CWE-1281 Base Incomplete

Sequence of Processor Instructions Leads to Unexpected Behavior

Certain sequences of valid and invalid processor instructions can cause the CPU to lock up or behave unpredictably, often requiring a hard reset to recover.

Definition

What is CWE-1281?

Certain sequences of valid and invalid processor instructions can cause the CPU to lock up or behave unpredictably, often requiring a hard reset to recover.
This issue arises when a processor's instruction set and internal logic aren't rigorously designed and tested. When the CPU encounters specific, problematic combinations of instructions—even if individual instructions are legal—it can enter a locked state or exhibit other erratic behavior instead of safely throwing an exception. This flaw sits at the intersection of hardware design and software execution, where the processor fails to handle edge-case instruction sequences gracefully. From a security perspective, this creates a critical vulnerability. An unprivileged user or program could deliberately craft these harmful instruction sequences to trigger a denial-of-service condition by freezing the CPU. Effective mitigation relies on hardware vendors identifying and correcting these logic flaws through microcode updates or processor revisions, as software workarounds are often limited.
Auswirkungen in der Praxis

Real-world CVEs caused by CWE-1281

  • CVE-2021-26339

    A bug in AMD CPU's core logic allows a potential DoS by using a specific x86 instruction sequence to hang the processor

  • CVE-1999-1476

    A bug in some Intel Pentium processors allow DoS (hang) via an invalid "CMPXCHG8B" instruction, causing a deadlock

Wie Angreifer es ausnutzen

Angreiferpfad Schritt für Schritt

  1. 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. [REF-1331]

  2. 2

    The Cyrix Coma bug was capable of trapping a Cyrix 6x86, 6x86L, or 6x86MX processor in an infinite loop. An infinite loop on a processor is not necessarily an issue on its own, as interrupts could stop the loop. However, on select Cyrix processors, the x86 Assembly 'xchg' instruction was designed to prevent interrupts. On these processors, if the loop was such that a new 'xchg' instruction entered the instruction pipeline before the previous one exited, the processor would become deadlocked. [REF-1323]

  3. 3

    The Motorola MC6800 microprocessor contained the first documented instance of a Halt and Catch Fire instruction - an instruction that causes the normal function of a processor to stop. If the MC6800 was given the opcode 0x9D or 0xDD, the processor would begin to read all memory very quickly, in sequence, and without executing any other instructions. This will cause the processor to become unresponsive to anything but a hard reset. [REF-1324]

  4. 4

    The example code is taken from the commit stage inside the processor core of the HACK@DAC'19 buggy CVA6 SoC [REF-1342]. To ensure the correct execution of atomic instructions, the CPU must guarantee atomicity: no other device overwrites the memory location between the atomic read starts and the atomic write finishes. Another device may overwrite the memory location only before the read operation or after the write operation, but never between them, and finally, the content will still be consistent.

  5. 5

    Atomicity is especially critical when the variable to be modified is a mutex, counting semaphore, or similar piece of data that controls access to shared resources. Failure to ensure atomicity may result in two processors accessing a shared resource simultaneously, permanent lock-up, or similar disastrous behavior.

Verwundbares Codebeispiel

Vulnerable x86 Assembly

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. [REF-1331]

Verwundbar x86 Assembly 
lock cmpxchg8b eax
Sicheres Codebeispiel

Secure Verilog

Refrain from interrupting if the intention is to commit an atomic instruction that should not be interrupted. This can be done by adding a condition to check whether the current committing instruction is atomic. [REF-1343]

Sicher Verilog 
```
if (csr_exception_i.valid && csr_exception_i.cause[63] && !amo_valid_commit_o && commit_instr_i[0].fu != CSR) begin** 
  ```
  	 exception_o = csr_exception_i;
  	 exception_o.tval = commit_instr_i[0].ex.tval;
   end
What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Präventions-Checkliste

How to prevent CWE-1281

  • 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.
  • 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].
Erkennungssignale

How to detect CWE-1281

SAST High

Führe statische Analyse (SAST) auf der Codebasis aus und suche im Datenfluss nach dem unsicheren Muster.

