Common Weakness Enumeration

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CWE-1281: Sequence of Processor Instructions Leads to Unexpected Behavior (Halt and Catch Fire)

Weakness ID: 1281
Abstraction: Base
Structure: Simple
Status: Incomplete
Presentation Filter:
+ Description
Specific combinations of processor instructions lead to undesirable behavior such as locking the processor until a hard reset performed.
+ Extended Description

If the instruction set architecture (ISA) and processor logic are not designed carefully, and tested thoroughly, certain combinations of instructions may lead to locking the processor or other unexpected and undesirable behavior. Upon encountering unimplemented instruction opcodes or illegal instruction operands the processor should throw an exception and carry on without negatively impacting security. However, specific combinations of legal and illegal instructions may cause unexpected behavior with security implications such as allowing unprivileged programs to completely lock the CPU.

Some examples are the Pentium f00f bug, MC6800 HCF, the Cyrix comma bug, and more generally other "Halt and Catch Fire" instructions.

+ Relationships

The table(s) below 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)
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.691Insufficient Control Flow Management
+ Relevant to the view "Hardware Design" (CWE-1194)
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.1201Core and Compute Issues
+ Modes Of Introduction

The 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.

Architecture and DesignUnexpected behavior from certain instruction combinations can arise from bugs in the ISA
ImplementationUnexpected behavior from certain instruction combinations can arise because of implementation details such as speculative execution, caching etc.
+ Applicable Platforms
The listings below show 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.


Class: Language-Independent (Undetermined Prevalence)

Operating Systems

Class: OS-Independent (Undetermined Prevalence)


Class: Architecture-Independent (Undetermined Prevalence)


Class: Technology-Independent (Undetermined Prevalence)

+ Common Consequences

The table below 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.


Technical Impact: Varies by Context

+ Demonstrative Examples

Example 1

The Pentium F00F bug is a real-world example of how a sequence of instructions can lock a processor. The “cmpxchg8b” instruction compares contents of registers with a memory location. The operand is expected to be a memory location, but in the bad code snippet it is the eax register. Because the specified operand is illegal, an exception is generated, which is the correct behavior and not a security issue in itself. However, when prefixed with the “lock” instruction, the processor deadlocks because locked memory transactions require a read and write pair of transactions to occur before the lock on the memory bus is released. The exception causes a read to occur but there is no corresponding write, as there would have been if a legal operand had been supplied to the cmpxchg8b instruction.

(bad code)
Example Language: Other 
lock cmpxchg8b eax
+ Observed Examples
A bug in Intel Pentium processor (MMX and Overdrive) allows local users to cause a denial of service (hang) in Intel-based operating systems such as Windows NT and Windows 95, via an invalid instruction, aka the "Invalid Operand with Locked CMPXCHG8B Instruction" problem.
+ Potential Mitigations

Phase: Testing

Implement a rigorous testing strategy that incorporates randomization to explore instruction sequences that are unlikely to appear in normal workloads in order to identify halt and catch fire instruction sequences.

Phase: Patching and Maintenance

Patch operating system to avoid running Halt and Catch Fire type sequences or to mitigate the damage caused by unexpected behavior. See [REF-1108].
+ References
[REF-1108] Intel Corporation. "Deep Dive: Retpoline: A Branch Target Injection Mitigation". <>.
+ Content History
+ Submissions
Submission DateSubmitterOrganization
2020-05-15Nicole FernTortuga Logic
+ Modifications
Modification DateModifierOrganization
2020-08-20CWE Content TeamMITRE
updated Related_Attack_Patterns
2021-03-15CWE Content TeamMITRE
updated Potential_Mitigations
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Page Last Updated: March 15, 2021