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Date: Thu, 4 Aug 2016 10:16:53 +0100
From: Cory Benfield <>
Subject: CVE-2016-6580, Python Priority: DoS via Unlimited Stream Insertion

DoS via Unlimited Stream Insertion

Hyper Project security advisory, August 4th 2016.


A HTTP/2 implementation built using the priority library could be targetted by
a malicious peer by having that peer assign priority information for every
possible HTTP/2 stream ID. The priority tree would happily continue to store
the priority information for each stream, and would therefore allocate
unbounded amounts of memory. Attempting to actually *use* a tree like this
would also cause extremely high CPU usage to maintain the tree.

We are not aware of any active exploits of this vulnerability, but as this
class of attack was publicly described in this report[1], users should assume
that they are at imminent risk of this kind of attack.


This issue has been given the name CVE-2016-6580.

Affected Versions

This issue affects all versions of the priority library prior to 1.2.0.

The Solution

In version 1.2.0, the priority library limits the maximum number of streams
that can be inserted into the tree. By default this limit is 1000, but it is

If it is necessary to backport a patch, the patch can be found in
this GitHub pull request[2].


We suggest you take the following actions immediately, in order of preference:

1. Update priority to 1.2.0 immediately, and consider revising the maximum
   number of streams downward to a suitable value for your application.
2. Backport the patch made available on GitHub.
3. Manually enforce a limit on the number of priority settings you'll allow at


This class of vulnerability was publicly reported in this report[1] on the
3rd of August. We requested a CVE ID from Mitre the same day.

Priority 1.2.0 was released on the 4th of August, at the same time as the
publication of this advisory.


Cory Benfield, on behalf of the Python Hyper project.


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