CWE-1281 Base Incompleto

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.

Definición

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.
Impacto en el mundo real

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

Cómo lo explotan los atacantes

Ruta del atacante paso a paso

  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.

Ejemplo de código vulnerable

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]

Vulnerable x86 Assembly 
lock cmpxchg8b eax
Ejemplo de código seguro

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]

Seguro 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.
Lista de prevención

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].
Señales de detección

How to detect CWE-1281

SAST High

Ejecuta análisis estático (SAST) sobre el código buscando el patrón inseguro en el flujo de datos.

DAST Moderate

Ejecuta pruebas dinámicas de seguridad de aplicaciones (DAST) contra el endpoint en vivo.

Runtime Moderate

Vigila los logs en tiempo de ejecución para detectar trazas de excepción inusuales, entradas malformadas o intentos de bypass de autorización.

Code review Moderate

Revisión de código: marca cualquier código nuevo que maneje entrada desde esta superficie sin usar los helpers validados del framework.

Auto-corrección de Plexicus

Plexicus detecta automáticamente CWE-1281 y abre un PR de corrección en menos de 60 segundos.

Codex Remedium escanea cada commit, identifica esta debilidad concreta y entrega un pull request listo para revisión con el parche. Sin tickets. Sin traspasos.

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Preguntas frecuentes

Frequently asked questions

¿Qué es 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.

¿Qué gravedad tiene CWE-1281?

MITRE no ha publicado una calificación de probabilidad de explotación para esta debilidad. Trátala como de impacto medio hasta que tu modelo de amenazas demuestre lo contrario.

¿Qué lenguajes o plataformas se ven afectados por CWE-1281?

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

¿Cómo puedo prevenir CWE-1281?

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].

¿Cómo detecta y corrige Plexicus CWE-1281?

El motor SAST de Plexicus detecta la firma de flujo de datos para CWE-1281 en cada commit. Cuando hay coincidencia, nuestro agente Codex Remedium abre un PR de corrección con el código corregido, las pruebas y un resumen de una línea para el revisor.

¿Dónde puedo aprender más sobre CWE-1281?

MITRE publica la definición canónica en https://cwe.mitre.org/data/definitions/1281.html. También puedes consultar la documentación de OWASP y NIST para guías relacionadas.

Debilidades relacionadas

Weaknesses related to CWE-1281

CWE-691 Padre

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 Hermano

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 Hermano

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 Hermano

Deployment of Wrong Handler

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

CWE-431 Hermano

Missing Handler

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

CWE-662 Hermano

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 Hermano

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 Hermano

Incorrect Behavior Order

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

CWE-705 Hermano

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…

Recursos

Further reading

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