CVE-2025-64076 identifies two critical vulnerabilities in the cbor2 Python library's C extension decoder, specifically within the decode_definite_long_string() function. The first issue is an integer underflow caused by an incorrect variable reference and missing state reset in the chunk processing loop. This leads to the buffer_length variable not being reset after consuming UTF-8 characters, causing chunk_length calculations to produce negative values. These negative values are passed as signed integers to the read() method, potentially triggering out-of-bounds reads and unlimited read operations, which can exhaust system resources. The second issue is a memory leak due to the failure to release Python object references (Py_DECREF) for chunk objects allocated in each iteration of the processing loop. For CBOR strings exceeding 65536 bytes, this leak accumulates proportionally to the payload size, enabling memory exhaustion attacks. Both vulnerabilities can be exploited remotely without authentication by sending specially crafted CBOR data containing definite-length text strings with multi-byte UTF-8 characters positioned at 65536-byte chunk boundaries. Successful exploitation results in denial of service through process crashes (manifesting as CBORDecodeEOF exceptions) or memory exhaustion. This affects any application using cbor2's C extension to process untrusted CBOR data, such as web APIs, IoT data collectors, and message queue processors. The vulnerabilities were addressed and fixed in cbor2 version 5.7.1, which resets buffer_length correctly and ensures proper reference count management to prevent memory leaks.

For European organizations, the impact of CVE-2025-64076 is significant, especially for those relying on cbor2 for processing CBOR-encoded data in critical infrastructure, IoT deployments, or cloud-based APIs. Exploitation can lead to denial of service conditions, causing application crashes or memory exhaustion, which disrupts service availability and may impact operational continuity. In sectors such as healthcare, manufacturing, and smart city infrastructure, where IoT data collectors and message queues are prevalent, these disruptions could have cascading effects on safety and productivity. Additionally, denial of service attacks could be leveraged as part of multi-vector campaigns targeting European enterprises, potentially amplifying the impact on critical services. Since exploitation requires no authentication or user interaction, attackers can remotely trigger these conditions by sending malicious CBOR payloads, increasing the risk of widespread exploitation if vulnerable systems are exposed to untrusted data sources.

European organizations should immediately upgrade all instances of the cbor2 library to version 5.7.1 or later, which contains the fixes for these vulnerabilities. For environments where immediate upgrade is not feasible, implement strict input validation and filtering to block or sanitize CBOR payloads containing large definite-length text strings with multi-byte UTF-8 characters, especially those near 65536-byte chunk boundaries. Deploy runtime application monitoring to detect abnormal memory usage patterns or frequent CBORDecodeEOF exceptions indicative of exploitation attempts. Network-level protections such as Web Application Firewalls (WAFs) or Intrusion Prevention Systems (IPS) should be configured to detect and block suspicious CBOR traffic patterns. Additionally, isolate systems processing untrusted CBOR data within segmented network zones to limit potential impact. Regularly audit dependencies and maintain an up-to-date software bill of materials (SBOM) to quickly identify and remediate vulnerable library versions. Finally, educate development and security teams about the risks associated with CBOR data processing and the importance of timely patching.