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.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
---|---|---|---|---|---|
IDS06-J | Medium | Unlikely | Medium | P4 | L3 |
Automated Detection
Static analysis tools that perform taint analysis can diagnose some violations of this rule.
Tool | Version | Checker | Description |
---|---|---|---|
The Checker Framework | 2.1.3 | Tainting Checker | Trust and security errors (see Chapter 8) |
Parasoft Jtest | 2024.1 | CERT.IDS06.VAFS | Ensure the correct number of arguments for varargs methods with format strings |
Klocwork | 2024.2 | SV.EXEC SV.EXEC.DIR SV.EXEC.ENV SV.EXEC.LOCAL SV.EXEC.PATH | Implemented |
Related Guidelines
SEI CERT Perl Coding Standard | IDS30-PL. Exclude user input from format strings |
Injection [RST] | |
CWE-134, Uncontrolled Format String |
Bibliography
[API 2006] | |
Chapter 6, "Formatted Output" | |
[Seacord 2015] |
2 Comments
Yitzhak Mandelbaum
Why do we forbid user input entirely (from format strings), rather than requiring sanitization, as we do in other rules?
David Svoboda
Mainly because we hadn't expressed things in terms of sanitization when we crafted this rule.
It helps that many (most?) systems that use format strings use string literals, or read the format strings from some 'strings' file.