Unexpected Sign Extension

Opis

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.

Skutki

• Integrity, Confidentiality, Availability, Other

  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.

Zapobieganie

• 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.

Przykłady podatności

• CVE-2018-10887 Chain: unexpected sign extension (CWE-194) leads to integer overflow (CWE-190), causing an out-of-bounds read (CWE-125)
• CVE-1999-0234 Sign extension error produces -1 value that is treated as a command separator, enabling OS command injection.
• CVE-2003-0161 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.
• CVE-2007-4988 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.
• CVE-2006-1834 chain: signedness error allows bypass of a length check; later sign extension makes exploitation easier.
• CVE-2005-2753 Sign extension when manipulating Pascal-style strings leads to integer overflow and improper memory copy.

Słabość nadrzędna

• CWE-681 Incorrect Conversion between Numeric Types

Kiedy powstaje

• Implementation

Dotyczy

C, C++

Uwagi

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.

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.

Źródła

1. C Language Issues for Application Security — John McDonald, Mark Dowd, Justin Schuh, 2008
2. Integral Security — Robert Seacord, 2006
3. The CLASP Application Security Process — Secure Software, Inc., 2005