{"cve":"CVE-2016-6581","nvd":{"published":"2017-01-10T15:59:00.423","modified":"2026-06-17T00:51:24.253","status":"Modified","description":"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.","cvss_v4":null,"cvss_v3":{"version":"3.0","vector":"CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","score":7.5},"cwes":["CWE-399"],"references":["http://www.securityfocus.com/bid/92315","https://python-hyper.org/hpack/en/latest/security/CVE-2016-6581.html","http://www.securityfocus.com/bid/92315","https://python-hyper.org/hpack/en/latest/security/CVE-2016-6581.html"],"products":["python:hpack","python:hyper"],"source":"nvd","synced_at":"2026-10-19T20:41:25.729257544+00:00"},"epss":{"cve":"CVE-2016-6581","score":0.01877,"percentile":0.7942,"date":"2026-10-31"},"kev":null,"osv":[{"id":"GHSA-ffq8-576r-v26g","url":"https://osv.dev/vulnerability/GHSA-ffq8-576r-v26g","summary":"HPACK Denial of Service vulnerability (HPACK Bomb)","modified":"2024-11-30T05:39:01.605415Z","packages":[{"ecosystem":"PyPI","name":"hpack","purl":"pkg:pypi/hpack","ranges":[">=1.0.0 <2.3.0"],"fixed":["2.3.0"]}]},{"id":"PYSEC-2017-87","url":"https://osv.dev/vulnerability/PYSEC-2017-87","summary":"","modified":"2023-11-08T03:58:32.800713Z","packages":[{"ecosystem":"PyPI","name":"hpack","purl":"pkg:pypi/hpack","ranges":["<2.3.0"],"fixed":["2.3.0"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}