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Vulnerability · Published Sep 23, 2026 · Assigned by GitHub_M

CVE-2026-59980hpack: Unbounded variable integer decoding can cause run-away computation on malformed input

Medium 6.3

In plain English

A medium-severity uncontrolled resource consumption in hpack from python-hyper. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 23% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 4.0)6.3Medium
Exploit odds (30 days)0.3%top 77.0% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

Official 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.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
Debian:12python-hpackno fix listedDEBIAN-CVE-2026-59980
Debian:13python-hpackno fix listedDEBIAN-CVE-2026-59980
Debian:14python-hpackno fix listedDEBIAN-CVE-2026-59980
PyPIhpack4.2.0GHSA-8v8h-hg4w-mvq2
PyPIhpack4.2.0PYSEC-2026-4053
Ubuntu:22.04:LTSpython-hpackno fix listedUBUNTU-CVE-2026-59980
Ubuntu:24.04:LTSpython-hpackno fix listedUBUNTU-CVE-2026-59980
Ubuntu:26.04:LTSpython-hpackno fix listedUBUNTU-CVE-2026-59980
Ubuntu:26.10python-hpackno fix listedUBUNTU-CVE-2026-59980

Affected products (as reported)

VendorProductVersions
python-hyper hpack < 4.2.0

Weakness: CWE-400 · 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

References