Integer coercion refers to a set of flaws pertaining to the type casting, extension, or truncation of primitive data types. Several flaws fall under the category of integer coercion errors. For the most part, these errors in and of themselves result only in availability and data integrity issues. However, in some circumstances, they may result in other, more complicated security related flaws, such as buffer overflow conditions. 1000 Weakness ChildOf 681 699 Weakness ChildOf 682 1000 Weakness CanAlsoBe 195 1000 Weakness CanAlsoBe 196 1000 Weakness CanAlsoBe 197 1000 Weakness CanAlsoBe 194 734 Category ChildOf 738 868 Category ChildOf 872 Within C, it might be that "coercion" is semantically different than "casting", possibly depending on whether the programmer directly specifies the conversion, or if the compiler does it implicitly. This has implications for the presentation of this node and others, such as CWE-681, and whether there is enough of a difference for these nodes to be split. Implementation Medium Availability DoS: resource consumption (CPU) DoS: resource consumption (memory) DoS: crash / exit / restart Integer coercion often leads to undefined states of execution resulting in infinite loops or crashes. Integrity Confidentiality Availability Execute unauthorized code or commands In some cases, integer coercion errors can lead to exploitable buffer overflow conditions, resulting in the execution of arbitrary code. Integrity Other Other Integer coercion errors result in an incorrect value being stored for the variable in question. Requirements A language which throws exceptions on ambiguous data casts might be chosen. Architecture and Design Design objects and program flow such that multiple or complex casts are unnecessary Implementation Ensure that any data type casting that you must used is entirely understood in order to reduce the plausibility of error in use. The following code is intended to read an incoming packet from a socket and extract one or more headers. C 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); 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. The following code reads a maximum size and performs a sanity check on that size. It then performs a strncpy, assuming it will not exceed the boundaries of the array. While the use of "short s" is forced in this particular example, short int's are frequently used within real-world code, such as code that processes structured data. C int GetUntrustedInt () { return(0x0000FFFF); } void main (int argc, char **argv) { char path[256]; char *input; int i; short s; unsigned int sz; i = GetUntrustedInt(); s = i; /* s is -1 so it passes the safety check - CWE-697 */ if (s > 256) { DiePainfully("go away!\n"); } /* s is sign-extended and saved in sz */ sz = s; /* output: i=65535, s=-1, sz=4294967295 - your mileage may vary */ printf("i=%d, s=%d, sz=%u\n", i, s, sz); input = GetUserInput("Enter pathname:"); /* strncpy interprets s as unsigned int, so it's treated as MAX_INT (CWE-195), enabling buffer overflow (CWE-119) */ strncpy(path, input, s); path[255] = '\0'; /* don't want CWE-170 */ printf("Path is: %s\n", path); } This code first exhibits an example of CWE-839, allowing "s" to be a negative number. When the negative short "s" is converted to an unsigned integer, it becomes an extremely large positive integer. When this converted integer is used by strncpy() it will lead to a buffer overflow (CWE-119). Michael Howard David LeBlanc John Viega 24 Deadly Sins of Software Security "Sin 7: Integer Overflows." Page 119 McGraw-Hill 2010 Mark Dowd John McDonald Justin Schuh The Art of Software Security Assessment Chapter 6, "Sign Extension", Page 248. 1st Edition Addison Wesley 2006 Integer coercion error Understand integer conversion rules INT02-C Do not use input functions to convert character data if they cannot handle all possible inputs INT05-C Ensure that integer conversions do not result in lost or misinterpreted data INT31-C Understand integer conversion rules INT02-CPP Do not use input functions to convert character data if they cannot handle all possible inputs INT05-CPP Ensure that integer conversions do not result in lost or misinterpreted data INT31-CPP CLASP CWE Content Team MITRE 2008-09-08 updated Applicable_Platforms, Common_Consequences, Maintenance_Notes, Relationships, Other_Notes, Taxonomy_Mappings CWE Content Team MITRE 2008-11-24 updated Relationships, Taxonomy_Mappings CWE Content Team MITRE 2009-12-28 updated Description, Other_Notes CWE Content Team MITRE 2010-04-05 updated Demonstrative_Examples CWE Content Team MITRE 2011-06-01 updated Common_Consequences CWE Content Team MITRE 2011-09-13 updated Relationships, Taxonomy_Mappings CWE Content Team MITRE 2012-05-11 updated Demonstrative_Examples, References CWE Content Team MITRE 2012-10-30 updated Potential_Mitigations