What’s Vulnerable › Vulnerabilities

CVE-2016-6581

• Severity: high (CVSS 7.5 v3.0)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.8% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2017-01-10 · updated 2026-06-17
• Weakness: CWE-399

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.

Affected products

• Python Hpack: 1.0, 2.0, 2.0.1, 2.1.1, 2.2
• Python Hyper: 0.4, 0.6

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

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