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An attacker who can fully or partially control the contents of a format string can crash the Perl interpreter , or cause a denial of service. She can also modify values, perhaps by using the %n|| conversion specifier, and use these values to divert control flow. Their capabilities are not as strong as in C [Seacord 2005]; nonetheless the danger is sufficiently great that the formatted output functions {{sprintf() and printf() should never be passed unsanitized format strings.
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In this invocation, the malicious user name user%n was incomprorated incorporated into the $prompt string. When fed to the printf() call inside validate_password(), the %n instructed Perl to fill the first format string argument with the number of characters printed. This , which caused Perl to set the $is_ok variable to 4. Since it is now nonzero, the program incorrectly grants access to the user.
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Automated Detection
Perl's Taint taint mode provides partial detection of unsanitized input in format strings.
Perl's warnings can detect if a call to printf() or sprintf() contains the wrong number of format string arguments.
Tool | Diagnostic |
|---|---|
Warnings | Missing argument in .*printf |
| Taint mode | Insecure dependency in .*printf |
Related Guidelines
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| CWE-134, "Uncontrolled format string" |
Bibliography
| [Christey 2005] | Format string vulnerabilities in Perl programs |
|---|---|
| [Seacord 2005] | Chapter 6, "Formatted Output" |
| [VU#948385] |
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