hpack: Unbounded variable integer decoding can cause run-away computation on malformed input (CVE-2026-59980) | HOL Guard CVE

Medium · CVSS 6.3CVE-2026-59980

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

Answer in brief

CVE-2026-59980 records a Medium severity (CVSS 6.3) vulnerability in hpack: Unbounded variable integer decoding can cause run-away computation on malformed input. The current sources do not mark it as known exploited. The current feed maps python-hyper/hpack (generic). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Sep 23, 2026Updated Sep 23, 2026Source checked Sep 23, 2026First seen by HOL Sep 23, 2026Material review Sep 23, 2026

Upstream Advisory

Key facts

Risk: Medium · CVSS 6.3
Exploitation: Not marked as known exploited
Affected software: 1 mapped package or product
Fix availability: Available

Why this deserves its current priority

CVSS is 6.3. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps python-hyper/hpack (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
python-hyper/hpackgeneric	<4.2.0	4.2.0

Recommended response

Check inventory. Check lockfiles and deployed manifests for python-hyper/hpack.
Review the reported fix. Update python-hyper/hpack to 4.2.0 if you use the affected versions. Test the change in a non-production environment first.

Evidence timeline and material changes

Published upstream
Sep 23, 2026
Evidence: source:cvelist:source_dates:source-dates:record
Source modified
Sep 23, 2026
Evidence: source:cvelist:source_dates:source-dates:record
First seen by HOL
Sep 23, 2026

Sources and claim methodology

Upstream source 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.

Quoted source text, attributed separately from HOL analysis.