IDS06-J. Exclude unsanitized user input from format strings

The java.io package includes a PrintStream class that has two equivalent formatting methods format() and printf().  System.out is a PrintStream object, allowing PrintStream methods to be invoked on the standard output stream. The risks from using these methods are not as high as from using similar functions in C or C++  [Seacord 2013]. The standard library implementations throw an exception when any conversion argument fails to match the corresponding format specifier.  Although this helps mitigate against exploits, if malicious user input is accepted in a format string, it can cause information leakage or denial of service. Consequently, unsanitized input from an untrusted source must never 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.

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 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.

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
    // Perform comparison with c, 
    // if it doesn't match, print the following line
    System.out.printf("%s did not match! "
        + " HINT: It was issued on %1$terd of some month", args[0],c);
  }
}

Risk Assessment

Allowing user input to taint a format string may cause information leaks or denial of service.

Automated Detection

Static analysis tools that perform taint analysis can diagnose some violations of this rule.

Related Guidelines

Bibliography