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

> Attackers can exploit timing side-channels in RustCrypto cmov on Cortex-M0/M1 targets to potentially recover sensitive cryptographic data.

- **Severity:** High 8.9 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 15 Jan 2026
- **Updated:** 15 Jan 2026
- **Also known as:** GHSA-2gqc-6j2q-83qp, RUSTSEC-2026-0003, RUSTSEC-2026-0385

## What is CVE-2026-23519?

CVE-2026-23519 is a high-severity vulnerability in RustCrypto utils, published on 15 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-203 (Observable Discrepancy), CWE-208 (Observable Timing Discrepancy)

## Which versions are affected by CVE-2026-23519?

4 open source packages ship an affected version, including ctutils and cmov. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `ctutils` | crates.io | From 0.0.0-0 | 0.4.4 |
| `cmov` | crates.io | All versions | 0.4.4 |
| `cmov` | crates.io | All versions | 0.4.4 |
| `cmov` | crates.io | From 0.0.0-0 | 0.4.4 |

Products: Rustcrypto Cmov.

## How do I fix CVE-2026-23519?

Upgrade ctutils to 0.4.4 or later.

## Is CVE-2026-23519 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-23519 exists.

## Other vulnerabilities in RustCrypto utils

- [CVE-2026-50185](https://insights.sbomify.com/CVE-2026-50185/): RustCrypto Cmov/CmovEq on aarch64 can produce wrong results if high-bits of registers are set (low 2.0)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-23519 and fix it if it is affected.

What it is: CVE-2026-23519 is a high-severity vulnerability in RustCrypto utils, published on 15 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-23519 exists.

Affected packages and fixed versions:
- ctutils (crates.io): affected from 0.0.0-0; fixed in 0.4.4
- cmov (crates.io): affected all versions; fixed in 0.4.4
- cmov (crates.io): affected all versions; fixed in 0.4.4
- cmov (crates.io): affected from 0.0.0-0; fixed in 0.4.4

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-23519/ (Markdown: https://insights.sbomify.com/CVE-2026-23519/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-23519
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-23519
- OSV: https://osv.dev/vulnerability/CVE-2026-23519

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-23519/
Summaries are generated and may contain mistakes; the official description above is the record.