MSC35-J. Understand how escape characters are interpreted when String literals are compiled

Many classes, including Pattern and those that support XML and SQL based actions by passing String arguments to methods, allow inclusion of escape sequences in character and string literals. According to [[JLS 05]] section 3.10.6 "Escape Sequences for Character and String Literals":

The character and string escape sequences allow for the representation of some nongraphic characters as well as the single quote, double quote, and backslash characters in character literals (§3.10.4) and string literals (§3.10.5).

In order to correctly use escape sequences pertaining to String literals, an understanding of how they are interpreted is essential. For example, SQL statements written in Java, sometimes require certain special escape characters or sequences (for instance, sequences containing \t, \n, \r). In SQL queries, all escape sequences must be preceded by an extra backslash for correct interpretation.

As yet another example, consider the Pattern class that finds extensive use in performing regular expression related tasks. In Java, a given String literal used for pattern matching is compiled into an instance of the Pattern type. If the pattern to be matched contains an undesired escape sequence such as a '\n', to avoid it being interpreted by the Java bytecode compiler as an escape sequence, the Pattern class requires the literal to be preceded by a backslash ('[]\n', which now correctly denotes back references instead of a new line).

In general, for a particular escape character of the form '\X', the equivalent Java representation is "[]\X". As an aside, this particular condition gains remarkable importance in automatic exploit signature detection systems and filters that rely on patter matching.

Noncompliant Code Example

In the following example, a method splitWords() finds matches between the String literal and the input sequence. Since '\b' is the escape sequence for a word boundary, the misleading notion that String literals can be used as is, can convince the implementer that the pattern matches to word boundaries and thus splits a given string into individual words. Instead, the string WORDS silently compiles to a backspace character.

import java.util.regex.Pattern;

public class BadSplitter {
  private final String WORDS = "\b"; // Intend to split on word boundaries

  public String[] splitWords(String input){
    Pattern p = Pattern.compile(WORDS);
    String[] input_array = p.split(input);
    return input_array;
  }
}

Compliant Solution

This compliant solution shows the correctly escaped value of the String literal WORDS that results in a regular expression designed to split on word boundaries.

import java.util.regex.Pattern;

public class GoodSplitter {
  private final String WORDS = "\\b"; // Will allow splitting on word boundaries

  public String[] split(String input){
    Pattern p = Pattern.compile(WORDS);
    String[] input_array = p.split(input);
    return input_array;
  }
}

Risk Assessment

Incorrect usage of escape characters in String literals can result in misinterpretation and potential corruption of data.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[JLS 05]] 3.10.6 Escape Sequences for Character and String Literals
[[API 06]] Class Pattern "Backslashes, escapes, and quoting"
[[API 06]] Package java.sql