8.9

CVSS Score

4.0

-

CVSS Score

Basic Information

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CVE-2026-23519

CVE-2026-23519: RustCrypto cmov: thumbv6m-none-eabi compiler emits non-constant time assembly when using cmovnz

RustCrypto CMOV provides conditional move CPU intrinsics which are guaranteed on major platforms to execute in constant-time and not be rewritten as branches by the compiler. Prior to 0.4.4, the thumbv6m-none-eabi (Cortex M0, M0+ and M1) compiler emits non-constant time assembly when using cmovnz (portable version). This vulnerability is fixed in 0.4.4.

(GitHub Advisory)

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CVE-2026-23519

CVE-2026-23519:

8.9

CVSS Score

4.0

-

CVSS Score

Basic Information

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Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
cmov	rust	< 0.4.4	0.4.4

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability is a timing side-channel issue in the cmov crate, specifically in the portable implementation of the Cmov trait. The root cause lies in the bitnz! macro, which, prior to the patch, did not use core::hint::black_box internally. This allowed the Rust compiler, particularly for the thumbv6m-none-eabi target, to optimize what should have been a constant-time conditional move into a branching instruction (bne). This introduces a data-dependent timing difference that could be exploited.

The patch rectifies this by incorporating black_box within the bitnz! macro, which prevents the compiler from performing this optimization and preserves the constant-time nature of the operation.

The functions Cmov::cmovnz and Cmov::cmovz are directly vulnerable as they utilize the bitnz! macro for creating a mask for the conditional move. The functions CmovEq::cmovne and CmovEq::cmoveq are also affected because they are part of the API that relies on the vulnerable cmovnz function.

While the provided patch specifically shows changes for the u64 implementation, the vulnerability description and Proof of Concept indicate that other integer types like u8 are also affected. Given the generic nature of the bitnz! macro, it's highly probable that implementations for other integer types (u16, u32, u128, usize) using this portable implementation are also vulnerable.

Vulnerable functions

Cmov::cmovnz

cmov/src/portable.rs

The `cmovnz` function uses the `bitnz!` macro to compute a mask for a conditional move. Before the patch, the `bitnz!` macro's result was not wrapped in `core::hint::black_box` at its source, allowing the compiler to optimize the code into a non-constant-time branch instruction for the `thumbv6m-none-eabi` target. This creates a timing side-channel vulnerability. The patch addresses this by adding `black_box` inside the `bitnz!` macro.

Cmov::cmovz

cmov/src/portable.rs

Similar to `cmovnz`, the `cmovz` function uses the vulnerable `bitnz!` macro to compute a mask, making it susceptible to the same timing side-channel vulnerability on the `thumbv6m-none-eabi` target due to compiler optimizations.

CmovEq::cmovne

cmov/src/portable.rs

This function calls the vulnerable `cmovnz` function. An attacker triggering this function would cause the non-constant-time code in `cmovnz` to be executed, exposing the timing side-channel.

CmovEq::cmoveq

cmov/src/portable.rs

This function also calls the vulnerable `cmovnz` function, making it an entry point for triggering the timing side-channel vulnerability.

Vulnerability Intelligence
Miggo AI

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WAF Rule

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Reasoning

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