CVE-2025-53629 is a high-severity vulnerability identified in the yhirose cpp-httplib library, a widely used C++11 single-file header-only HTTP/HTTPS library. This vulnerability affects all versions prior to 0.23.0. The issue arises from improper handling of HTTP requests that use the 'Transfer-Encoding: chunked' header. Specifically, the library allocates memory dynamically based on the chunked data size without imposing any limits or throttling mechanisms. An attacker can exploit this by sending specially crafted chunked requests that cause the server to allocate excessive amounts of memory, potentially exhausting server resources and leading to denial of service (DoS). The vulnerability is classified under CWE-770 (Allocation of Resources Without Limits or Throttling), indicating a failure to control resource consumption. Notably, this vulnerability does not impact confidentiality or integrity but severely affects availability. The CVSS v3.1 score is 7.5 (high), reflecting that the vulnerability can be exploited remotely over the network without authentication or user interaction. Although no known exploits are currently reported in the wild, the vulnerability is related to CVE-2025-53628, suggesting a pattern of similar resource exhaustion issues in the cpp-httplib library. The fix was introduced in version 0.23.0, which presumably implements proper limits on memory allocation for chunked transfer encoding requests.

For European organizations, the impact of CVE-2025-53629 can be significant, especially for those relying on cpp-httplib in their web servers, embedded systems, or networked applications. Exploitation can lead to denial of service by exhausting server memory, causing application crashes or system instability. This can disrupt business operations, degrade service availability, and potentially lead to financial losses or reputational damage. Organizations in sectors with high availability requirements, such as finance, healthcare, telecommunications, and critical infrastructure, are particularly vulnerable. Since cpp-httplib is a lightweight library often embedded in custom or IoT applications, the vulnerability could affect a broad range of devices and services, including those with limited monitoring or patching capabilities. Additionally, the lack of authentication or user interaction for exploitation increases the risk of automated attacks or scanning by malicious actors. The absence of known exploits currently provides a window for proactive mitigation, but the high severity score warrants urgent attention.

1. Immediate upgrade to cpp-httplib version 0.23.0 or later, where the vulnerability is fixed. 2. If upgrading is not immediately feasible, implement network-level protections such as rate limiting and deep packet inspection to detect and block suspicious chunked transfer encoding requests. 3. Employ application-layer firewalls or Web Application Firewalls (WAFs) configured to monitor and restrict unusually large or malformed HTTP chunked requests. 4. Monitor server resource usage closely to detect abnormal memory consumption patterns indicative of exploitation attempts. 5. Conduct thorough code audits for any custom implementations using cpp-httplib to ensure no other resource allocation vulnerabilities exist. 6. Develop incident response plans that include detection and mitigation strategies for resource exhaustion attacks. 7. Engage with vendors or third-party providers to confirm patch deployment and assess exposure. 8. For embedded or IoT devices using cpp-httplib, coordinate firmware updates or mitigations to address the vulnerability promptly.