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Interpretation of Java format strings is stricter than that in languages such as C. The implementations in the standard libraries throw appropriate exceptions when any conversion argument fails to match the corresponding flag. This approach reduces opportunities for malicious exploits. Nevertheless, malicious user input can exploit format strings and can cause information leaks or denial of service. Therefore strings from an untrusted source shall not be incorporated into format strings.

Noncompliant Code Example

This noncompliant code example demonstrates an information leak issue. It accepts a credit card expiration date as an input argument and uses it within the format string. In the absence of proper input validation, an attacker can determine the date against which the input is being verified, perhaps by supplying an input that includes one of the format string arguments %1$tm, %1$te or %1$tY.

class Format {
  static Calendar c = new GregorianCalendar(1995, GregorianCalendar.MAY, 23);
  public static void main(String[] args) {  
    // args[0] is the credit card expiration date
    // args[0] can contain either %1$tm, %1$te or %1$tY as malicious arguments
    // First argument prints 05 (May), second prints 23 (day) and third prints 1995 (year)
    // Perform comparison with c, if it doesn't match print the following line
    System.out.printf(args[0] + " did not match! HINT: It was issued on %1$terd of some month", c);
  }
}

In the absence of proper input validation, an attacker can determine the date against which the input is being verified, perhaps by supplying an input that includes one of the format string arguments %1$tm, %1$te, or %1$tY.

Compliant Solution

This compliant solution ensures that user generated input is excluded from format strings.

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

Other Languages

This guideline appears in the C Secure Coding Standard as FIO30-C. Exclude user input from format strings.

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