CVE-2026-26209: cbor2 provides encoding and decoding for the Concise Binary Object Representation (CBOR) serialization format

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HIGH 7.5REMOTEUPGRADE

CVE-2026-26209 is a high-severity flaw (CVSS 7.5) affecting cbor2. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

cbor2 provides encoding and decoding for the Concise Binary Object Representation (CBOR) serialization format. Versions prior to 5.9.0 are vulnerable to a Denial of Service (DoS) attack caused by uncontrolled recursion when decoding deeply nested CBOR structures. This vulnerability affects both the pure Python implementation and the C extension `_cbor2`. The C extension relies on Python's internal recursion limits `Py_EnterRecursiveCall` rather t

Why this priority

High severity
Public exploit reference
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.4% (top 66%)
CISA exploited list	No
Weakness	CWE-674
Published	2026-03-23
Last modified	2026-06-17

Publisher (CNA): GitHub, Inc. · Vendor: agronholm

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-03-23 19:16 UTCPublished CVE record published
2026-06-17 10:25 UTCUpdated Record last modified

agronholm cbor2 UPGRADE

Affected: < 5.9.0

Fixed in: 5.9.0

Safe to downgrade to: no unaffected older release

1. Check installed version

pip show cbor2

2. UPGRADE to 5.9.0

pip install --upgrade "cbor2>=5.9.0"

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-26209?

cbor2: upgrade to 5.9.0. Exact commands for each OS are on this page.

Is CVE-2026-26209 being exploited?

CVE-2026-26209 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-26209 affect?

CVE-2026-26209 affects cbor2. Severity is high (CVSS 7.5). cbor2 provides encoding and decoding for the Concise Binary Object Representation (CBOR) serialization format. Versions prior to 5.9.0 are vulnerable to a Denial of Service (DoS) attack caused by uncontrolled recursion when decoding deeply

How urgent is CVE-2026-26209?

Priority 45/100. High severity; Public exploit reference; Remotely exploitable, no login, no user action

Software pages

Python vulnerabilities

References

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