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Advisory record · GHSA-8v8h-hg4w-mvq2

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

CVSS 6.3 MediumEPSS 0.0030 · 23rd percentile

Published by the GitHub Advisory Database at 24 Sept 2026, 19:43 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

PackageEcosystemAffectedFixed in
hpackPyPI<4.2.04.2.0
Identifiers
GHSA-8v8h-hg4w-mvq2 · CVE-2026-59980 · PYSEC-2026-4053
Weakness (CWE)
CWE-400
Tags
Published
24 Sept 2026
Updated
1 Oct 2026

Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

The hpack package, versions before 4.2.0, is vulnerable to unbounded variable integer decoding. An attacker who can send malformed HTTP/2 header data to the decoder, typically through an application using hpack or h2, can force quadratic-time computation that blocks further processing. Upgrade hpack to 4.2.0 or later, which limits integer decoding to uint32. Until then, reject or sanitize header input containing long runs of 0xFF bytes.

Written 1 Nov 2026.

Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

Severity and exploitation

ScoreRatingCVSSVectorSource
6.3 Medium 4.0 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 CNA (GitHub_M)
  • CISA KEV Not listed.
  • EPSS 0.0030, the 23rd percentile, on 13 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

Timeline

  1. 31 Oct 2026, 14:09 UTC First seen by agentprime
  2. 31 Oct 2026, 14:09 UTC Confirmed by OSV

Sources

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.