Unsigned to Signed Conversion Error

Opis

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

Although less frequent an issue than signed-to-unsigned conversion, unsigned-to-signed conversion can be the perfect precursor to dangerous buffer underwrite conditions that allow attackers to move down the stack where they otherwise might not have access in a normal buffer overflow condition. Buffer underwrites occur frequently when large unsigned values are cast to signed values, and then used as indexes into a buffer or for pointer arithmetic.

Skutki

• Availability

  DoS: Crash, Exit, or Restart

  Incorrect sign conversions generally lead to undefined behavior, and therefore crashes.

• Integrity

  Modify Memory

  If a poor cast lead to a buffer overflow or similar condition, data integrity may be affected.

• Integrity, Confidentiality, Availability, Access Control

  Execute Unauthorized Code or Commands; Bypass Protection Mechanism

  Improper signed-to-unsigned conversions without proper checking can sometimes trigger buffer overflows which can be used to execute arbitrary code. This is usually outside the scope of a program's implicit security policy.

Zapobieganie

• Requirements

  Choose a language which is not subject to these casting flaws.

• Architecture and Design

  Design object accessor functions to implicitly check values for valid sizes. Ensure that all functions which will be used as a size are checked previous to use as a size. If the language permits, throw exceptions rather than using in-band errors.

• Implementation

  Error check the return values of all functions. Be aware of implicit casts made, and use unsigned variables for sizes if at all possible.

Słabość nadrzędna

• CWE-681 Incorrect Conversion between Numeric Types

Może też być

• CWE-124 Buffer Underwrite ('Buffer Underflow')
• CWE-120 Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')

Kiedy powstaje

• Implementation

Dotyczy

C, C++

Źródła

1. The Art of Software Security Assessment — Mark Dowd, John McDonald, Justin Schuh, Chapter 6, "Type Conversions", Page 223, Addison Wesley, 2006
2. The CLASP Application Security Process — Secure Software, Inc., 2005