GHSA-f3w6-w445-fwh9: 
Ruby vulnerability analysis and mitigation

Impact

A Rails application that processes attachments with ImageMagick may allow an unauthenticated attacker to write arbitrary files on the server and, when attachments are stored with the Disk service, to read arbitrary files readable by the application process. Those files typically include secret_key_base and credentials for external systems, which may in turn allow escalation to remote code execution or lateral movement to those systems.

Details

ImageMagick selects a coder from the content of the file it is given, and some coders act on files named inside the image rather than on the image itself. Active Storage constrained none of ImageMagick's coders, and the ImageMagick analyzer runs when a file is attached, so an attacker who can upload a crafted file may be able to invoke one. Attaching the upload is sufficient; no variant, preview, or analyze call is required. One specific attack chain has been reported to us, but we do not assume it is the only one that exists. The fixed versions ship an ImageMagick security policy and prepend its directory to MAGICK_CONFIGURE_PATH through MiniMagick.cli_env, so every ImageMagick process that MiniMagick spawns applies it. The policy denies the MSL scripting coder, the coders that read or delete files other than the image, fetch remote or local references, or display to a screen, every coder backed by Ghostscript together with the gs delegate, and @filename indirection. ImageMagick applies every policy.xml on its search path, so the shipped policy adds to the host's policy rather than replacing it. The policy applies to every ImageMagick command that MiniMagick runs, not only those of Active Storage. See the activestorage CHANGELOG and "ImageMagick Security Policy" in the Active Storage guide.

Affected applications

An application is affected if it meets all of these requirements:

• Uses ImageMagick for Active Storage image processing. This is config.active_storage.variant_processor = :mini_magick, which is the default for applications on load_defaults 6.1 or earlier.
• Runs ImageMagick under a security policy that does not already deny the coders involved. The policies that Debian and Ubuntu package do not.
• Allows file uploads from untrusted users.

Mitigation

• Upgrade to a fixed version of activestorage.
• An application whose variant processor is :mini_magick must use mini_magick >= 5.1, which applying the policy requires. With an older mini_magick, such an application raises an exception during boot.
• An application that renders PostScript or PDF through ImageMagick must re-allow the gs delegate and the coders it uses in the host's policy.xml.
• An application that replaces MiniMagick.cli_env after boot, clears the environment with MiniMagick.cli_prefix, or installs its own MiniMagick::Shell does not receive the shipped policy and must carry its rules in ImageMagick's system policy. GraphicsMagick does not read ImageMagick policies.
• If attachments are stored with the Disk service, change secret_key_base and every secret readable by the application process, including the master key and everything config/credentials.yml.enc holds, storage and database credentials, and third-party tokens. Do not retain an exposed secret as a fallback.

Workarounds

Copy the rules of the policy that the fixed versions ship, activestorage/lib/active_storage/image_magick/policy.xml, into ImageMagick's system policy, for example /etc/ImageMagick-7/policy.xml. At a minimum, deny the MSL coder, the Ghostscript coders, and the gs delegate:

<policy domain="coder" rights="none" pattern="MSL"/>
<policy domain="delegate" rights="none" pattern="gs"/>
<policy domain="coder" rights="none" pattern="{PS,PS2,PS3,EPS,EPS2,EPS3,EPI,EPSF,EPSI,EPT,EPT2,EPT3,PDF,AI,EPDF,PDFA,POCKETMOD,XPS,PCL}"/>

Releases

The fixed releases are available at the normal locations. Patches for those who cannot upgrade at once:

• 8.1: https://github.com/rails/rails/commit/468aa9a93fc98f5476b6821770f4ccfcb7f7d328
• 8.0: https://github.com/rails/rails/commit/ea8c40121ce53a3d0255cd4eb6d13bfe3bc01776

Versions affected

• activestorage < 8.0.5.2
• activestorage >= 8.1, < 8.1.4.1

Credit

This issue was responsibly reported by HackerOne researcher ryotak.

References

• ImageMagick security policy