Summary
hpack: Unbounded variable integer decoding can cause run-away computation on malformed input
Advisory details
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.
References
- https://github.com/advisories/GHSA-8v8h-hg4w-mvq2
- https://github.com/python-hyper/hpack/security/advisories/GHSA-8v8h-hg4w-mvq2
- https://nvd.nist.gov/vuln/detail/CVE-2026-59980
- https://github.com/python-hyper/hpack/commit/8cfb02c547740e16dbfe7aba77bad84b297cec2c
- https://cs.opensource.google/go/x/net/+/master:http2/hpack/hpack.go;l=468;drc=8e2b117aee74f6b86c207a808b0255de45c0a18a
- https://docs.rs/http2/0.5.19/src/http2/hpack/decoder.rs.html#403
- https://github.com/python-hyper/hpack/releases/tag/v4.2.0
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