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Response snapshot Nov 5, 2026, 22:51 UTC

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VulnerabilityCVE-2025-64076

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...

Multiple vulnerabilities exist in cbor2 through version 5.7.0 in the decode_definite_long_string() function of the C extension decoder (source/decoder.

At a glance

Exploited
Not known
Severity
7.5 High (CVSS 3.1)
Chance of exploitation
0.5% in the next 30 days
Sources
1 source

What to do

Check the vendor advisory for affected versions and fixes.

Read NVD CVE-2025-64076 (opens a new tab)

What happened

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.

Continue reading

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.

Scores and how they were measured

CVSS base severity

7.5

Version 3.1As of

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

0.5%

Scored Percentile 42.2%

Model and measurement source (opens a new tab)

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    NVD CVE-2025-64076 (opens a new tab)

    NVD record: description, CVSS (the CNA score is preferred), affected products, and publication date. Not evidence of exploitation.

    Published Checked

  2. Primary source

    NVD CVE API 2.0 (opens a new tab)

    The NVD API page that supplied this CVE record.

    Published Date not suppliedChecked

  3. Primary source

    NVD CVE API 2.0 (opens a new tab)

    The NVD API page that supplied this CVE record.

    Published Date not suppliedChecked

  4. Primary source

    NVD CVE API 2.0 (opens a new tab)

    The NVD API page that supplied this CVE record.

    Published Date not suppliedChecked

  5. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2025-64076, scored 2026-11-05. This is a model estimate, not proof of exposure.

    Published Checked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, November 5). 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... (CVE-2025-64076). https://kevwire.com/record/CVE-2025-64076

MLA

"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... (CVE-2025-64076)." KEVWire, 5 Nov. 2026, kevwire.com/record/CVE-2025-64076.

Chicago

KEVWire. "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... (CVE-2025-64076)." Last modified November 5, 2026. https://kevwire.com/record/CVE-2025-64076.

Evidence summary (plain text)

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... (Nov 18, 2025).
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.
CVSS 7.5 (version 3.1; evidence as of Nov 5, 2026; https://nvd.nist.gov/vuln/detail/CVE-2025-64076).
EPSS 0.5% for the following 30 days, scored Nov 5, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2025-64076&date=2026-11-05).
Sources:
NVD CVE-2025-64076: https://nvd.nist.gov/vuln/detail/CVE-2025-64076
NVD CVE API 2.0: https://services.nvd.nist.gov/rest/json/cves/2.0?lastModStartDate=2026-11-05T13:41:02.345Z&lastModEndDate=2026-11-05T15:20:59.749Z&resultsPerPage=500&startIndex=0
NVD CVE API 2.0: https://services.nvd.nist.gov/rest/json/cves/2.0?lastModStartDate=2026-11-05T13:11:13.160Z&lastModEndDate=2026-11-05T14:40:57.328Z&resultsPerPage=500&startIndex=0
NVD CVE API 2.0: https://services.nvd.nist.gov/rest/json/cves/2.0?lastModStartDate=2026-11-05T12:41:01.195Z&lastModEndDate=2026-11-05T14:10:57.298Z&resultsPerPage=500&startIndex=0
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2025-64076&date=2026-11-05
Record ID: CVE-2025-64076.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Nov 5, 2026, 19:11 UTC. Ingestion is not an independent editorial investigation.

NVD status: Analyzed. Publication in the NVD is not evidence of exploitation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

Record JSON

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Record details

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