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Fix ReDoS Vulnerability in PermitScrubber and Add Performance Test #186

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Fix ReDoS Vulnerability in PermitScrubber and Add Performance Test #186

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26a34b1
Fix ReDoS vulnerability in PermitScrubber by optimizing regex
ch4n3-yoon Aug 13, 2024
f461d2a
Add linear performance test to verify ReDoS mitigation in PermitScrubber
ch4n3-yoon Aug 13, 2024
e1c1537
Fix: Add missing require 'benchmark' to scrubbers_test.rb
ch4n3-yoon Aug 13, 2024
713241f
Revert "Add linear performance test to verify ReDoS mitigation in Per…
ch4n3-yoon Aug 14, 2024
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2 changes: 1 addition & 1 deletion lib/rails/html/scrubbers.rb
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Original file line number Diff line number Diff line change
Expand Up @@ -150,7 +150,7 @@ def scrub_attribute(node, attr_node)
Loofah::HTML5::Scrub.scrub_attribute_that_allows_local_ref(attr_node)
end

if Loofah::HTML5::SafeList::SVG_ALLOW_LOCAL_HREF.include?(node.name) && attr_name == "xlink:href" && attr_node.value =~ /^\s*[^#\s].*/m
if Loofah::HTML5::SafeList::SVG_ALLOW_LOCAL_HREF.include?(node.name) && attr_name == "xlink:href" && attr_node.value =~ /^\s*[^#].*/m
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I think the meaning of the regexp is changed. It began to match to any character that starts with /\s/.

I'm not sure what the original regexp want to match to. maybe:

  • /^[^\S\n]*[^#\s]/: nearly or identical to the original regexp
  • /\A\s*[^#\s]/: matches to "\n\n a..." but doesn't match to " #\n #\n a"

or perhaps using attr_node.value !~ allow_pattern would be more simple?

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This regular expression originally came from the html5lib library:

https://github.com/html5lib/html5lib-ruby/blob/master/lib/html5/sanitizer.rb#L141-L143

The goal of this line is to accept "local links" (like #some-id) but not full URLs, with initial whitespace being ignored. There are test cases for this behavior in Loofah, which rails-html-sanitizer originally copied this code from.

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@flavorjones flavorjones Aug 20, 2024 •
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So @tompng is correct, this patch changes the behavior in unwanted ways. One example test case is:

<animatemotion xlink:href='\n#foo'>/</animatemotion>

Specifically, in this case the xlink:href attribute should not be removed but is being removed because:

val = "\n#foo"
# => "\n#foo"

val =~ /^\s*[^#\s].*/
# => nil

val =~ /^\s*[^#].*/
# => 0

attr_node.remove
end

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1 change: 1 addition & 0 deletions test/scrubbers_test.rb
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@@ -1,5 +1,6 @@
# frozen_string_literal: true

require "benchmark"
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Please remove this line, it's no longer necessary.

require "minitest/autorun"
require "rails-html-sanitizer"

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