{"id":"PYSEC-2026-4053","summary":"hpack: Unbounded variable integer decoding can cause run-away computation on malformed input","details":"### Impact\n\nUsers 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.\n\n### Patches\nA fix is available in python-hyper/hpack v4.2.0 to restricted variable integer decoding to uint32 to prevent run-away computation.\n\n### Workarounds\nSanitize input to hpack decoder for long sequences of `0xFF` values to prevent malicious use.","aliases":["CVE-2026-59980","GHSA-8v8h-hg4w-mvq2"],"modified":"2026-10-01T17:45:03.832893385Z","published":"2026-10-01T16:38:39.106369Z","references":[{"type":"WEB","url":"https://github.com/python-hyper/hpack/security/advisories/GHSA-8v8h-hg4w-mvq2"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-59980"},{"type":"WEB","url":"https://github.com/python-hyper/hpack/commit/8cfb02c547740e16dbfe7aba77bad84b297cec2c"},{"type":"WEB","url":"https://cs.opensource.google/go/x/net/+/master:http2/hpack/hpack.go;l=468;drc=8e2b117aee74f6b86c207a808b0255de45c0a18a"},{"type":"WEB","url":"https://docs.rs/http2/0.5.19/src/http2/hpack/decoder.rs.html#403"},{"type":"PACKAGE","url":"https://github.com/python-hyper/hpack"},{"type":"WEB","url":"https://github.com/python-hyper/hpack/releases/tag/v4.2.0"},{"type":"PACKAGE","url":"https://pypi.org/project/hpack"},{"type":"ADVISORY","url":"https://github.com/advisories/GHSA-8v8h-hg4w-mvq2"}],"affected":[{"package":{"name":"hpack","ecosystem":"PyPI","purl":"pkg:pypi/hpack"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"4.2.0"}]}],"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/pypa/advisory-database/blob/main/vulns/hpack/PYSEC-2026-4053.yaml"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V4","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"}]}