From the following link:
Noncompliant Code Example (Right Shift)
The result of
E1 >> E2 is
E2 bit positions. If
E1 has an unsigned type or if
E1 has a signed type and a nonnegative value, the value of the result is the integral part of the quotient of E1 / 2E2. If
E1 has a signed type and a negative value, the resulting value is implementation defined and can be either an arithmetic (signed) shift:
Or a logical (unsigned) shift:
This noncompliant code example fails to test whether the right operand is greater than or equal to the width of the promoted left operand, allowing undefined behavior.
unsigned int ui1; unsigned int ui2; unsigned int uresult; /* Initialize ui1 and ui2 */ uresult = ui1 >> ui2;
Making assumptions about whether a right shift is implemented as an arithmetic (signed) shift or a logical (unsigned) shift can also lead to vulnerabilities. See recommendation INT13-C. Use bitwise operators only on unsigned operands.