The software 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.
Time of Introduction
Technical Impact: DoS: crash / exit /
Incorrect sign conversions generally lead to undefined behavior, and
Technical Impact: Modify memory
If a poor cast lead to a buffer overflow or similar condition, data
integrity may be affected.
Technical Impact: Execute unauthorized code or
commands; Bypass protection
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.
Likelihood of Exploit
Choose a language which is not subject to these casting flaws.
Phase: 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.
Error check the return values of all functions. Be aware of implicit
casts made, and use unsigned variables for sizes if at all