Difference between revisions of "Sign extension error"

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Revision as of 13:35, 16 May 2006


Overview

If one extends a signed number incorrectly, if negative numbers are used, an incorrect extension may result.

Consequences

  • Integrity: If one attempts to sign extend a negative variable with an unsigned extension algorithm, it will produce an incorrect result.
  • Authorization: Sign extension errors - if they are used to collect information from smaller signed sources - can often create buffer overflows and other memory based problems.

Exposure period

  • Requirements section: The choice to use a language which provides a framework to deal with this could be used.
  • Implementation: A logical flaw of this kind might lead to any number of other flaws.

Platform

  • Languages: C or C++
  • Operating platforms: Any

Required resources

Any

Severity

High

Likelihood of exploit

High

Avoidance and mitigation

  • Implementation: Use a sign extension library or standard function to extend signed numbers.
  • Implementation: When extending signed numbers fill in the new bits with 0 if the sign bit is 0 or fill the new bits with 1 if the sign bit is 1.

Discussion

Sign extension errors - if they are used to collect information from smaller signed sources - can often create buffer overflows and other memory based problems.

Examples

In C:

struct fakeint {
  short f0;
  short zeros;
};
struct fakeint strange;
struct fakeint strange2;

strange.f0=-240;
strange2.f0=240;

strange2.zeros=0;
strange.zeros=0;

printf("%d %d\n",strange.f0,strange);
printf("%d %d\n",strange2.f0,strange2);

Related problems

Not available.


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