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.

CVSS 4.0

6.3CVSS: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

0.3% chance of exploitation in the next 30 days; higher than 22% of CVEs (scores of 2026-10-25)

CWE

CWE-400

Published

2026-09-23

Modified

2026-09-30

Record from

the NVD, synced 2026-10-19

Packages (OSV)

References

• 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/commit/8cfb02c547740e16dbfe7aba77bad84b297cec2c
• https://github.com/python-hyper/hpack/security/advisories/GHSA-8v8h-hg4w-mvq2

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