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cwe:cn:definition:134

CWE-134:Uncontrolled Format String

Description Summary

The software uses externally-controlled format strings in printf-style functions, which can lead to buffer overflows or data representation problems.

Modes of Introduction

The programmer rarely intends for a format string to be user-controlled at all. This weakness is frequently introduced in code that constructs log messages, where a constant format string is omitted.

In cases such as localization and internationalization, the language-specific message repositories could be an avenue for exploitation, but the format string issue would be resultant, since attacker control of those repositories would also allow modification of message length, format, and content.

Other Notes

While Format String vulnerabilities typically fall under the Buffer Overflow category, technically they are not overflowed buffers. The Format String vulnerability is fairly new (circa 1999) and stems from the fact that there is no realistic way for a function that takes a variable number of arguments to determine just how many arguments were passed in. The most common functions that take a variable number of arguments, including C-runtime functions, are the printf() family of calls. The Format String problem appears in a number of ways. A *printf() call without a format specifier is dangerous and can be exploited. For example, printf(input); is exploitable, while printf(y, input); is not exploitable in that context. The result of the first call, used incorrectly, allows for an attacker to be able to peek at stack memory since the input string will be used as the format specifier. The attacker can stuff the input string with format specifiers and begin reading stack values, since the remaining parameters will be pulled from the stack. Worst case, this improper use may give away enough control to allow an arbitrary value (or values in the case of an exploit program) to be written into the memory of the running program.

Frequently targeted entities are file names, process names, identifiers.

Format string problems are a classic C/C++ issue that are now rare due to the ease of discovery. One main reason format string vulnerabilities can be exploited is due to the %n operator. The %n operator will write the number of characters, which have been printed by the format string therefore far, to the memory pointed to by its argument. Through skilled creation of a format string, a malicious user may use values on the stack to create a write-what-where condition. Once this is achieved, he can execute arbitrary code. Other operators can be used as well; for example, a %9999s operator could also trigger a buffer overflow, or when used in file-formatting functions like fprintf, it can generate a much larger output than intended.

Likelihood of Exploit

Very High

Common Consequences

Scope Technical Impace Note
ConfidentialityRead memoryFormat string problems allow for information disclosure which can severely simplify exploitation of the program.
Integrity
Confidentiality
Availability
Execute unauthorized code or commandsFormat string problems can result in the execution of arbitrary code.

Detection Methods

Detection Method - 1

Automated Static Analysis

This weakness can often be detected using automated static analysis tools. Many modern tools use data flow analysis or constraint-based techniques to minimize the number of false positives.

Automated static analysis might not be able to recognize when proper input validation is being performed, leading to false positives - i.e., warnings that do not have any security consequences or do not require any code changes.

Automated static analysis might not be able to detect the usage of custom API functions or third-party libraries that indirectly invoke SQL commands, leading to false negatives - especially if the API/library code is not available for analysis.

This is not a perfect solution, since 100% accuracy and coverage are not feasible.
2013/05/30 09:36

Detection Method - 2

Black Box

Since format strings often occur in rarely-occurring erroneous conditions (e.g. for error message logging), they can be difficult to detect using black box methods. It is highly likely that many latent issues exist in executables that do not have associated source code (or equivalent source.

Detection Method - 3

Automated Static Analysis - Binary / Bytecode

According to SOAR, the following detection techniques may be useful:

Detection Method - 4

Manual Static Analysis - Binary / Bytecode

According to SOAR, the following detection techniques may be useful:

Detection Method - 5

=== Dynamic Analysis with automated results interpretation === According to SOAR, the following detection techniques may be useful:

Detection Method - 6

Dynamic Analysis with manual results interpretation

According to SOAR, the following detection techniques may be useful:

Detection Method - 7

Manual Static Analysis - Source Code

According to SOAR, the following detection techniques may be useful:

Detection Method - 8

Automated Static Analysis - Source Code

According to SOAR, the following detection techniques may be useful:

Detection Method - 9

Architecture / Design Review

According to SOAR, the following detection techniques may be useful:

Potential Mitigations

Mitigation - 1

Requirements

Choose a language that is not subject to this flaw.

Mitigation - 2

Implementation

Ensure that all format string functions are passed a static string which cannot be controlled by the user and that the proper number of arguments are always sent to that function as well. If at all possible, use functions that do not support the %n operator in format strings. [R.134.1] [R.134.2]

Mitigation - 3

Build and Compilation

Heed the warnings of compilers and linkers, since they may alert you to improper usage.

Demonstrative Examples

Example - 1

The following program prints a string provided as an argument.

#include <stdio.h> 
 
void printWrapper(char *string) { 
 
printf(string); 
 
} 
 
int main(int argc, char **argv) { 
 
char buf[5012]; 
memcpy(buf, argv[1], 5012); 
printWrapper(argv[1]); 
return (0); 
 
} 

The example is exploitable, because of the call to printf() in the printWrapper() function. Note: The stack buffer was added to make exploitation more simple.

Example - 2

The following code copies a command line argument into a buffer using snprintf().

int main(int argc, char **argv){ 
char buf[128]; 
... 
snprintf(buf,128,argv[1]); 
 
} 

This code allows an attacker to view the contents of the stack and write to the stack using a command line argument containing a sequence of formatting directives. The attacker can read from the stack by providing more formatting directives, such as %x, than the function takes as arguments to be formatted. (In this example, the function takes no arguments to be formatted.) By using the %n formatting directive, the attacker can write to the stack, causing snprintf() to write the number of bytes output thus far to the specified argument (rather than reading a value from the argument, which is the intended behavior). A sophisticated version of this attack will use four staggered writes to completely control the value of a pointer on the stack.

Example - 3

Certain implementations make more advanced attacks even easier by providing format directives that control the location in memory to read from or write to. An example of these directives is shown in the following code, written for glibc:

printf("%d %d %1$d %1$d\n", 5, 9); 

This code produces the following output: 5 9 5 5 It is also possible to use half-writes (%hn) to accurately control arbitrary DWORDS in memory, which greatly reduces the complexity needed to execute an attack that would otherwise require four staggered writes, such as the one mentioned in the first example.

Observed Examples

Reference Description
CVE-2002-1825format string in Perl program
CVE-2001-0717format string in bad call to syslog function
CVE-2002-0573format string in bad call to syslog function
CVE-2002-1788format strings in NNTP server responses
CVE-2006-2480Format string vulnerability exploited by triggering errors or warnings, as demonstrated via format string specifiers in a .bmp filename.
CVE-2007-2027Chain: untrusted search path enabling resultant format string by loading malicious internationalization messages
cwe/cn/definition/134.txt · 最后更改: 2014/09/04 14:25 (外部编辑)