Unsigned to signed conversion error

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ASDR Table of Contents

Last revision (mm/dd/yy): 02/19/2009


An unsigned-to-signed conversion error takes place when a large unsigned primitive is used as an signed value - usually as a size variable.


  • Availability: Incorrect sign conversions generally lead to undefined behavior, and therefore crashes.
  • Integrity: If a poor cast lead to a buffer underwrite, data integrity may be affected.
  • Access control (instruction processing): Improper unsigned-to-signed conversions, often create buffer underwrite conditions which can be used to execute arbitrary code. This is usually outside the scope of a program's implicit security policy.

Exposure period

  • Requirements specification: The choice could be made to use a language that is not susceptible to these issues.
  • Design: Accessor functions may be designed to mitigate some of these logical issues.
  • Implementation: Many logic errors can lead to this condition. It can be exacerbated by lack of or misuse of mitigating technologies.


  • Languages: C, C++, Fortran, Assembly
  • Operating platforms: All

Required resources




Likelihood of exploit

Low to Medium

Although less frequent an issue than signed-to-unsigned casting, unsigned-to-signed casting 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.

Risk Factors



While not exploitable, the following program is an excellent example of how implicit casts, while not changing the value stored, significantly changes its use:

#include <stdio.h>
int main() {   
  int value;    
  value = (int)(~((int)0) ^ (1 << (sizeof(int)*8)));    

  printf("Max unsigned int: %u %1$x\nNow signed: %1$d %1$x\n",
  return (0);

The above code produces the following output:

Max unsigned int: 4294967295 ffffffff
Now signed: -1 ffffffff

Note how the hex value remains unchanged.

Related Attacks

Related Vulnerabilities

Related Controls

  • Requirements specification: The choice could be made to use a language that is not susceptible to these issues.
  • Design: Ensure that interacting functions retain the same types and that only safe type casts must occur. If possible, use intelligent marshalling routines to translate between objects.
  • Implementation: Use out-of-data band channels for transmitting error messages if unsigned size values must be transmitted. Check all errors.
  • Build: Pay attention to compiler warnings which may alert you to improper type casting.

Related Technical Impacts