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

Type: advisorySource: OSV1 source record

### Impact Users of the python-hyper/hpack library, most commonly used as downstream dependency of the python-hyper/h2 library (an HTTP/2 client and server implementation). 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. ### Patches A fix is available in python-hyper/hpack v4.2.0 to restricted variable integer decoding to uint32 to prevent run-away computation. ### Workarounds Sanitize input to hpack decoder for long sequences of `0xFF` values to prevent malicious use.

Announced Source revision 1
Original source ↗

GHSA-8v8h-hg4w-mvq2

Primary source · Revision 1

Imported from structured advisory data. Source references identify upstream records or fields; verify affected versions and your deployed configuration.

Security context

Source reference: osv:GHSA-8v8h-hg4w-mvq2

hpack

Source reference: /affected/0/package/name

Affected versions
1.0.0

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1.0.1

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1.1.0

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2.0.0

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2.0.1

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2.1.0

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2.1.1

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2.2.0

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2.3.0

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3.0.0

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4.0.0

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4.1.0

Source reference: /affected/0/versions

Fixed versions
4.2.0

Source reference: /affected/0/ranges

Prerequisites
Not stated in this record

Structured affected-version data

Original package identities and version rules from this source.

[
  {
    "ranges": [
      {
        "type": "ECOSYSTEM",
        "events": [
          {
            "introduced": "0"
          },
          {
            "fixed": "4.2.0"
          }
        ]
      }
    ],
    "package": {
      "name": "hpack",
      "purl": "pkg:pypi/hpack",
      "ecosystem": "PyPI"
    },
    "versions": [
      "1.0.0",
      "1.0.1",
      "1.1.0",
      "2.0.0",
      "2.0.1",
      "2.1.0",
      "2.1.1",
      "2.2.0",
      "2.3.0",
      "3.0.0",
      "4.0.0",
      "4.1.0"
    ],
    "database_specific": {
      "source": "https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/09/GHSA-8v8h-hg4w-mvq2/GHSA-8v8h-hg4w-mvq2.json",
      "last_known_affected_version_range": "<= 4.1.0"
    }
  }
]
Related CVEs
CVE-2026-59980

Source reference: /aliases

Publisher severity

No explicit severity was provided.

Linked CVE context

These ratings describe the referenced vulnerabilities and products. They are not a severity rating for every application using this dependency.

CVE-2026-59980 · Medium

hpack

Event
#62521 · Revision 1
Added to Heist Feed
Publisher date
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