When HTML is sanitized using Sanitize's "relaxed" config or a custom config that allows certain elements, some content in a <math> or <svg> element may not be sanitized correctly even if math and svg are not in the allowlist.
You are likely to be vulnerable to this issue if you use Sanitize's relaxed config or a custom config that allows one or more of the following HTML elements:
iframemathnoembednoframesnoscriptplaintextscriptstylesvgxmp
Impact
Using carefully crafted input, an attacker may be able to sneak arbitrary HTML through Sanitize, potentially resulting in XSS (cross-site scripting) or other undesired behavior when that HTML is rendered in a browser.
Releases
This problem has been fixed in Sanitize 5.2.1.
Workarounds
If upgrading is not possible, a workaround is to override the default value of Sanitize's :remove_contents config option with the following value, which ensures that the contents of math and svg elements (among others) are removed entirely when those elements are not in the allowlist:
%w[iframe math noembed noframes noscript plaintext script style svg xmp]
For example, if you currently use Sanitize's relaxed config, you can create a custom config object that overrides the default value of :remove_contents like this:
custom_config = Sanitize::Config.merge(
Sanitize::Config::RELAXED,
:remove_contents => %w[iframe math noembed noframes noscript plaintext script style svg xmp]
)
You would then pass this custom config to Sanitize when sanitizing HTML.
For more information
If you have any questions or comments about this advisory:
Credits
Many thanks to Michał Bentkowski of Securitum for reporting this bug and helping to verify the fix.
References
When HTML is sanitized using Sanitize's "relaxed" config or a custom config that allows certain elements, some content in a
<math>or<svg>element may not be sanitized correctly even ifmathandsvgare not in the allowlist.You are likely to be vulnerable to this issue if you use Sanitize's relaxed config or a custom config that allows one or more of the following HTML elements:
iframemathnoembednoframesnoscriptplaintextscriptstylesvgxmpImpact
Using carefully crafted input, an attacker may be able to sneak arbitrary HTML through Sanitize, potentially resulting in XSS (cross-site scripting) or other undesired behavior when that HTML is rendered in a browser.
Releases
This problem has been fixed in Sanitize 5.2.1.
Workarounds
If upgrading is not possible, a workaround is to override the default value of Sanitize's
:remove_contentsconfig option with the following value, which ensures that the contents ofmathandsvgelements (among others) are removed entirely when those elements are not in the allowlist:For example, if you currently use Sanitize's relaxed config, you can create a custom config object that overrides the default value of
:remove_contentslike this:You would then pass this custom config to Sanitize when sanitizing HTML.
For more information
If you have any questions or comments about this advisory:
Credits
Many thanks to Michał Bentkowski of Securitum for reporting this bug and helping to verify the fix.
References