CWE-1421 Base Incomplete

Exposure of Sensitive Information in Shared Microarchitectural Structures during Transient Execution

This vulnerability occurs when a processor's speculative execution (transient operations) can temporarily access restricted data from another security domain. This sensitive information can leave…

Definition

What is CWE-1421?

This vulnerability occurs when a processor's speculative execution (transient operations) can temporarily access restricted data from another security domain. This sensitive information can leave traces in shared hardware structures like CPU caches, where an attacker could potentially retrieve it using a covert channel attack.
Modern processors use hardware features like virtual memory and privilege rings to create secure boundaries between applications, operating systems, and virtual machines. These features are designed to prevent unauthorized access to sensitive data. However, underlying hardware components like CPU caches are often shared across these boundaries for performance reasons, creating a potential conflict between security design and hardware optimization. During speculative execution, the processor may temporarily bypass these security boundaries and access protected data, leaving microarchitectural footprints such as cache state changes. An attacker who can trigger this speculative access and then monitor these hardware side effects through timing analysis or other covert channels can infer the victim's confidential information. This could include private application data, kernel secrets, memory addresses, or system configuration details that should remain isolated.
Real-world impact

Real-world CVEs caused by CWE-1421

  • CVE-2017-5715

    A fault may allow transient user-mode operations to access kernel data cached in the L1D, potentially exposing the data over a covert channel.

  • CVE-2018-3615

    A fault may allow transient non-enclave operations to access SGX enclave data cached in the L1D, potentially exposing the data over a covert channel.

  • CVE-2019-1135

    A TSX Asynchronous Abort may allow transient operations to access architecturally restricted data, potentially exposing the data over a covert channel.

How attackers exploit it

Step-by-step attacker path

  1. 1

    Some processors may perform access control checks in parallel with memory read/write operations. For example, when a user-mode program attempts to read data from memory, the processor may also need to check whether the memory address is mapped into user space or kernel space. If the processor performs the access concurrently with the check, then the access may be able to transiently read kernel data before the check completes. This race condition is demonstrated in the following code snippet from [REF-1408], with additional annotations:

  2. 2

    Vulnerable processors may return kernel data from a shared microarchitectural resource in line 4, for example, from the processor's L1 data cache. Since this vulnerability involves a race condition, the mov in line 4 may not always return kernel data (that is, whenever the check "wins" the race), in which case this demonstration code re-attempts the access in line 6. The accessed data is multiplied by 4KB, a common page size, to make it easier to observe via a cache covert channel after the transmission in line 7. The use of cache covert channels to observe the side effects of transient execution has been described in [REF-1408].

  3. 3

    Many commodity processors share microarchitectural fill buffers between sibling hardware threads on simultaneous multithreaded (SMT) processors. Fill buffers can serve as temporary storage for data that passes to and from the processor's caches. Microarchitectural Fill Buffer Data Sampling (MFBDS) is a vulnerability that can allow a hardware thread to access its sibling's private data in a shared fill buffer. The access may be prohibited by the processor's ISA, but MFBDS can allow the access to occur during transient execution, in particular during a faulting operation or an operation that triggers a microcode assist. More information on MFBDS can be found in [REF-1405] and [REF-1409].

  4. 4

    Some processors may allow access to system registers (for example, system coprocessor registers or model-specific registers) during transient execution. This scenario is depicted in the code snippet below. Under ordinary operating circumstances, code in exception level 0 (EL0) is not permitted to access registers that are restricted to EL1, such as TTBR0_EL1. However, on some processors an earlier mis-prediction can cause the MRS instruction to transiently read the value in an EL1 register. In this example, a conditional branch (line 2) can be mis-predicted as "not taken" while waiting for a slow load (line 1). This allows MRS (line 3) to transiently read the value in the TTBR0_EL1 register. The subsequent memory access (line 6) can allow the restricted register's value to become observable, for example, over a cache covert channel. Code snippet is from [REF-1410]. See also [REF-1411].

Vulnerable code example

Vulnerable x86 Assembly

Some processors may perform access control checks in parallel with memory read/write operations. For example, when a user-mode program attempts to read data from memory, the processor may also need to check whether the memory address is mapped into user space or kernel space. If the processor performs the access concurrently with the check, then the access may be able to transiently read kernel data before the check completes. This race condition is demonstrated in the following code snippet from [REF-1408], with additional annotations:

Vulnerable x86 Assembly 
1 ; rcx = kernel address, rbx = probe array
 2 xor rax, rax # set rax to 0
 3 retry:
 4 mov al, byte [rcx] # attempt to read kernel memory
 5 shl rax, 0xc # multiply result by page size (4KB)
 6 jz retry # if the result is zero, try again
 7 mov rbx, qword [rbx + rax] # transmit result over a cache covert channel
Secure code example

Secure pseudo

Secure pseudo 
// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
  const safe = validateAndEscape(input);
  return executeWithGuards(safe);
}
What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Prevention checklist

