CWE-195: Signed to Unsigned Conversion Error

In this example the variable amount can hold a negative value when it is returned. Because the function is declared to return an unsigned int, amount will be implicitly converted to unsigned.

If the error condition in the code above is met, then the return value of readdata() will be 4,294,967,295 on a system that uses 32-bit integers.

unsigned int readdata () {int amount = 0;...if (result == ERROR)amount = -1;...return amount;}

In this example, depending on the return value of accecssmainframe(), the variable amount can hold a negative value when it is returned. Because the function is declared to return an unsigned value, amount will be implicitly cast to an unsigned number.

If the return value of accessmainframe() is -1, then the return value of readdata() will be 4,294,967,295 on a system that uses 32-bit integers.

unsigned int readdata () {int amount = 0;...amount = accessmainframe();...return amount;}

The following code is intended to read an incoming packet from a socket and extract one or more headers.

The code performs a check to make sure that the packet does not contain too many headers. However, numHeaders is defined as a signed int, so it could be negative. If the incoming packet specifies a value such as -3, then the malloc calculation will generate a negative number (say, -300 if each header can be a maximum of 100 bytes). When this result is provided to malloc(), it is first converted to a size_t type. This conversion then produces a large value such as 4294966996, which may cause malloc() to fail or to allocate an extremely large amount of memory (CWE-195). With the appropriate negative numbers, an attacker could trick malloc() into using a very small positive number, which then allocates a buffer that is much smaller than expected, potentially leading to a buffer overflow.

DataPacket *packet;int numHeaders;PacketHeader *headers;                      sock=AcceptSocketConnection();ReadPacket(packet, sock);numHeaders =packet->headers;                      if (numHeaders > 100) {ExitError("too many headers!");}headers = malloc(numHeaders * sizeof(PacketHeader);ParsePacketHeaders(packet, headers);

This example processes user input comprised of a series of variable-length structures. The first 2 bytes of input dictate the size of the structure to be processed.

The programmer has set an upper bound on the structure size: if it is larger than 512, the input will not be processed. The problem is that len is a signed short, so the check against the maximum structure length is done with signed values, but len is converted to an unsigned integer for the call to memcpy() and the negative bit will be extended to result in a huge value for the unsigned integer. If len is negative, then it will appear that the structure has an appropriate size (the if branch will be taken), but the amount of memory copied by memcpy() will be quite large, and the attacker will be able to overflow the stack with data in strm.

char* processNext(char* strm) {char buf[512];short len = *(short*) strm;strm += sizeof(len);if (len <= 512) {memcpy(buf, strm, len);process(buf);return strm + len;}else {return -1;}}

In the following example, it is possible to request that memcpy move a much larger segment of memory than assumed:

If returnChunkSize() happens to encounter an error it will return -1. Notice that the return value is not checked before the memcpy operation (CWE-252), so -1 can be passed as the size argument to memcpy() (CWE-805). Because memcpy() assumes that the value is unsigned, it will be interpreted as MAXINT-1 (CWE-195), and therefore will copy far more memory than is likely available to the destination buffer (CWE-787, CWE-788).

int returnChunkSize(void *) {                                                                                 /* if chunk info is valid, return the size of usable memory,                                                                                    * else, return -1 to indicate an error                                                                                    */                            ...                      }int main() {...memcpy(destBuf, srcBuf, (returnChunkSize(destBuf)-1));...}

This example shows a typical attempt to parse a string with an error resulting from a difference in assumptions between the caller to a function and the function's action.

The buffer length ends up being -1, resulting in a blown out stack. The space character after the colon is included in the function calculation, but not in the caller's calculation. This, unfortunately, is not usually so obvious but exists in an obtuse series of calculations.

int proc_msg(char *s, int msg_len) 		 {                    		    // Note space at the end of the string - assume all strings have preamble with space 		    int pre_len = sizeof("preamble: "); 		    char buf[pre_len - msg_len]; 		    ... Do processing here if we get this far 		   		  } 		  char *s = "preamble: message\n"; 		  char *sl = strchr(s, ':');        // Number of characters up to ':' (not including space) 		  int jnklen = sl == NULL ? 0 : sl - s;    // If undefined pointer, use zero length 		  int ret_val = proc_msg ("s",  jnklen);    // Violate assumption of preamble length, end up with negative value, blow out stack