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Incomplete multi-character sanitization

ID: rb/incomplete-multi-character-sanitization
Kind: problem
Security severity: 7.8
Severity: warning
Precision: high
Tags:
   - correctness
   - security
   - external/cwe/cwe-020
   - external/cwe/cwe-080
   - external/cwe/cwe-116
Query suites:
   - ruby-code-scanning.qls
   - ruby-security-extended.qls
   - ruby-security-and-quality.qls

Click to see the query in the CodeQL repository

Sanitizing untrusted input is a common technique for preventing injection attacks and other security vulnerabilities. Regular expressions are often used to perform this sanitization. However, when the regular expression matches multiple consecutive characters, replacing it just once can result in the unsafe text re-appearing in the sanitized input.

Attackers can exploit this issue by crafting inputs that, when sanitized with an ineffective regular expression, still contain malicious code or content. This can lead to code execution, data exposure, or other vulnerabilities.

Recommendation

To prevent this issue, it is highly recommended to use a well-tested sanitization library whenever possible. These libraries are more likely to handle corner cases and ensure effective sanitization.

If a library is not an option, you can consider alternative strategies to fix the issue. For example, applying the regular expression replacement repeatedly until no more replacements can be performed, or rewriting the regular expression to match single characters instead of the entire unsafe text.

Example

Consider the following Ruby code that aims to remove all HTML comment start and end tags:

str.gsub(/<!--|--!?>/, "")

Given the input string “<!<!— comment —>>”, the output will be “<!– comment –>”, which still contains an HTML comment.

One possible fix for this issue is to apply the regular expression replacement repeatedly until no more replacements can be performed. This ensures that the unsafe text does not re-appear in the sanitized input, effectively removing all instances of the targeted pattern:

def remove_html_comments(input)  
  previous = nil  
  while input != previous  
    previous = input  
    input = input.gsub(/<!--|--!?>/, "")  
  end  
  input  
end

Example

Another example is the following regular expression intended to remove script tags:

str.gsub(/<script\b[^<]*(?:(?!<\/script>)<[^<]*)*<\/script>/, "")

If the input string is “<scrip<script>is removed</script>t>alert(123)</script>”, the output will be “<script>alert(123)</script>”, which still contains a script tag.

A fix for this issue is to rewrite the regular expression to match single characters (”<” and “>”) instead of the entire unsafe text. This simplifies the sanitization process and ensures that all potentially unsafe characters are removed:

def remove_all_html_tags(input)  
  input.gsub(/<|>/, "")  
end

Another potential fix is to use the popular sanitize gem. It keeps most of the safe HTML tags while removing all unsafe tags and attributes.

require 'sanitize'

def sanitize_html(input)
  Sanitize.fragment(input)
end

Example

Lastly, consider a path sanitizer using the regular expression /\.\.\//:

str.gsub(/\.\.\//, "")

The regular expression attempts to strip out all occurences of /../ from str. This will not work as expected: for the string /./.././, for example, it will remove the single occurrence of /../ in the middle, but the remainder of the string then becomes /../, which is another instance of the substring we were trying to remove.

A possible fix for this issue is to use the File.sanitize function from the Ruby Facets gem. This library is specifically designed to handle path sanitization, and should handle all corner cases and ensure effective sanitization:

require 'facets'  
  
def sanitize_path(input)  
  File.sanitize(input)  
end

References

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