CVE-2025-63649: 
Linux Alpine vulnerability analysis and mitigation

CVE-2025-63649 is an out-of-bounds read vulnerability in the http_parser_transfer_encoding_chunked function located in mk_server/mk_http_parser.c of the Monkey HTTP Server. It affects Monkey through commit f37e984 (versions up to and including 1.8.5) and allows unauthenticated remote attackers to cause a Denial of Service (DoS) by sending a crafted POST request. The CVE was requested and assigned in October 2025 and publicly disclosed in January 2026. It carries a CVSS v3.1 base score of 7.5 (High) (Feedly, Archer Advisory).

The vulnerability is classified as CWE-125 (Out-of-bounds Read) and resides in the chunked transfer encoding parser within mk_server/mk_http_parser.c. When processing a crafted HTTP POST request with malformed chunked transfer encoding, the http_parser_transfer_encoding_chunked function reads beyond the bounds of an allocated buffer, leading to a crash. No authentication or user interaction is required, and the attack is conducted entirely over the network with low complexity. A proof-of-concept (PoC) archive (monkey-poc.zip) was included in the original GitHub issue report (Monkey Issue #426, Archer Advisory).

Successful exploitation causes the Monkey HTTP Server process to crash, resulting in a complete loss of availability for any services hosted on the affected server. There is no impact on confidentiality or integrity, as the vulnerability only triggers an out-of-bounds read that leads to a DoS condition rather than memory disclosure or code execution. The vulnerability is part of a broader set of nine issues discovered in the same codebase, some of which (e.g., CVE-2025-63658) carry potential for remote code execution (Archer Advisory).

A PoC exploit (in the form of a ZIP archive with crafted request payloads) was publicly released alongside the vulnerability disclosure in the GitHub issue (Monkey Issue #426). There is no evidence of in-the-wild exploitation or threat actor attribution at this time. The EPSS score is approximately 0.018% (0.000180), indicating a low probability of exploitation in the near term. CVE-2025-63649 is not currently listed in the CISA Known Exploited Vulnerabilities (KEV) catalog (Feedly).

1. Reconnaissance: Identify internet-facing Monkey HTTP Server instances running commit f37e984 or earlier (version ≤ 1.8.5) using network scanning tools such as Shodan, Censys, or nmap with HTTP banner grabbing.
2. Craft malicious POST request: Construct an HTTP POST request with a malformed Transfer-Encoding: chunked body — for example, a chunk size field containing invalid or oversized values that cause the parser to read beyond buffer boundaries.
3. Send the request: Deliver the crafted POST request to any HTTP endpoint on the target Monkey server (e.g., POST / HTTP/1.1 with Transfer-Encoding: chunked and a malformed chunk sequence).
4. Trigger DoS: The http_parser_transfer_encoding_chunked function in mk_http_parser.c performs an out-of-bounds read, causing the server process to crash and become unavailable (Monkey Issue #426, Archer Advisory).

• Network: Unexpected HTTP POST requests with malformed Transfer-Encoding: chunked bodies targeting the Monkey HTTP Server; repeated connection attempts from a single source IP followed by server unavailability.
• Logs: Monkey HTTP Server access logs showing POST requests immediately preceding a crash or restart event; error logs indicating segmentation faults or abnormal process termination in the HTTP parser.
• Process: Sudden termination of the monkey server process (e.g., exit code indicating SIGSEGV or SIGABRT); automatic restart loops if a process supervisor (e.g., systemd) is configured.

As of the disclosure date (January 2026), no official patch has been released by the Monkey HTTP Server project for this vulnerability (Archer Advisory). Organizations using Monkey HTTP Server should consider the following interim mitigations:

• Network-level filtering: Deploy a Web Application Firewall (WAF) or reverse proxy to inspect and block malformed chunked transfer encoding requests before they reach the Monkey server.
• Access restriction: Limit exposure of the Monkey HTTP Server to trusted networks or IP ranges using firewall rules.
• Monitor for updates: Track the Monkey GitHub repository for patches addressing this and the related CVEs (CVE-2025-63650 through CVE-2025-63658).
• Consider alternatives: Given the number of unpatched vulnerabilities in the codebase, evaluate migrating to an actively maintained HTTP server.

The vulnerability was noted by security aggregators including Red Hat's CVE tracking page and VulDB shortly after disclosure (Red Hat CVE). Social media posts on Mastodon and Bluesky by @thehackerwire referenced the CVE in early 2026. Community reaction has been limited, consistent with the niche deployment footprint of the Monkey HTTP Server. The lack of an official vendor fix has been highlighted in the advisory, which notes that all nine reported vulnerabilities remain unpatched (Archer Advisory).