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CWE-195: Signed to Unsigned Conversion Error

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

Description Summary

The software uses a signed primitive and performs a cast to an unsigned primitive, which can produce an unexpected value if the value of the signed primitive can not be represented using an unsigned primitive.

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.

Often, functions will return negative values to indicate a failure. When the result of a function is to be used as a size parameter, using these negative return values can have unexpected results. For example, if negative size values are passed to the standard memory copy or allocation functions they will be implicitly cast to a large unsigned value. This may lead to an exploitable buffer overflow or underflow condition.

+ Time of Introduction
  • Implementation
+ Applicable Platforms

Languages

C

C++

+ Common Consequences
ScopeEffect

Technical Impact: Unexpected state

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.

Example 4

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.

(Bad Code)
Example Language:
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;
}
}

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.

Example 5

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

(Bad Code)
Example Language:
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));
...
}

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).

+ Observed Examples
ReferenceDescription
Chain: 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
ChildOfCategoryCategory885SFP Cluster: Risky Values
Software Fault Pattern (SFP) Clusters (primary)888
CanPrecedeWeakness ClassWeakness Class119Improper Restriction of Operations within the Bounds of a Memory Buffer
Research Concepts1000
CanFollowWeakness BaseWeakness Base839Numeric Range Comparison Without Minimum Check
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
+ References
[REF-7] Mark Dowd, John McDonald and Justin Schuh. "The Art of Software Security Assessment". Chapter 6, "Type Conversions", Page 223.. 1st Edition. Addison Wesley. 2006.
+ Content History
Submissions
Submission DateSubmitterOrganizationSource
Externally Mined
Modifications
Modification DateModifierOrganizationSource
2008-09-08MITREInternal
updated Applicable_Platforms, Common_Consequences, Relationships, Other_Notes, Taxonomy_Mappings
2009-05-27MITREInternal
updated Demonstrative_Examples
2009-10-29MITREInternal
updated Common_Consequences, Description, Other_Notes, Relationships
2010-02-16MITREInternal
updated Demonstrative_Examples
2010-04-05MITREInternal
updated Demonstrative_Examples
2011-03-29MITREInternal
updated Relationships
2011-06-01MITREInternal
updated Common_Consequences
2011-06-27MITREInternal
updated Common_Consequences
2012-05-11MITREInternal
updated Demonstrative_Examples, References, Relationships
2014-06-23MITREInternal
updated Demonstrative_Examples, Description
Page Last Updated: June 23, 2014