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146 Web Application Penetration Testing process, and using fuzzing techniques. Gray Box testing When reviewing code, one must realize that there are several avenues where heap related vulnerabilities may arise. Code that seems innocuous at the first glance can actually be vulnerable under certain conditions. Since there are several variants of this vulnerability, we will cover only the issues that are predominant. Most of the time, heap buffers are considered safe by a lot of developers who do not hesitate to perform insecure operations like strcpy( ) on them. The myth that a stack overflow and instruction pointer overwrite are the only means to execute arbitrary code proves to be hazardous in case of code shown below:- int main(int argc, char *argv[]) { …… } vulnerable(argv[1]); return 0; int vulnerable(char *buf) { } …….. HANDLE hp = HeapCreate(0, 0, 0); HLOCAL chunk = HeapAlloc(hp, 0, 260); strcpy(chunk, buf); ‘’’ Vulnerability’’’ return 0; In this case, if buf exceeds 260 bytes, it will overwrite pointers in the adjacent boundary tag, facilitating the overwrite of an arbitrary memory location with 4 bytes of data once the heap management routine kicks in. Lately, several products, especially anti-virus libraries, have been affected by variants that are combinations of an integer overflow and copy operations to a heap buffer. As an example, consider a vulnerable code snippet, a part of code responsible for processing TNEF filetypes, from Clam Anti Virus 0.86.1, source file tnef.c and function tnef_message( ): string = cli_malloc(length + 1); ‘’’ Vulnerability’’’ if(fread(string, 1, length, fp) != length) {‘’’ Vulnerability’’’ free(string); return -1; } The malloc in line 1 allocates memory based on the value of length, which happens to be a 32 bit integer. In this particular example, length is user-controllable and a malicious TNEF file can be crafted to set length to ‘-1’, which would result in malloc( 0 ). Therefore, this malloc would allocate a small heap buffer, which would be 16 bytes on most 32 bit platforms (as indicated in malloc.h). And now, in line 2, a heap overflow occurs in the call to fread( ). The 3rd argument, in this case length, is expected to be a size_t variable. But if it’s going to be ‘-1’, the argument wraps to 0xFFFFFFFF, thus copying 0xFFFFFFFF bytes into the 16 byte buffer. Static code analysis tools can also help in locating heap related vulnerabilities such as “double free” etc. A variety of tools like RATS, Flawfinder and ITS4 are available for analyzing C-style languages. Tools • OllyDbg: “A windows based debugger used for analyzing buffer overflow vulnerabilities” - http://www.ollydbg.de • Spike, A fuzzer framework that can be used to explore vulnerabilities and perform length testing - http://www.immunitysec.com/downloads/SPIKE2.9.tgz • Brute Force Binary Tester (BFB), A proactive binary checker - http://bfbtester.sourceforge.net • Metasploit, A rapid exploit development and Testing frame work - http://www.metasploit.com References Whitepapers • w00w00: “Heap Overflow Tutorial” - http://www.cgsecurity.org/exploit/heaptut.txt • David Litchfield: “Windows Heap Overflows” - http://www.blackhat.com/presentations/win-usa-04/bhwin-04-litchfield/bh-win-04-litchfield.ppt Testing for Stack Overflow Summary Stack overflows occur when variable size data is copied into fixed length buffers located on the program stack without any bounds checking. Vulnerabilities of this class are generally considered to be of high severity since their exploitation would mostly permit arbitrary code execution or Denial of Service. Rarely found in interpreted platforms, code written in C and similar languages is often ridden with instances of this vulnerability. In fact almost every platform is vulnerable to stack overflows with the following notable exceptions: • J2EE – as long as native methods or system calls are not invoked • .NET – as long as /unsafe or unmanaged code is not invoked (such as the use of P/Invoke or COM Interop) • PHP – as long as external programs and vulnerable PHP extensions written in C or C++ are not called can suffer from stack overflow issues. Stack overflow vulnerabilities often allow an attacker to directly take control of the instruction pointer and, therefore, alter the execution of the program and execute arbitrary code. Besides
147 Web Application Penetration Testing overwriting the instruction pointer, similar results can also be obtained by overwriting other variables and structures, like Exception Handlers, which are located on the stack. On opening the executable with the supplied arguments and continuing execution the following results are obtained. How to Test Black Box testing The key to testing an application for stack overflow vulnerabilities is supplying overly large input data as compared to what is expected. However, subjecting the application to arbitrarily large data is not sufficient. It becomes necessary to inspect the application’s execution flow and responses to ascertain whether an overflow has actually been triggered or not. Therefore, the steps required to locate and validate stack overflows would be to attach a debugger to the target application or process, generate malformed input for the application, subject the application to malformed input, and inspect responses in a debugger. The debugger allows the tester to view the execution flow and the state of the registers when the vulnerability gets triggered. On the other hand, a more passive form of testing can be employed, which involves inspecting assembly code of the application by using disassemblers. In this case, various sections are scanned for signatures of vulnerable assembly fragments. This is often termed as reverse engineering and is a tedious process. As a simple example, consider the following technique employed while testing an executable “sample.exe” for stack overflows: #include int main(int argc, char *argv[]) { char buff[20]; printf(“copying into buffer”); strcpy(buff,argv[1]); return 0; } File sample.exe is launched in a debugger, in our case OllyDbg. As shown in the registers window of the debugger, the EIP or Extended Instruction Pointer, which points to the next instruction to be executed, contains the value ‘41414141’. ‘41’ is a hexadecimal representation for the character ‘A’ and therefore the string ‘AAAA’ translates to 41414141. This clearly demonstrates how input data can be used to overwrite the instruction pointer with user-supplied values and control program execution. A stack overflow can also allow overwriting of stack-based structures like SEH (Structured Exception Handler) to control code execution and bypass certain stack protection mechanisms. As mentioned previously, other methods of testing such vulnerabilities include reverse engineering the application binaries, which is a complex and tedious process, and using fuzzing techniques. Gray Box testing When reviewing code for stack overflows, it is advisable to search for calls to insecure library functions like gets(), strcpy(), strcat() etc which do not validate the length of source strings and blindly copy data into fixed size buffers. For example consider the following function:- void log_create(int severity, char *inpt) { Since the application is expecting command line arguments, a large sequence of characters such as ‘A’, can be supplied in the argument field shown above. char b[1024];
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