COMMON WEAKNESS ENUMERATIONDraft

CWE-196

Unsigned to Signed Conversion Error

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.

Abstraction

Variant

Structure

Simple

Likelihood of Exploit

Medium

Understand the problem

What is the problem?

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.

Extended description

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.

Root cause

How and when does the problem arise?

Lifecycle phase

Implementation

Risk

What does successful exploitation lead to?

DoS: Crash, Exit, or Restart

Affected security scopes: Availability

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

Affected security scopes: Integrity

If a poor cast lead to a buffer overflow or similar condition, data integrity may be affected.
Execute Unauthorized Code or CommandsBypass Protection Mechanism

Affected security scopes: Integrity, Confidentiality, Availability, Access Control

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.

Defence

How is it prevented and fixed?

Requirements

Security requirement

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

Architecture and Design

Architecture and design approach

Detail
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

Implementation-phase defence

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

Verification

How is it detected?

Automated Static Analysis

High
Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)