CVE-2016-6581 json
HPACK Denial of Service vulnerability (HPACK Bomb)
published 2017-01-10 9 y ago modified 2026-06-17 4 mo ago also GHSA-ffq8-576r-v26g , PYSEC-2017-87 , EUVD-2017-0059
- cvss
- 7.5
- version 3.0, NVD
- cisa kev
- Not listed
- not in CISA's catalog
- epss
- 1.9%
- EPSS 1.9% chance of exploitation in 30 days, percentile 79%
- public exploits
- 0
- none known
Summary
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.
Patch / fix
- pypi hpack 2.3.0
- python-hpack Debian 14, Debian 13, Debian 12 2.3.0-1
debian advisories DEBIAN-CVE-2016-6581
- python-hpack openSUSE Tumbleweed 4.2.0-1.1
- python-hpack openSUSE Tumbleweed 4.0.0-2.7
opensuse advisories openSUSE-SU-2026:11556-1openSUSE-SU-2024:14140-1
Exploit prediction
1.88% chance of exploitation in the next 30 days, higher than 79% of all scored CVEs (FIRST EPSS, 2026-11-08).
CVSS scores
| assigned by | version | score | vector |
|---|---|---|---|
| NVD | 3.0 | 7.5 | CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H |
| NVD | 2.0 | 7.8 | AV:N/AC:L/Au:N/C:N/I:N/A:C |
| GitHub | 3.1 | 7.5 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H |
Affected packages
| ecosystem | package | affected | fixed in |
|---|---|---|---|
| pypi | hpack | >=1.0.0, <2.3.0 <2.3.0 |
2.3.0 |
Classification
- Weakness
-
- CWE-399 Resource Management Errors
References
-
python-hyper.org
/hpack/en/latest/security/CVE-2016-6581.html
web, mitigation, vendor-advisory
-
securityfocus.com
/bid/92315
vdb-entry, third-party-advisory, web
-
nvd.nist.gov
/vuln/detail/CVE-2016-6581
advisory
-
github.com
/advisories/GHSA-ffq8-576r-v26g
advisory
-
github.com
/pypa/advisory-database/tree/main/vulns/hpack/PYSEC-2017-87.yaml
web
-
github.com
/python-hyper/hpack
package
-
web.archive.org
/web/20200227233559/http://www.securityfocus.com/bid/92315
web
-
python-hyper.org
/projects/hpack/en/stable/security/CVE-2016-6581.html
web
Sources
- CVE List CVE-2016-6581
- GitHub Advisory GHSA-ffq8-576r-v26g
- NVD CVE-2016-6581
- OSV PYSEC-2017-87
Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.