects Monkey through commit f37e984 (all versions up to and including 1.8.5). CVE ID was requested and assigned in October 2025, with public disclosure in January 2026. It carries a CVSS v3.1 base score of 7.5 (High) (Feedly, Archer Advisory).

The root cause is a use-after-free memory safety flaw (CWE-416) in the mk_string_char_search function located in mk_core/mk_string.c of Monkey HTTP Server at commit f37e984. When processing a specially crafted HTTP request, the server accesses memory that has already been freed, leading to undefined behavior and a crash. The attack vector is network-accessible, requires no authentication or user interaction, and has low attack complexity, making it trivially exploitable by any remote attacker. This CVE is part of a broader set of nine vulnerabilities discovered in the same codebase by researcher archersec, including additional use-after-free, out-of-bounds read, and stack overflow issues (Archer Advisory, GitHub Issue).

Successful exploitation causes the Monkey HTTP Server process to crash or become unresponsive, resulting in a complete loss of availability for any services hosted on the affected server. There is no impact to confidentiality or integrity — the vulnerability is limited to a Denial of Service condition. Because no authentication is required and the attack can be launched remotely over the network, any internet-exposed Monkey HTTP Server instance is at risk of repeated 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) demonstrating the vulnerability on Ubuntu 20.04. As of the time of disclosure, there is no evidence of active in-the-wild exploitation. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. The EPSS score is approximately 0.51%, indicating a low but non-negligible probability of exploitation in the near term (Archer Advisory, GitHub Issue, Feedly).

1. Reconnaissance: Identify internet-facing Monkey HTTP Server instances using tools like Shodan or Censys, searching for the Monkey server banner. Target instances running commit f37e984 or versions ≤ 1.8.5.
2. Craft malicious HTTP request: Construct a specially crafted HTTP request designed to trigger the use-after-free condition in the mk_string_char_search function. The exact payload structure is detailed in the PoC archive (monkey-poc.zip) referenced in the GitHub issue.
3. Send the request: Transmit the crafted HTTP request to the target server on its listening port (typically TCP 80 or 443). No authentication credentials are required.
4. Trigger DoS: The server's mk_string_char_search function in mk_core/mk_string.c accesses freed memory, causing the server process to crash or become unresponsive, resulting in a Denial of Service (Archer Advisory, GitHub Issue).

• Network: Unusual or malformed HTTP requests to the Monkey HTTP Server from unexpected source IPs; repeated connection attempts followed by server unresponsiveness.
• Logs: Monkey HTTP Server access logs showing requests with anomalous headers or payloads immediately preceding a crash; error logs indicating segmentation faults or memory corruption errors in mk_core/mk_string.c.
• Process: Unexpected termination or restart of the Monkey HTTP Server process (monkey); core dump files generated in the server's working directory following a crash.
• System: Increased frequency of service restarts or watchdog alerts for the Monkey HTTP Server daemon (Archer Advisory).

As of the time of disclosure, no official patch has been released by the Monkey project maintainers — the vulnerability remains unfix in all versions through commit f37e984 (≤ 1.8.5). Organizations should implement network-level controls (e.g., firewall rules, reverse proxy filtering) to restrict HTTP access to the Monkey server from trusted sources only. Consider replacing Monkey HTTP Server with an actively maintained alternative if a patch is not forthcoming. Monitor affected systems for unexpected crashes or service restarts that may indicate exploitation attempts (Archer Advisory, Feedly).

The vulnerability was reported by researcher archersec, who disclosed a total of nine vulnerabilities in Monkey HTTP Server through a coordinated advisory in January 2026, following CVE assignment in October 2025. The GitHub issue (#426) was closed without an official fix from the Monkey project maintainers, raising concerns about the project's maintenance status. Red Hat has tracked the CVE in their security advisory database. No significant broader media coverage or notable community debate has been observed beyond the initial disclosure (Archer Advisory, GitHub Issue, Red Hat Advisory).