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HpackApplication · Python

CVE-2016-6581

HIGH · 7.5 CVSS v3.0 Published 2017-01-10
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
84/100
Remediation priority · High
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

The Python HPACK library versions 1.0.0 through 2.2.0 contain a vulnerability in their HTTP/2 implementation that allows an HPACK Bomb attack. Attackers insert a header field sized exactly to the dynamic header table, then send repeated header blocks that expand it, achieving a 4,096:1 compression ratio (16KB decompresses to 64MB), causing memory exhaustion and denial of service.

MitigationUpgrade the Python HPACK library to version 2.2.1 or later, which contains bounds checking to limit dynamic table expansion. Alternatively, implement application-layer limits on header decompression sizes.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
HpackApplication
Affected:= 1.0= 2.0= 2.0.1= 2.1.1= 2.2
HyperApplication
Affected:= 0.4= 0.6

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check installed HPACK library version
    Run 'pip show python-hpack' or import hpmm; print(hpmm.__version__) to determine the installed version
    Affected if Version is 1.0.0, 2.0, 2.0.1, 2.1.1, or 2.2 (any version from 1.0.0 through 2.2.0 is affected)
  2. Check installed Hyper library version
    Run 'pip show hyper' or import hyper; print(hyper.__version__) to determine the installed version
    Affected if Version is 0.4 or 0.6 (these versions bundle the vulnerable HPACK library)
  3. Confirm HTTP/2 usage in the application
    Inspect application code and configuration for HTTP/2 connections (look for hyper.http20, h2, or other HTTP/2 client/server implementations)
    Affected if The application uses HTTP/2 protocol with the affected HPACK library for header compression
  4. Check dynamic header table configuration
    Inspect any HPACK-related configuration files or code that sets dynamic table size limits (search for 'max_table_size' or similar settings)
    Affected if No bounds checking is configured on dynamic table size, allowing unlimited expansion
  5. Monitor memory usage during HTTP/2 traffic
    Observe application memory consumption while processing HTTP/2 requests with large header blocks
    Affected if Memory grows excessively (to 64MB or more) when receiving relatively small compressed header blocks, indicating HPACK bomb vulnerability

The environment is affected if either the python-hpack library (versions 1.0.0-2.2.0) or hyper library (versions 0.4 or 0.6) is installed and HTTP/2 with HPACK header compression is in use without custom memory limits.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

From vendor data
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Upgrade the Python HPACK library to version 2.2.1 or later, which contains bounds checking to limit dynamic table expansion. Alternatively, implement application-layer limits on header decompression sizes.

Fix this in Hpack Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing6.0 h
  • Review / QA2.0 h
14.0 hours of engineering $2,380
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2016-6581 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
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  • Version or environment caveats, and links to real fixes
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