Code from Example 630 refactored to use a static format string in Java

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Example 631 Code from Example 630 refactored to use a static format string
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while (fgets(buf, sizeof buf, f)) { lreply(200, "%s", buf); } void lreply(int n, const char* fmt, ) { char buf[BUFSIZ]; vsnprintf(buf, sizeof buf, fmt, ap); }
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A Classic Format String Attack
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At the heart of many format string exploits is the %n formatting directive, which causes the number of characters already processed to be written to memory as an integer value This directive is a black sheep most formatting directives pertain to the way input parameters are interpreted The %n directive alone calls for the processing function to write data out to one of the function arguments The capability to write data is a valuable attack vector Another important property of the %n directive is that it writes the number of bytes that should have been written instead of the actual number that were written This is important when output is written to a xed-size string and is truncated due to a lack of available space Even with the capability to write values to memory, an attacker must have two things in order to mount a highly effective exploit: Control over the location in memory to which the %n directive writes Control over the value the %n directive writes Because of the way stack frames are constructed, the rst challenge can often be easily overcome simply by placing the target address for the %n directive in the format string being processed Because the characters will likely be copied unchanged onto the stack, careful tinkering with the number conversion speci cations will cause the formatted string function to interpret the attacker supplied value as the address of an integer in memory and write the number of bytes process to the speci ed location Either analysis of the executing binary or trial-and-error attempts will be necessary to determine the location of the target address Now that the attacker can control the location in memory to which the %n directive will write, all that remains is to stuff an interesting value there This challenge is easier to overcome because the attacker can specify an arbitrary eld width as part of many conversation speci cations The combination of eld-width speci ers and the property that the %n directive will write out the number of characters that would have been processed, regardless of any truncation that might have occurred, gives the attacker the capability to construct arbitrarily large values An even more sophisticated version of this attack uses four staggered %n writes to completely control the value of a pointer To see an attack in action, Tim Newsham s original explication on format string vulnerabilities, Format String Attacks, illustrates the steps involved in a canonical format string attack [Newsham, 2000]
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Better String Classes and Libraries Like the language itself, native strings in C were designed to value efficiency and simplicity over robustness and security Null-terminated strings are memory efficient but error prone In this section, we discuss string libraries (many of which provide alternative string representations) that can eliminate many of the causes of buffer overflow vulnerabilities in string handling code Alternatives to native C strings do not need to claim security as a feature to prevent buffer overflows If you are using C++, use the string representation defined in the standard STL namespace as std::string The std::string class provides a layer of abstraction above the underlying string representation and provides methods for performing most string operations without the risk of introducing buffer overflow vulnerabilities Example 632 shows a simple block of code that uses std::string to count the number of occurrences of term in a line of input
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Example 632 Code that uses std::string to count the number of occurrences of a substring in a line of input
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std::string in; int i = 0; int count = 0; getline(cin, in, '\n'); for(i = infind(term, 0); i != string::npos; i = infind(term, i)) { count++; i++; } cout<<count;
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In Microsoft environments where the use of the STL is frowned on, the ATL/MFC CString string class and CStringT template class provide effective handling of strings and should be used to avoid many of the risks of buffer over ow inherent in C-style strings Example 633 shows the same block of code from Example 632 rewritten to use CString
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