CVE-2025-13836 is a vulnerability identified in the Python Software Foundation's CPython implementation affecting its HTTP client behavior. When an HTTP response is read from a server, if the client does not specify the amount of data to read, the default mechanism uses the Content-Length header to determine how much data to load into memory. A malicious server can exploit this by sending a response with an excessively large Content-Length value, causing the client to allocate and read a large amount of data. This can lead to out-of-memory (OOM) conditions or denial of service (DoS) by exhausting system resources. The vulnerability does not require authentication or user interaction and can be triggered remotely by any server responding to HTTP requests made by the vulnerable client. The affected product is CPython, the reference implementation of Python, which is widely used in many applications and services. The vulnerability is rated with a CVSS 4.0 score of 6.3 (medium severity), reflecting the network attack vector, low attack complexity, no privileges required, no user interaction, and limited impact on availability. There are no known exploits in the wild at the time of publication, and no patches have been linked yet. The issue primarily impacts applications that rely on CPython's HTTP client without explicitly limiting the read size, potentially causing resource exhaustion and service disruption.

For European organizations, the primary impact of CVE-2025-13836 is the risk of denial of service or service degradation due to resource exhaustion when interacting with malicious or compromised HTTP servers. This can affect web services, APIs, and backend systems that use Python's HTTP client libraries without specifying read limits. Critical infrastructure, financial services, healthcare, and government sectors that rely heavily on Python for automation, data processing, or web communication could experience outages or degraded performance. While confidentiality and integrity are not directly impacted, availability disruptions can lead to operational downtime and potential cascading effects on dependent services. Organizations with large-scale Python deployments or those integrating third-party HTTP services are particularly at risk. The lack of authentication or user interaction requirements means the attack surface is broad, and any client-server communication using vulnerable CPython versions could be exploited remotely.

To mitigate CVE-2025-13836, organizations should immediately review and update their Python HTTP client usage to explicitly specify read limits when processing HTTP responses, avoiding reliance on the Content-Length header alone. Developers should audit codebases for instances where HTTP response reading occurs without defined size constraints and implement safeguards such as maximum buffer sizes or timeout controls. Monitoring and alerting on abnormal memory usage or application crashes related to HTTP client operations can help detect exploitation attempts. Organizations should track updates from the Python Software Foundation and apply security patches promptly once available. Additionally, employing network-level protections such as web application firewalls (WAFs) to detect and block suspicious HTTP responses with abnormally large Content-Length headers can reduce exposure. For critical systems, consider isolating Python-based HTTP clients in controlled environments with resource limits to prevent system-wide impact.