CodeQL documentation

Incomplete string escaping or encoding

ID: rb/incomplete-sanitization
Kind: problem
Security severity: 7.8
Severity: warning
Precision: high
   - 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 such as SQL injection or cross-site scripting. Usually, this is done by escaping meta-characters such as quotes in a domain-specific way so that they are treated as normal characters.

However, directly using the String#sub method to perform escaping is notoriously error-prone. Common mistakes include only replacing the first occurrence of a meta-character, or backslash-escaping various meta-characters but not the backslash itself.

In the former case, later meta-characters are left undisturbed and can be used to subvert the sanitization. In the latter case, preceding a meta-character with a backslash leads to the backslash being escaped, but the meta-character appearing un-escaped, which again makes the sanitization ineffective.

Even if the escaped string is not used in a security-critical context, incomplete escaping may still have undesirable effects, such as badly rendered or confusing output.


Use a (well-tested) sanitization library if at all possible. These libraries are much more likely to handle corner cases correctly than a custom implementation.

An even safer alternative is to design the application so that sanitization is not needed. Otherwise, make sure to use String#gsub rather than String#sub, to ensure that all occurrences are replaced, and remember to escape backslashes if applicable.


As an example, assume that we want to embed a user-controlled string account_number into a SQL query as part of a string literal. To avoid SQL injection, we need to ensure that the string does not contain un-escaped single-quote characters. The following method attempts to ensure this by doubling single quotes, and thereby escaping them:

def escape_quotes(s)
  s.sub "'", "''"

As written, this sanitizer is ineffective: String#sub will replace only the first occurrence of that string.

As mentioned above, the method escape_quotes should be replaced with a purpose-built sanitizer, such as ActiveRecord::Base::sanitize_sql in Rails, or by using ORM methods that automatically sanitize parameters.

If this is not an option, escape_quotes should be rewritten to use the String#gsub method instead:

def escape_quotes(s)
  s.gsub "'", "''"


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