No exploitation reported

CVE-2016-6581

No exploitation reported by the sources we read.

Published2017-01-10
WeaknessCWE-399

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

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.

Timeline

dates in UTC

2017-01-1015:59CVE publishedNVD

Observed activity

isMalicious corpus · updated 2026-10-23 18:48 UTC

Indicators tagged with this CVE, per day0 indicators over 7 days

Block these indicators

Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2016-6581?format=txt
CSV export: https://api.ismalicious.com/edl/cve/CVE-2016-6581?format=csv
TAXII collection of exploited CVEs
The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD

Python hpack

Branch	Affected	Fixed in	Source
1.0	= 1.0	no fix published	NVD
2.0	= 2.0	no fix published	NVD
2.0	= 2.0.1	no fix published	NVD
2.1	= 2.1.1	no fix published	NVD
2.2	= 2.2	no fix published	NVD

Python hyper

Branch	Affected	Fixed in	Source
0.4	= 0.4	no fix published	NVD
0.6	= 0.6	no fix published	NVD

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Fix

Advisories and patches

python-hyper.orgMitigation · Vendor Advisory

Detection

Nuclei template: none so far
Exploit-DB: no entry
Proof-of-concept repositories: none listed

Sources

NVDCVSS 3.07.5read 2026-10-23
NVDCVSS 2.07.8read 2026-10-23
NVDweakness (CWE)CWE-399read 2026-10-23
FIRST EPSSEPSS score1.8 %2026-10-22

Technical details

CVSS v3 Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Published: 2017-01-10
Last Modified: 2026-06-17
NVD status: Modified

Frequently asked questions

What is CVE-2016-6581?

Is CVE-2016-6581 actively exploited?

Active exploitation of CVE-2016-6581 has not been confirmed. Its EPSS score was 1.8% on 2026-10-22, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2016-6581?

CVE-2016-6581 has a CVSS v3 base score of 7.5 (HIGH severity), with vector string CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.

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