CVE-2016-6581: Python Hpack

High severity, CVSS 7.5. EPSS: 1.8% chance of exploitation in the next 30 days.

A HTTP/2 implementation built using any version of the Python HPACK library between v1.0.0 and v2.2.0 could be targeted for a denial of service attack, specifically a so-called "HPACK Bomb" attack. This attack occurs when an attacker inserts a header field that is exactly the size of the HPACK dynamic header table into the dynamic header table. The attacker can then send a header block that is simply repeated requests to expand that field in the dynamic table. This can lead to a gigantic compression ratio of 4,096 or better, meaning that 16kB of data can decompress to 64MB of data on the target machine.

Affected products

• Python Hpack: version 1.0 only; version 2.0 only; version 2.0.1 only; version 2.1.1 only; version 2.2 only
• Python Hyper: version 0.4 only; version 0.6 only

Published 2017-01-10. Last modified 2026-06-17.