CWE-196Unsigned to Signed Conversion Error

Description

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.

Common consequences

• 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.

Potential mitigations

• 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.

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