How to prevent CWE-1421

  • Architecture and Design Hardware designers may choose to engineer the processor's pipeline to prevent architecturally restricted data from being used by operations that can execute transiently.
  • Architecture and Design Hardware designers may choose not to share microarchitectural resources that can contain sensitive data, such as fill buffers and store buffers.
  • Architecture and Design Hardware designers may choose to sanitize specific microarchitectural state (for example, store buffers) when the processor transitions to a different context, such as whenever a system call is invoked. Alternatively, the hardware may expose instruction(s) that allow software to sanitize microarchitectural state according to the user or system administrator's threat model. These mitigation approaches are similar to those that address CWE-226; however, sanitizing microarchitectural state may not be the optimal or best way to mitigate this weakness on every processor design.
  • Architecture and Design The hardware designer can attempt to prevent transient execution from causing observable discrepancies in specific covert channels.
  • Architecture and Design Software architects may design software to enforce strong isolation between different contexts. For example, kernel page table isolation (KPTI) mitigates the Meltdown vulnerability [REF-1401] by separating user-mode page tables from kernel-mode page tables, which prevents user-mode processes from using Meltdown to transiently access kernel memory [REF-1404].
  • Build and Compilation If the weakness is exposed by a single instruction (or a small set of instructions), then the compiler (or JIT, etc.) can be configured to prevent the affected instruction(s) from being generated, and instead generate an alternate sequence of instructions that is not affected by the weakness.
  • Build and Compilation Use software techniques (including the use of serialization instructions) that are intended to reduce the number of instructions that can be executed transiently after a processor event or misprediction.
  • Implementation System software can mitigate this weakness by invoking state-sanitizing operations when switching from one context to another, according to the hardware vendor's recommendations.
Detection signals

How to detect CWE-1421

Manual Analysis Moderate

This weakness can be detected in hardware by manually inspecting processor specifications. Features that exhibit this weakness may include microarchitectural predictors, access control checks that occur out-of-order, or any other features that can allow operations to execute without committing to architectural state. Academic researchers have demonstrated that new hardware weaknesses can be discovered by examining publicly available patent filings, for example [REF-1405] and [REF-1406]. Hardware designers can also scrutinize aspects of the instruction set architecture that have undefined behavior; these can become a focal point when applying other detection methods.

Automated Analysis Moderate

This weakness can be detected (pre-discovery) in hardware by employing static or dynamic taint analysis methods [REF-1401]. These methods can label data in one context (for example, kernel data) and perform information flow analysis (or a simulation, etc.) to determine whether tainted data can appear in another context (for example, user mode). Alternatively, stale or invalid data in shared microarchitectural resources can be marked as tainted, and the taint analysis framework can identify when transient operations encounter tainted data.

Automated Analysis High

Software vendors can release tools that detect presence of known weaknesses (post-discovery) on a processor. For example, some of these tools can attempt to transiently execute a vulnerable code sequence and detect whether code successfully leaks data in a manner consistent with the weakness under test. Alternatively, some hardware vendors provide enumeration for the presence of a weakness (or lack of a weakness). These enumeration bits can be checked and reported by system software. For example, Linux supports these checks for many commodity processors: $ cat /proc/cpuinfo | grep bugs | head -n 1 bugs : cpu_meltdown spectre_v1 spectre_v2 spec_store_bypass l1tf mds swapgs taa itlb_multihit srbds mmio_stale_data retbleed

Fuzzing Opportunistic

Academic researchers have demonstrated that this weakness can be detected in hardware using software fuzzing tools that treat the underlying hardware as a black box ([REF-1406], [REF-1430])

Plexicus auto-fix

Plexicus auto-detects CWE-1421 and opens a fix PR in under 60 seconds.

Codex Remedium scans every commit, identifies this exact weakness, and ships a reviewer-ready pull request with the patch. No tickets. No hand-offs.

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Frequently asked questions

Frequently asked questions

What is CWE-1421?

This vulnerability occurs when a processor's speculative execution (transient operations) can temporarily access restricted data from another security domain. This sensitive information can leave traces in shared hardware structures like CPU caches, where an attacker could potentially retrieve it using a covert channel attack.

How serious is CWE-1421?

MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.

What languages or platforms are affected by CWE-1421?

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

How can I prevent CWE-1421?

Hardware designers may choose to engineer the processor's pipeline to prevent architecturally restricted data from being used by operations that can execute transiently. Hardware designers may choose not to share microarchitectural resources that can contain sensitive data, such as fill buffers and store buffers.

How does Plexicus detect and fix CWE-1421?

Plexicus's SAST engine matches the data-flow signature for CWE-1421 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.

Where can I learn more about CWE-1421?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/1421.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-1421

CWE-1420 Parent

Exposure of Sensitive Information during Transient Execution

Transient execution vulnerabilities occur when a processor speculatively runs operations that don't officially commit, potentially leaking…

CWE-1422 Sibling

Exposure of Sensitive Information caused by Incorrect Data Forwarding during Transient Execution

This vulnerability occurs when a CPU incorrectly forwards outdated or incorrect data during speculative execution. This allows sensitive…

CWE-1423 Sibling

Exposure of Sensitive Information caused by Shared Microarchitectural Predictor State that Influences Transient Execution

This vulnerability occurs when separate software components, like different processes or virtual machines, share the processor's internal…

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Further reading

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