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CWE-194: Unexpected Sign Extension

 
Unexpected Sign Extension
Weakness ID: 194 (Weakness Base)Status: Incomplete
+ Description

Description Summary

The software performs an operation on a number that causes it to be sign extended when it is transformed into a larger data type. When the original number is negative, this can produce unexpected values that lead to resultant weaknesses.
+ Time of Introduction
  • Implementation
+ Applicable Platforms

Languages

C

C++

+ Common Consequences
ScopeEffect

Technical Impact: Read memory; Modify memory; Other

When an unexpected sign extension occurs in code that operates directly on memory buffers, such as a size value or a memory index, then it could cause the program to write or read outside the boundaries of the intended buffer. If the numeric value is associated with an application-level resource, such as a quantity or price for a product in an e-commerce site, then the sign extension could produce a value that is much higher (or lower) than the application's allowable range.

+ Likelihood of Exploit

High

+ Demonstrative Examples

Example 1

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: 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).

+ Observed Examples
ReferenceDescription
Sign extension error produces -1 value that is treated as a command separator, enabling OS command injection.
Product uses "char" type for input character. When char is implemented as a signed type, ASCII value 0xFF (255), a sign extension produces a -1 value that is treated as a program-specific separator value, effectively disabling a length check and leading to a buffer overflow. This is also a multiple interpretation error.
chain: signed short width value in image processor is sign extended during conversion to unsigned int, which leads to integer overflow and heap-based buffer overflow.
chain: signedness error allows bypass of a length check; later sign extension makes exploitation easier.
Sign extension when manipulating Pascal-style strings leads to integer overflow and improper memory copy.
+ Potential Mitigations

Phase: Implementation

Avoid using signed variables if you don't need to represent negative values. When negative values are needed, perform sanity checks after you save those values to larger data types, or before passing them to functions that are expecting unsigned values.

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
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
CanAlsoBeCategoryCategory192Integer Coercion Error
Research Concepts1000
CanAlsoBeWeakness BaseWeakness Base197Numeric Truncation Error
Research Concepts1000
+ Relationship Notes

Sign extension errors can lead to buffer overflows and other memory-based problems. They are also likely to be factors in other weaknesses that are not based on memory operations, but rely on numeric calculation.

+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
CLASPSign extension error
+ References
John McDonald, Mark Dowd and Justin Schuh. "C Language Issues for Application Security". 2008-01-25. <http://www.informit.com/articles/article.aspx?p=686170&seqNum=6>.
Robert Seacord. "Integral Security". 2006-11-03. <http://www.ddj.com/security/193501774>.
+ Maintenance Notes

This entry is closely associated with signed-to-unsigned conversion errors (CWE-195) and other numeric errors. These relationships need to be more closely examined within CWE.

+ Content History
Submissions
Submission DateSubmitterOrganizationSource
Externally Mined
Modifications
Modification DateModifierOrganizationSource
2008-09-08MITREInternal
updated Applicable_Platforms, Common_Consequences, Description, Relationships, Taxonomy_Mappings
2008-11-05MITREInternal
complete rewrite of the entire entry
2008-11-24MITREInternal
updated Common_Consequences, Demonstrative_Examples, Description, Maintenance_Notes, Name, Observed_Examples, Potential_Mitigations, References, Relationship_Notes, Relationships
2009-05-27MITREInternal
updated Demonstrative_Examples
2009-10-29MITREInternal
updated Demonstrative_Examples
2010-04-05MITREInternal
updated Demonstrative_Examples
2010-12-13MITREInternal
updated Applicable_Platforms
2011-06-01MITREInternal
updated Common_Consequences
2012-05-11MITREInternal
updated Demonstrative_Examples, Relationships
Previous Entry Names
Change DatePrevious Entry Name
2008-04-11Sign Extension Error
2008-11-24Incorrect Sign Extension
Page Last Updated: June 23, 2014
 

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