hpack is an HTTP/2 Header Encoding for Python. Prior to version 4.2.0, unbounded variable integer decoding can cause run-away computation on malformed input leading to O(n^2) runtime, effectively blocking further processing with large enough unsanitized input. A fix is available in python-hyper/hpack v4.2.0 to restricted variable integer decoding to uint32 to prevent run-away computation. As a workaround, sanitize input to hpack decoder for long sequences of `0xFF` values to prevent malicious use.

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Fixes

Solution

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Workaround

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History

Thu, 24 Sep 2026 14:30:00 +0000

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}


Thu, 24 Sep 2026 09:45:00 +0000

Type Values Removed Values Added
First Time appeared Python-hyper
Python-hyper hpack
Vendors & Products Python-hyper
Python-hyper hpack

Thu, 24 Sep 2026 00:15:00 +0000

Type Values Removed Values Added
Weaknesses CWE-770
References
Metrics threat_severity

None

cvssV3_1

{'score': 7.5, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H'}

threat_severity

Important


Wed, 23 Sep 2026 22:30:00 +0000

Type Values Removed Values Added
Description hpack is an HTTP/2 Header Encoding for Python. Prior to version 4.2.0, unbounded variable integer decoding can cause run-away computation on malformed input leading to O(n^2) runtime, effectively blocking further processing with large enough unsanitized input. A fix is available in python-hyper/hpack v4.2.0 to restricted variable integer decoding to uint32 to prevent run-away computation. As a workaround, sanitize input to hpack decoder for long sequences of `0xFF` values to prevent malicious use.
Title hpack: Unbounded variable integer decoding can cause run-away computation on malformed input
Weaknesses CWE-400
References
Metrics cvssV4_0

{'score': 6.3, 'vector': 'CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N'}


Projects

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cve-icon MITRE

Status: PUBLISHED

Assigner: GitHub_M

Published:

Updated: 2026-09-24T13:25:08.900Z

Reserved: 2026-07-07T19:41:00.004Z

Link: CVE-2026-59980

cve-icon Vulnrichment

Updated: 2026-09-24T13:24:43.707Z

cve-icon NVD

Status : Received

Published: 2026-09-23T23:17:11.473

Modified: 2026-09-24T14:17:37.727

Link: CVE-2026-59980

cve-icon Redhat

Severity : Important

Publid Date: 2026-09-23T22:07:35Z

Links: CVE-2026-59980 - Bugzilla

cve-icon OpenCVE Enrichment

Updated: 2026-09-24T09:09:11Z

Weaknesses