Testing for DoS Buffer Overflows (OWASP-DS-003)

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OWASP Testing Guide v3 Table of Contents

This article is part of the OWASP Testing Guide v3. The entire OWASP Testing Guide v3 can be downloaded here.

OWASP at the moment is working at the OWASP Testing Guide v4: you can browse the Guide here

Contents


Brief Summary

In this test we check whether it is possible to cause a denial of service condition by overflowing one or more data structures of the target application.

Description of the Issue

Any language where the developer has direct responsibility for managing memory allocation, most notably C & C++, has the potential for a buffer overflow. While the most serious risk related to a buffer overflow is the ability to execute arbitrary code on the server, the first risk comes from the denial of service that can happen if the application crashes. Buffer overflows are discussed in more detail elsewhere in this testing document, but we will briefly give an example as it relates to an application denial of service.

The following is a simplified example of vulnerable code in C:

void overflow (char *str) {
   char buffer[10];
   strcpy(buffer, str); // Dangerous!
}

int main () {
  char *str = "This is a string that is larger than the buffer of 10";
  overflow(str);
}

If this code example were executed, it would cause a segmentation fault and dump core. The reason is that strcpy would try to copy 53 characters into an array of 10 elements only, overwriting adjacent memory locations. While this example above is an extremely simple case, the reality is that in a web based application there may be places where the user input is not adequately checked for its length, making this kind of attack possible.

Black Box Testing

Refer to the Testing for Buffer Overflow section for how to submit a range of lengths to the application looking for possible locations that may be vulnerable. As it relates to a DoS, if you have received a response (or a lack of) that makes you believe that the overflow has occurred, attempt to make another request to the server and see if it still responds.

Gray Box Testing

Please refer to the Testing for Buffer Overflow section of the Guide for detailed information on this testing.