Signed to Unsigned Conversion Error
Weakness ID: 195 (Weakness Variant)Status: Draft
+ Description

Description Summary

A signed-to-unsigned conversion error takes place when a signed primitive is used as an unsigned value, usually as a size variable.

Extended Description

It is dangerous to rely on implicit casts between signed and unsigned numbers because the result can take on an unexpected value and violate assumptions made by the program.

+ Time of Introduction
  • Implementation
+ Applicable Platforms

Languages

C

C++

+ Common Consequences
ScopeEffect
Availability

Conversion between signed and unsigned values can lead to a variety of errors, but from a security standpoint is most commonly associated with integer overflow and buffer overflow vulnerabilities.

+ Demonstrative Examples

Example 1

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.

(Bad Code)
Example Language:
unsigned int readdata () {
int amount = 0;
...
if (result == ERROR)
amount = -1;
...
return amount;
}

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.

Example 2

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.

(Bad Code)
Example Language:
unsigned int readdata () {
int amount = 0;
...
amount = accessmainframe();
...
return amount;
}

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.

Example 3

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.

+ Observed Examples
ReferenceDescription
CVE-2007-4268Chain: integer signedness passes signed comparison, leads to heap overflow
+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfCategoryCategory189Numeric Errors
Development Concepts699
ChildOfWeakness BaseWeakness Base681Incorrect Conversion between Numeric Types
Development Concepts (primary)699
Research Concepts (primary)1000
CanPrecedeWeakness ClassWeakness Class119Failure to Constrain Operations within the Bounds of a Memory Buffer
Research Concepts1000
CanAlsoBeCategoryCategory192Integer Coercion Error
Research Concepts1000
CanAlsoBeWeakness BaseWeakness Base197Numeric Truncation Error
Research Concepts1000
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
CLASPSigned to unsigned conversion error
+ Content History
Submissions
Submission DateSubmitterOrganizationSource
CLASPExternally Mined
Modifications
Modification DateModifierOrganizationSource
2008-09-08CWE Content TeamMITREInternal
updated Applicable Platforms, Common Consequences, Relationships, Other Notes, Taxonomy Mappings
2009-05-27CWE Content TeamMITREInternal
updated Demonstrative Examples
2009-10-29CWE Content TeamMITREInternal
updated Common Consequences, Description, Other Notes, Relationships