Security Policy • Zero Configuration Security Policy • Other Security Considerations

ImageMagick includes a security policy configuration file, policy.xml. It is useful for limiting the resources consumed by ImageMagick and can help prevent a denial-of-service or other exploits.

As an example, suppose you download an image from the internet and unbeknownst to you its been crafted to generate a 20000 by 20000 pixel image. ImageMagick attempts to allocate enough resources (memory, disk) and your system will likely deny the resource request and exit. However, its also possible that your computer might be temporarily sluggish or unavailable or ImageMagick may abort. To prevent such a scenario, you can set limits in the `policy.xml` configuration file. You may ask why ImageMagick does not already include reasonable limits? Simply because what is reasonable in your environment, might not be reasonable to someone else. For example, you may have ImageMagick sandboxed where security is not a concern, whereas another user may use ImageMagick to process images on their publically accessible website. Or ImageMagick is running on a host with 1TB of memory whereas another ImageMagick instance is running on an iPhone. By policy, permitting giga-pixel image processing on the large memory host makes sense, not so much for the resource constrained iPhone. If you utilize ImageMagick from a public website, you may want to increase security by preventing usage of the MVG or HTTPS coders. Only you can decide what are reasonable limits taking in consideration your environment. We provide this policy with reasonable limits and encourage you to modify it to suit your local environment:

    <policymap>
      <policy domain="resource" name="temporary-path" value="/tmp"/>
      <policy domain="resource" name="memory" value="256MiB"/>
      <policy domain="resource" name="map" value="512MiB"/>
      <policy domain="resource" name="width" value="8KP"/>
      <policy domain="resource" name="height" value="8KP"/>
      <policy domain="resource" name="area" value="16KP"/>
      <policy domain="resource" name="disk" value="1GiB"/>
      <policy domain="resource" name="file" value="768"/>
      <policy domain="resource" name="thread" value="2"/>
      <policy domain="resource" name="throttle" value="0"/>
      <policy domain="resource" name="time" value="120"/>
      <policy domain="system" name="precision" value="6"/>
      <policy domain="cache" name="shared-secret" stealth="true" value="replace with your secret phrase"/>
      <policy domain="coder" rights="none" pattern="MVG" />
      <policy domain="delegate" rights="none" pattern="HTTPS" />
      <policy domain="path" rights="none" pattern="@*"/>
    </policymap>

Since we process multiple simultaneous sessions, we do not want any one session consuming all the available memory.With this policy, large images are cached to disk. If the image is too large and exceeds the pixel cache disk limit, the program exits. In addition, we place a time limit to prevent any run-away processing tasks. If any one image has a width or height that exceeds 8192 pixels, an exception is thrown and processing stops. As of ImageMagick 7.0.1-8 and 6.9.4-6, you can prevent the use of any delegate or all delegates (set the pattern to "*"). Note, prior to these releases, use a domain of `coder` to prevent delegate usage (e.g. `domain="coder" rights="none" pattern="HTTPS"`). The policy also prevents indirect reads. If you want to, for example, read text from a file (e.g. `caption:@myCaption.txt`), you'll need to disable the `path` policy.

Here is what you can expect when you restrict the HTTPS coder, for example:

    $ convert https://www.imagemagick.org/images/wizard.png wizard.jpg
    convert: not authorized `HTTPS'
    convert: unable to open file: No such file or directory
    convert: no images defined `wizard.jpg'

As of ImageMagick version 7.0.4-7, you can conveniently deny access to all delegates and coders except for a small subset of proven web-safe image types. For example,

      <policy domain="delegate" rights="none" pattern="*" />
      <policy domain="coder" rights="none" pattern="*" />
      <policy domain="coder" rights="read | write" pattern="{GIF,JPEG,PNG,WEBP}" />

As of ImageMagick 7.0.5-5, you can allocate the pixel cache and some internal buffers with anonymous memory mapping rather than from heap. As a consequence, the pixels are initialized to zero. You can also securely delete any temporary files for increased security. The value is the number of times to shred (replace its content with random data before deleting) a temporary file. For example,

      <policy domain="system" name="memory-map" value="anonymous"/>
      <policy domain="cache" name="memory-map" value="anonymous"/>
      <policy domain="system" name="shred" value="1"/>

You can verify your policy changes are in effect with this command:

    -> identify -list policy
    Path: ImageMagick/policy.xml
      Policy: Resource
        name: time
        value: 120
      Policy: Resource
        name: throttle
        value: 0
      Policy: Resource
        name: thread
        value: 2
      Policy: Resource
        name: file
        value: 768
      Policy: Resource
        name: disk
        value: 1GiB
      Policy: Resource
        name: map
        value: 512MiB
      Policy: Resource
        name: memory
        value: 256MiB
      Policy: Resource
        name: area
        value: 16KP
      Policy: Resource
        name: height
        value: 8KP
      Policy: Resource
        name: width
        value: 8KP
      Policy: Resource
        name: temporary-path
        value: /tmp
      Policy: System
        name: precision
        value: 6
      Policy: Path
        rights: None
        pattern: @*

    Path: [built-in]
      Policy: Undefined
        rights: None

Notice the `Cache` policy is not listed due to the `stealth` property.

For additional details about resource limits and the policy configuration file, read Resources and Architecture.

## Zero Configuration Security Policy

A zero configuration build of ImageMagick does not permit external configuration files. To define your security policy, you must instead edit the `MagickCore/policy-private.h` source module, add your policy statements, and then build the ImageMagick distribution. Here is an example zero configuration security policy:

    static const char
      *ZeroConfigurationPolicy = \
    "<policymap> \
      <policy domain=\"coder\" rights=\"none\" pattern=\"MVG\"/> \
    </policymap>";

## Other Security Considerations

If you spot a security flaw in ImageMagick, contact us and select Security Issue as the issue. Alternatively, post your concern to GitHub. Be sure to include how to reproduce the security flaw and a link to any images needed to reproduce the flaw.

In addition to the security policy, you can make ImageMagick safer by ...

- keeping ImageMagick up-to-date. The latest releases have fixes for any security flaws we discovered in the past.
- sanitizing any filenames or command line options you pass to ImageMagick.
- running ImageMagick in a sanitized software container such as Docker.
- running ImageMagick as the least-privileged user (e.g. 'nobody').
- explicitly setting the image file type. For example, use the filename `png:image.png` rather than `image.png`. Without an explicit image type in the filename, ImageMagick guesses the image type.