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Common Weakness Enumeration

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ID

CWE CATEGORY: Integer Coercion Error

Category ID: 192
Status: Incomplete
+ Description

Description Summary

Integer coercion refers to a set of flaws pertaining to the type casting, extension, or truncation of primitive data types.

Extended Description

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.

+ Time of Introduction
  • Implementation
+ Applicable Platforms

Languages

C

C++

Java

.NET

+ Common Consequences
ScopeEffect
Availability

Technical Impact: 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

Technical Impact: 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

Technical Impact: Other

Integer coercion errors result in an incorrect value being stored for the variable in question.

+ Likelihood of Exploit

Medium

+ Demonstrative Examples

Example 1

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

(Bad Code)
Example Language:
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.

Example 2

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.

(Bad Code)
Example Language:
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).

+ Potential Mitigations

Phase: Requirements

A language which throws exceptions on ambiguous data casts might be chosen.

Phase: Architecture and Design

Design objects and program flow such that multiple or complex casts are unnecessary

Phase: Implementation

Ensure that any data type casting that you must used is entirely understood in order to reduce the plausibility of error in use.

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfWeakness BaseWeakness Base681Incorrect Conversion between Numeric Types
Research Concepts (primary)1000
ChildOfWeakness ClassWeakness Class682Incorrect Calculation
Development Concepts (primary)699
ChildOfCategoryCategory738CERT C Secure Coding Section 04 - Integers (INT)
Weaknesses Addressed by the CERT C Secure Coding Standard (primary)734
ChildOfCategoryCategory872CERT C++ Secure Coding Section 04 - Integers (INT)
Weaknesses Addressed by the CERT C++ Secure Coding Standard (primary)868
CanAlsoBeWeakness BaseWeakness Base194Unexpected Sign Extension
Research Concepts1000
CanAlsoBeWeakness VariantWeakness Variant195Signed to Unsigned Conversion Error
Research Concepts1000
CanAlsoBeWeakness VariantWeakness Variant196Unsigned to Signed Conversion Error
Research Concepts1000
CanAlsoBeWeakness BaseWeakness Base197Numeric Truncation Error
Research Concepts1000
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
CLASPInteger coercion error
CERT C Secure CodingINT02-CUnderstand integer conversion rules
CERT C Secure CodingINT05-CDo not use input functions to convert character data if they cannot handle all possible inputs
CERT C Secure CodingINT31-CEnsure that integer conversions do not result in lost or misinterpreted data
CERT C++ Secure CodingINT02-CPPUnderstand integer conversion rules
CERT C++ Secure CodingINT05-CPPDo not use input functions to convert character data if they cannot handle all possible inputs
CERT C++ Secure CodingINT31-CPPEnsure that integer conversions do not result in lost or misinterpreted data
+ References
[REF-17] Michael Howard, David LeBlanc and John Viega. "24 Deadly Sins of Software Security". "Sin 7: Integer Overflows." Page 119. McGraw-Hill. 2010.
[REF-7] Mark Dowd, John McDonald and Justin Schuh. "The Art of Software Security Assessment". Chapter 6, "Sign Extension", Page 248.. 1st Edition. Addison Wesley. 2006.
+ Maintenance Notes

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.

+ Content History
Submissions
Submission DateSubmitterOrganizationSource
CLASPExternally Mined
Modifications
Modification DateModifierOrganizationSource
2008-09-08CWE Content TeamMITREInternal
updated Applicable_Platforms, Common_Consequences, Maintenance_Notes, Relationships, Other_Notes, Taxonomy_Mappings
2008-11-24CWE Content TeamMITREInternal
updated Relationships, Taxonomy_Mappings
2009-12-28CWE Content TeamMITREInternal
updated Description, Other_Notes
2010-04-05CWE Content TeamMITREInternal
updated Demonstrative_Examples
2011-06-01CWE Content TeamMITREInternal
updated Common_Consequences
2011-09-13CWE Content TeamMITREInternal
updated Relationships, Taxonomy_Mappings
2012-05-11CWE Content TeamMITREInternal
updated Demonstrative_Examples, References
2012-10-30CWE Content TeamMITREInternal
updated Potential_Mitigations

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Page Last Updated: May 05, 2017