Unexpected Sign Extension
Incomplete Variant
Structure: Simple
Description
The product 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.
Common Consequences 1
Scope: IntegrityConfidentialityAvailabilityOther
Impact: Read MemoryModify MemoryOther
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.
Potential Mitigations 1
Phase: Implementation
Avoid using signed variables if you don't need to represent negative values. When negative values are needed, perform validation after you save those values to larger data types, or before passing them to functions that are expecting unsigned values.
Demonstrative Examples 1
ID : DX-23
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.Code Example:
Bad
C
cThis code first exhibits an example of Numeric Range Comparison Without Minimum Check, 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 (Improper Restriction of Operations within the Bounds of a Memory Buffer).
Observed Examples 6
CVE-2018-10887Chain: unexpected sign extension (Unexpected Sign Extension) leads to integer overflow (Integer Overflow or Wraparound), causing an out-of-bounds read (Out-of-bounds Read)
CVE-1999-0234Sign extension error produces -1 value that is treated as a command separator, enabling OS command injection.
CVE-2003-0161Product 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.
CVE-2007-4988chain: 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.
CVE-2006-1834chain: signedness error allows bypass of a length check; later sign extension makes exploitation easier.
CVE-2005-2753Sign extension when manipulating Pascal-style strings leads to integer overflow and improper memory copy.
References 3
C Language Issues for Application Security
John McDonald, Mark Dowd, and Justin Schuh
25-01-2008
ID: REF-161
Integral Security
Robert Seacord
03-11-2006
ID: REF-162
The CLASP Application Security Process
Secure Software, Inc.
2005
ID: REF-18
Likelihood of Exploit
High
Applicable Platforms
Languages:
C : UndeterminedC++ : Undetermined
Modes of Introduction
Implementation
Related Weaknesses
Taxonomy Mapping
- CLASP
- Software Fault Patterns
- CERT C Secure Coding
Notes
RelationshipSign 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.
MaintenanceThis entry is closely associated with signed-to-unsigned conversion errors (Signed to Unsigned Conversion Error) and other numeric errors. These relationships need to be more closely examined within CWE.