DAST Moderate

Führe dynamische Application-Security-Tests gegen den Live-Endpoint aus.

Runtime Moderate

Beobachte Runtime-Logs auf ungewöhnliche Exception-Traces, fehlerhafte Eingaben oder Versuche, Autorisierung zu umgehen.

Code review Moderate

Code Review: Markiere jeden neuen Code, der Eingaben von dieser Oberfläche ohne validierte Framework-Helper verarbeitet.

Plexicus Auto-Fix

Plexicus erkennt CWE-1281 automatisch und öffnet in unter 60 Sekunden einen Fix-PR.

Codex Remedium scannt jeden Commit, identifiziert genau diese Schwachstelle und liefert einen reviewer-ready Pull Request mit dem Patch. Keine Tickets. Keine Hand-offs.

Demo anfordern Plexicus kostenlos testen
Häufig gestellte Fragen

Frequently asked questions

Was ist CWE-1281?

Certain sequences of valid and invalid processor instructions can cause the CPU to lock up or behave unpredictably, often requiring a hard reset to recover.

Wie gravierend ist CWE-1281?

MITRE hat für diese Schwachstelle keine Exploit-Wahrscheinlichkeit veröffentlicht. Behandle sie als mittlere Auswirkung, bis dein Threat Model anderes belegt.

Welche Sprachen oder Plattformen sind von CWE-1281 betroffen?

MITRE lists the following affected platforms: Not OS-Specific, Not Architecture-Specific, Not Technology-Specific, Processor Hardware.

Wie kann ich CWE-1281 verhindern?

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. Patch operating system to avoid running Halt and Catch Fire type sequences or to mitigate the damage caused by unexpected behavior. See [REF-1108].

Wie erkennt und behebt Plexicus CWE-1281?

Die SAST-Engine von Plexicus erkennt die Datenfluss-Signatur von CWE-1281 bei jedem Commit. Bei einem Treffer öffnet unser Codex-Remedium-Agent einen Fix-PR mit korrigiertem Code, Tests und einer einzeiligen Zusammenfassung für den Reviewer.

Wo erfahre ich mehr über CWE-1281?

MITRE veröffentlicht die kanonische Definition unter https://cwe.mitre.org/data/definitions/1281.html. Für ergänzende Hinweise kannst du auch die OWASP- und NIST-Dokumentation heranziehen.

Verwandte Schwachstellen

Weaknesses related to CWE-1281

CWE-691 Parent

Insufficient Control Flow Management

This vulnerability occurs when a program's execution flow isn't properly managed, allowing attackers to bypass critical checks, trigger…

CWE-1265 Sibling

Unintended Reentrant Invocation of Non-reentrant Code Via Nested Calls

This vulnerability occurs when a non-reentrant function is called, and during its execution, another call is triggered that unexpectedly…

CWE-362 Sibling

Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')

A race condition occurs when multiple processes or threads access a shared resource simultaneously without proper coordination, creating a…

CWE-430 Sibling

Deployment of Wrong Handler

This vulnerability occurs when a system incorrectly assigns or routes an object to the wrong processing component.

CWE-431 Sibling

Missing Handler

This vulnerability occurs when a software component lacks the necessary code to properly handle an error or unexpected event.

CWE-662 Sibling

Improper Synchronization

This vulnerability occurs when a multi-threaded or multi-process application allows shared resources to be accessed by multiple threads or…

CWE-670 Sibling

Always-Incorrect Control Flow Implementation

This weakness occurs when a section of code is structured in a way that always executes incorrectly, regardless of input or conditions.…

CWE-696 Sibling

Incorrect Behavior Order

This weakness occurs when a system executes multiple dependent actions in the wrong sequence, leading to unexpected and potentially…

CWE-705 Sibling

Incorrect Control Flow Scoping

This vulnerability occurs when a program fails to return execution to the correct point in the code after finishing a specific operation…

Ressourcen

Further reading

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