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 and System.err are PrintStream objects, allowing PrintStream methods to be invoked on the standard output and error streams. 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 throwing an exception helps mitigate against exploits, if untrusted data is incorporated into a format string, it can result in an information leak or allow a denial-of-service attack. Consequently, unsanitized input from an untrusted source must never be incorporated into format strings.

Noncompliant Code Example

This noncompliant code example leaks information about a user's credit card. It incorporates untrusted data in a format string.

class Format {
  static Calendar c = new GregorianCalendar(1995, GregorianCalendar.MAY, 23);
  public static void main(String[] args) {  
    // args[0] should contain the credit card expiration date
    // but might contain %1$tm, %1$te or %1$tY format specifiers
    System.out.format(
      args[0] + " did not match! HINT: It was issued on %1$terd of some month", c
    );
  }
}

In the absence of proper input validation, an attacker can determine the date against which the input is verified by supplying an input string that includes the %1$tm, %1$te, or %1$tY format specifiers. In this example, these format specifiers print 05 (May), 23 (day), and 1995 (year), respectively.

Compliant Solution

This compliant solution excludes untrusted user input from the format string. Although arg[0] still may contain one or more format specifiers, they are now rendered inert. 

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.format(
      "%s did not match! HINT: It was issued on %terd of some month", 
      args[0], c
    );
  }
}

Risk Assessment

Incorporating untrusted data in a format string may result in information leaks or allow a denial-of-service attack.

Automated Detection

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

Related Guidelines

Bibliography

[API 2006]	Class Formatter
[Seacord 2013]	Chapter 6, "Formatted Output"
[Seacord 2015]	IDS06-J. Exclude unsanitized user input from format strings LiveLesson