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Fencer Vulns
high 7.5

CVE-2025-64076 json

cbor2 C extension decoder flaws can cause denial of service

published 2025-11-18 11 mo ago modified 2026-11-05 3 d ago also GHSA-9q9c-vjxh-p795 , PYSEC-2025-238 , EUVD-2025-198054

cvss
7.5
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.5%
EPSS 0.5% chance of exploitation in 30 days, percentile 42%
public exploits
0
none known

Summary

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.

Patch / fix

  • pypi cbor2 5.7.1
  • cbor2 Debian 14 5.7.1-1

debian advisories DEBIAN-CVE-2025-64076

  • python-cbor2 SLES for SAP applications 16.0, SLES 16.0 5.6.5-160000.3.1

suse advisories SUSE-SU-2025:21168-1

  • python-cbor2 openSUSE Leap 16.0 5.6.5-160000.3.1

opensuse advisories openSUSE-SU-2025:20133-1

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2025-64076 updated cvss 7.5

cbor2: cbor2: Integer Underflow and Memory Leak leading to Denial of Service

  • Red Hat All currently supported Red Hat products
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
Proof of concept is public
Automatable
no
Technical impact
partial
Assessed
11 mo ago

Exploit prediction

0.49% chance of exploitation in the next 30 days, higher than 42% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • github.com https://github.com/agronholm/cbor2/pull/265 referenced PoC, tagged "exploit" by a source, unverified
  • github.com https://github.com/agronholm/cbor2/issues/264 referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 3.1 7.5 CVSS:3.1/AC:L/AV:N/A:H/C:N/I:N/PR:N/S:U/UI:N
CNA 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
GitHub 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
OSV 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected packages

ecosystem package affected fixed in
pypi cbor2
<5.7.1
5.7.1

Classification

Weakness
  • CWE-191 Integer Underflow (Wrap or Wraparound)

References

Sources

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