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Common Weakness Enumeration

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ID

CWE-1335: Incorrect Bitwise Shift of Integer

Weakness ID: 1335
Abstraction: Base
Structure: Simple
Status: Draft
Presentation Filter:
+ Description
An integer value is specified to be shifted by a negative amount or an amount greater than or equal to the number of bits contained in the value causing an unexpected or indeterminate result.
+ Extended Description

Specifying a value to be shifted by a negative amount is undefined in various languages. Various computer architectures implement this action in different ways. The compilers and interpreters when generating code to accomplish a shift generally do not do a check for this issue.

Specifying an over-shift, a shift greater than or equal to the number of bits contained in a value to be shifted, produces a result which varies by architecture and compiler. In some languages, this action is specifically listed as producing an undefined result.

+ Relationships
Section HelpThis table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.
+ Relevant to the view "Research Concepts" (CWE-1000)
NatureTypeIDName
ChildOfPillarPillar - a weakness that is the most abstract type of weakness and represents a theme for all class/base/variant weaknesses related to it. A Pillar is different from a Category as a Pillar is still technically a type of weakness that describes a mistake, while a Category represents a common characteristic used to group related things.682Incorrect Calculation
+ Modes Of Introduction
Section HelpThe different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.
PhaseNote
ImplementationAdding shifts without properly verifying the size and sign of the shift amount.
+ Applicable Platforms
Section HelpThis listing shows possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.

Languages

C (Undetermined Prevalence)

C++ (Undetermined Prevalence)

C# (Undetermined Prevalence)

Java (Undetermined Prevalence)

JavaScript (Undetermined Prevalence)

Operating Systems

Class: OS-Independent (Undetermined Prevalence)

Technologies

Class: Technology-Independent (Undetermined Prevalence)

+ Common Consequences
Section HelpThis table specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.
ScopeImpactLikelihood
Integrity

Technical Impact: DoS: Crash, Exit, or Restart

+ Demonstrative Examples

Example 1

A negative shift amount for an x86 or x86_64 shift instruction will produce the number of bits to be shifted by taking a 2's-complement of the shift amount and effectively masking that amount to the lowest 6 bits for a 64 bit shift instruction.

(bad code)
Example Language:
unsigned int r = 1 << -5;

The example above ends up with a shift amount of -5. The hexadecimal value is FFFFFFFFFFFFFFFD which, when bits above the 6th bit are masked off, the shift amount becomes a binary shift value of 111101 which is 61 decimal. A shift of 61 produces a very different result than -5. The previous example is a very simple version of the following code which is probably more realistic of what happens in a real system.

(bad code)
Example Language:
int choose_bit(int reg_bit, int bit_number_from_elsewhere)
{
  if (NEED_TO_SHIFT)
  {
    reg_bit -= bit_number_from_elsewhere;
  }
  return reg_bit;
}
unsigned int handle_io_register(unsigned int *r)
{

  unsigned int the_bit = 1 << choose_bit(5, 10);
  *r |= the_bit;
  return the_bit;
}
(good code)
Example Language:
int choose_bit(int reg_bit, int bit_number_from_elsewhere)
{
  if (NEED_TO_SHIFT)
  {
    reg_bit -= bit_number_from_elsewhere;
  }
  return reg_bit;
}

unsigned int handle_io_register(unsigned int *r)
{
  int the_bit_number = choose_bit(5, 10);
  if ((the_bit_number > 0) && (the_bit_number < 63))
  {
    unsigned int the_bit = 1 << the_bit_number;
    *r |= the_bit;
  }
  return the_bit;
}

Note that the good example not only checks for negative shifts and disallows them but also for over-shifts. Not bit operation is done if the shift is out of bounds. Depending on the program, perhaps an error message should be logged.

+ Observed Examples
ReferenceDescription
An unexpected large value in the ext4 filesystem causes an overshift condition resulting in a divide by zero.
An unexpected large value in the ext4 filesystem causes an overshift condition resulting in a divide by zero - fix of CVE-2009-4307.
An overshift in a kernel a allowed out of bounds reads and writes resulting in a root takeover.
Program is not properly handling signed bitwise left-shifts causing an overlapping memcpy memory range error.
Compression function improperly executes a signed left shift of a negative integer.
Some kernels improperly handle right shifts of 32 bit numbers in a 64 bit register.
Putty has an incorrectly sized shift value resulting in an overshift.
LED driver overshifts under certain conditions resulting in a DoS.
+ Potential Mitigations

Phase: Implementation

Implicitly or explicitly add checks and mitigation for negative or over-shift values.
+ Content History
+ Submissions
Submission DateSubmitterOrganization
2021-03-29CWE Content TeamMITRE
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Page Last Updated: July 20, 2021