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.

References

• https://nvd.nist.gov/vuln/detail/CVE-2016-6581
• GHSA-ffq8-576r-v26g
• http://www.securityfocus.com/bid/92315
• http://python-hyper.org/projects/hpack/en/stable/security/CVE-2016-6581.html