Non-constant-time code generation on ARM32 targets

{
  "id": "09d35e50f0e8471e5f333e111ffde367ba712ae99180d9cc3b72d2e3a264836c",
  "url": "https://osv.dev/vulnerability/RUSTSEC-2026-0003",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "Non-constant-time code generation on ARM32 targets",
    "summary": "## Summary While the `cmov` crate has a special backend for `aarch64` which uses special CSEL instructions, on 32-bit ARM it uses a portable pure Rust fallback implementation. This implementation uses a combination of bitwise arithmetic and `core::hint::black_box` to attempt to coerce constant-time code generation out of the optimizer, but the implementation in v0.4.3 and earlier failed to do this on 32-bit ARM targets. ## Impact Branch instructions inserted by the LLVM optimizer on 32-bit targets can be leveraged using various microarchitectural sidechannels like cache timing attacks to learn secret information that `cmov` is designed to protect. ## Details The following assembly was emitted when using `Cmov::cmovnz`, a function which implements a conditional move when a provided value is non-zero: ```asm bne .LBB0_2 mvns r3, r3 ``` This includes a branch instruction `bne`: Branch if Not Equal. ## PoC The following code reproduces the issue: ```rust #![no_std] use cmov::Cmov; #[inline(never)] pub fn test_ct_cmov(a: &mut u8, b: u8, c: u8) { a.cmovnz(&b, c); } ``` ## Resolution `cmov` v0.4.4 includes a portable `black_box`-based tactical mitigation for the issue which coerced the compiler into producing the expected codegen, and additionally v0.4.5 added an `asm!` reimplementation of the problematic mask generation function for ARM32 targets which should guarantee that particular function never contains a branch on such targets.",
    "evidence": "osv:RUSTSEC-2026-0003",
    "products": [
      {
        "name": {
          "value": "cmov",
          "evidence": "/affected/0/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "0.4.4",
            "evidence": "/affected/0/ranges"
          }
        ],
        "affected_versions": []
      }
    ],
    "severity": "high",
    "structured": {
      "id": "RUSTSEC-2026-0003",
      "aliases": [
        "CVE-2026-23519",
        "GHSA-2gqc-6j2q-83qp",
        "RUSTSEC-2026-0003"
      ],
      "metrics": [
        {
          "type": "CVSS_V4",
          "score": "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"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "type": "SEMVER",
              "events": [
                {
                  "introduced": "0.0.0-0"
                },
                {
                  "fixed": "0.4.4"
                }
              ]
            }
          ],
          "package": {
            "name": "cmov",
            "purl": "pkg:cargo/cmov",
            "ecosystem": "crates.io"
          },
          "database_specific": {
            "cvss": "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",
            "source": "https://github.com/rustsec/advisory-db/blob/osv/crates/RUSTSEC-2026-0003.json",
            "categories": [
              "crypto-failure"
            ],
            "informational": null
          },
          "ecosystem_specific": {
            "affects": {
              "os": [],
              "arch": [],
              "functions": []
            },
            "affected_functions": null
          }
        }
      ],
      "modified": "2026-10-23T08:41:35.268663457Z",
      "provider": "osv",
      "references": [
        "https://crates.io/crates/cmov",
        "https://rustsec.org/advisories/RUSTSEC-2026-0003.html",
        "https://github.com/RustCrypto/utils/commit/55977257e7c82a309d5e8abfdd380a774f0f9778"
      ],
      "provider_metadata": {
        "license": "CC-BY-4.0"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-23519",
        "evidence": "/aliases"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 412606,
  "published_at": "2026-01-14T12:00:00+00:00"
}