# CVE-2026-59980 — python-hyper hpack

> As of 2026-09-24, CVE-2026-59980 is a Medium-severity (band derived from the CVSS v4.0 score, not stated by the CNA) vulnerability in python-hyper hpack, CVSS v4.0 6.3, EPSS 0.3% (20.2th percentile). No Threadlinqs-tracked threat campaign has been attributed to CVE-2026-59980 as of 2026-09-24; the identifier is re-checked against the Threadlinqs threat corpus on every daily ingest.

**Last updated:** 2026-09-24

## What is CVE-2026-59980?

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.

The record classifies CVE-2026-59980 under weakness class [CWE-400](https://intel.threadlinqs.com/cwe/CWE-400). 1 affected-product entry is recorded, across 1 vendor, listed below. The identifier was first published 12 days ago.

## Severity and exploitation probability

- **CVSS v4.0 base score:** 6.3 — Medium (derived from score) (`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`)
- **EPSS (FIRST):** 0.3% probability of exploitation in the next 30 days, higher than 20.2% of all scored CVEs
- **CISA KEV:** Not listed in the CISA Known Exploited Vulnerabilities catalog
- **Threadlinqs priority:** 4/10 — a Threadlinqs composite of the CVSS base score, the EPSS percentile and public exploit availability
- **Published:** 2026-09-23, last modified 2026-09-24

## Is CVE-2026-59980 being exploited?

It currently carries a trending score of 35 in the Threadlinqs vulnerability feed.

## Affected products and versions

- **python-hyper**: hpack

## How to fix CVE-2026-59980

No vendor patch reference has been recorded for CVE-2026-59980 in the tracked sources. Follow the references below for a fix, and treat the products listed above as exposed until the vendor states otherwise.

## Threat activity tracking CVE-2026-59980

No threat campaign in the Threadlinqs corpus currently references CVE-2026-59980, in its CVE list or as an indicator. The linkage is recomputed from the live corpus on every daily ingest, so this section fills in as soon as a tracked campaign cites the identifier.

## Sources

Enriched from CVE.org, NVD, FIRST EPSS. Last verified by Threadlinqs on 2026-09-25. This product uses the NVD API but is not endorsed or certified by the NVD.

**Vendor advisory and patch**

- [github.com](https://github.com/python-hyper/hpack/security/advisories/GHSA-8v8h-hg4w-mvq2)

**Other references**

- [github.com](https://github.com/python-hyper/hpack/commit/8cfb02c547740e16dbfe7aba77bad84b297cec2c)
- [cs.opensource.google](https://cs.opensource.google/go/x/net/+/master:http2/hpack/hpack.go;l=468;drc=8e2b117aee74f6b86c207a808b0255de45c0a18a)
- [docs.rs](https://docs.rs/http2/0.5.19/src/http2/hpack/decoder.rs.html#403)

Canonical: https://intel.threadlinqs.com/cve/CVE-2026-59980
Full detection coverage and IOCs for threats exploiting CVE-2026-59980 via the Threadlinqs MCP server (Purple tier): https://intel.threadlinqs.com/mcp