CVE-2025-64076 cbor2 C extension decoder flaws can cause denial of service
Remote attackers can cause denial of service via crashes or memory exhaustion by sending crafted CBOR data to applications using the cbor2 library.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-64076?
CVE-2025-64076 is a high-severity vulnerability in n/a, published on 18 Nov 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Multiple vulnerabilities exist in cbor2 through version 5.7.0 in the decode_definite_long_string() function of the C extension decoder (source/decoder.c): (1) Integer Underflow Leading to Out-of-Bounds Read (CWE-191, CWE-125): An incorrect variable reference and missing state reset in the chunk processing loop causes buffer_length to not be reset to zero after UTF-8 character consumption. This results in subsequent chunk_length calculations producing negative values (e.g., chunk_length = 65536 - buffer_length), which are passed as signed integers to the read() method, potentially triggering unlimited read operations and resource exhaustion. (2) Memory Leak via Missing Reference Count Release (CWE-401): The main processing loop fails to release Python object references (Py_DECREF) for chunk objects allocated in each iteration. For CBOR strings longer than 65536 bytes, this causes cumulative memory leaks proportional to the payload size, enabling memory exhaustion attacks through repeated processing of large CBOR payloads. 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 (CBORDecodeEOF exceptions) or memory exhaustion. The vulnerabilities affect all applications using cbor2's C extension to process untrusted CBOR data, including web APIs, IoT data collectors, and message queue processors. Fixed in commit 851473490281f82d82560b2368284ef33cf6e8f9 pushed with released version 5.7.1.
Weakness: CWE-191
Which versions are affected by CVE-2025-64076?
3 open source packages ship an affected version, including cbor2. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
cbor2 PyPI | All versions | 5.7.1 |
cbor2 PyPI | All versions | 5.7.1 |
cbor2 PyPI | All versions | 5.7.1 |
Products include Agronholm Cbor2.
How do I fix CVE-2025-64076?
Upgrade cbor2 to 5.7.1 or later.
Is CVE-2025-64076 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2025-64076 exists.
Other vulnerabilities in n/a
Sources
Also known as AZL-70516, DEBIAN-CVE-2025-64076, GHSA-9q9c-vjxh-p795, PYSEC-2025-238, UBUNTU-CVE-2025-64076.