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Formatting in Java is stricter as compared to traditional languages such as C. Exceptions are thrown if any conversion argument mismatches with the corresponding flag. Although not easily exploitable, it is possible for user input to taint the format string and cause information leakage or denial of service in some casesThe 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 demonstrates a careless information leak. The code accepts the credit card expiration date as an input argument and uses it within the format string. In the absence of proper input validation, an artful attacker can observe the very date that has been asked to be inputted. Any of the arguments %1$tm, %1$te or %1$tY can further such an attempt.leaks information about a user's credit card. It incorporates untrusted data in a format string.

import java.util.Calendar;
import java.util.GregorianCalendar;
import static java.util.Calendar.*;

class Format {
  static Calendar c = new GregorianCalendar(1995, GregorianCalendar.MAY, 23);
  public static void main(String[] args) {  
    // args[0] should iscontain the credit card expiration date
    //args[0] can but might contain either %1$tm, %1$te or %1$tY asformat malicious argumentsspecifiers
    //perform comparison with c, if it doesn't match print the following line
    System.out.printf(System.out.format(
      args[0] + " did not match! HINT: It was issued on %1$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

The perfect remedy to format string problems is to ensure that user generated input never shows up in format strings. This will safeguard the code from unforeseen exploitation.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 Perform comparison with c, 
    // if it doesn't match, print the following line
    System.out.printf("The inputformat(
      "%s did not match! HINT: It was issued on %1$terd%terd of some month", 
      args[0], c
    );
  }
}

Risk Assessment

Allowing user input to taint the Incorporating untrusted data in a format string may cause result in information leakage leaks or allow a denial-of-service attack.

Automated Detection

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TODO

Related Vulnerabilities

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

Other Languages

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

Tool	Version	Checker	Description
The Checker Framework	Include Page The Checker Framework_V The Checker Framework_V	Tainting Checker	Trust and security errors (see Chapter 8)
Parasoft Jtest	Include Page Parasoft_V Parasoft_V	CERT.IDS06.VAFS	Ensure the correct number of arguments for varargs methods with format strings
Klocwork	Include Page Klocwork_V Klocwork_V	SV.EXEC SV.EXEC.DIR SV.EXEC.ENV SV.EXEC.LOCAL SV.EXEC.PATH	Implemented

Related Guidelines

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Bibliography

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

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References

Wiki Markup\[[API 06|AA. Java References#API 06]\] [Class Formatter|http://java.sun.com/javase/6/docs/api/java/util/Formatter.html] \[[Seacord 05|AA. Java References#Seacord 05]\] Chapter 6, Formatted Output