CWE

Common Weakness Enumeration

A Community-Developed List of Software Weakness Types

CWE/SANS Top 25 Most Dangerous Software Errors
Home > CWE List > CWE- Individual Dictionary Definition (3.0)  
ID

CWE-194: Unexpected Sign Extension

Weakness ID: 194
Abstraction: Base
Structure: Simple
Status: Incomplete
Presentation Filter:
+ Description
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.
+ Relationships

The table(s) below shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.

+ Relevant to the view "Research Concepts" (CWE-1000)
NatureTypeIDName
ChildOfClassClass681Incorrect Conversion between Numeric Types
+ Relevant to the view "Development Concepts" (CWE-699)
NatureTypeIDName
ChildOfClassClass681Incorrect Conversion between Numeric Types
+ Modes Of Introduction

The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the software life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.

PhaseNote
Implementation
+ Applicable Platforms
The listings below show possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.

Languages

C: (Undetermined Prevalence)

C++: (Undetermined Prevalence)

+ Common Consequences

The table below specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.

ScopeImpactLikelihood
Integrity
Confidentiality
Availability
Other

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)
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).

+ 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.
+ Memberships
This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.
NatureTypeIDName
MemberOfCategoryCategory998SFP Secondary Cluster: Glitch in Computation
+ Notes

Maintenance

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.

Relationship

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
Software Fault PatternsSFP1Glitch in computation
CERT C Secure CodingINT31-CCWE More SpecificEnsure that integer conversions do not result in lost or misinterpreted data
+ References
[REF-161] 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>.
[REF-162] Robert Seacord. "Integral Security". 2006-11-03. <http://www.ddj.com/security/193501774>.
+ Content History
Submissions
Submission DateSubmitterOrganizationSource
CLASP
Modifications
Modification DateModifierOrganizationSource
2008-09-08CWE Content TeamMITRE
updated Applicable_Platforms, Common_Consequences, Description, Relationships, Taxonomy_Mappings
2008-11-05CWE Content TeamMITRE
complete rewrite of the entire entry
2008-11-24CWE Content TeamMITRE
updated Common_Consequences, Demonstrative_Examples, Description, Maintenance_Notes, Name, Observed_Examples, Potential_Mitigations, References, Relationship_Notes, Relationships
2009-05-27CWE Content TeamMITRE
updated Demonstrative_Examples
2009-10-29CWE Content TeamMITRE
updated Demonstrative_Examples
2010-04-05CWE Content TeamMITRE
updated Demonstrative_Examples
2010-12-13CWE Content TeamMITRE
updated Applicable_Platforms
2011-06-01CWE Content TeamMITRE
updated Common_Consequences
2012-05-11CWE Content TeamMITRE
updated Demonstrative_Examples, Relationships
2014-07-30CWE Content TeamMITRE
updated Relationships, Taxonomy_Mappings
2017-11-08CWE Content TeamMITRE
updated References, Taxonomy_Mappings
Previous Entry Names
Change DatePrevious Entry Name
2008-04-11Sign Extension Error
2008-11-24Incorrect Sign Extension

More information is available — Please select a different filter.
Page Last Updated: November 14, 2017