CVE Catalog

CVE-2026-59980

MediumCVSS 6.3
Published: Updated: Translated: NVD NIST

Exploitation Probability (EPSS)

Low risk
0.30%

21th percentile - higher than 21% of all known CVEs

Summary

The hpack library (HTTP/2 header encoding for Python) prior to version 4.2.0 does not bound variable-length integer decoding. Malformed input can trigger runaway computation with O(n^2) runtime, blocking further processing. The fix in version 4.2.0 restricts decoding to uint32.

Risk Assessment

An attacker can send crafted input to the hpack decoder and cause CPU exhaustion and denial of service in services processing HTTP/2 traffic.

Recommendation

Upgrade hpack to version 4.2.0 or later. As a workaround, sanitize input and reject long sequences of 0xFF bytes before passing them to the decoder.

Related vulnerabilities

Original NVD description (English source)

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.

Vulnerability data from NVD (NIST) · CISA KEV · EPSS