CVE-2026-23519: RustCrypto Utilities cmov `thumbv6m-none-eabi` Compiler Emits Non-constant Time Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-2gqc-6j2q-83qp",
  "modified" : "2026-01-23T21:46:10Z",
  "published" : "2026-01-15T18:17:15Z",
  "aliases" : [ "CVE-2026-23519" ],
  "summary" : "RustCrypto Utilities cmov: `thumbv6m-none-eabi` compiler emits non-constant time assembly when using `cmovnz`",
  "details" : "### Summary\n\n`thumbv6m-none-eabi` (Cortex M0, M0+ and M1) compiler emits non-constant time assembly when using `cmovnz` (portable version). I did not found any other target with the same behaviour but I did not go through all targets supported by Rust. \n\n### Details\n\nIt seems that, [during `mask` computation](https://github.com/RustCrypto/utils/blob/9e555db060c80f4669d804f448a524a37d201b32/cmov/src/portable.rs#L78), an LLVM optimisation pass is detecting that [`bitnz`](https://github.com/RustCrypto/utils/blob/9e555db060c80f4669d804f448a524a37d201b32/cmov/src/portable.rs#L13) is returning 0 or 1, that can be interpreted as a boolean. This intermediate value is not masked by a call to `black_box` and thus the subsequent [`.wrapping_sub(1)`](https://github.com/RustCrypto/utils/blob/9e555db060c80f4669d804f448a524a37d201b32/cmov/src/portable.rs#L78C1-L78C84) can be interpreted as a conditional bitwise conditional not.\n\n### PoC\n\nThis is an attempt at having a minimal faulty code. In a library crate with an up-to-date `cmov` as only dependency, the content of `src/lib.rs` is:\n\n```rust\n#![no_std]\nuse cmov::Cmov;\n\n#[inline(never)]\npub fn test_ct_cmov(a: &mut u8, b: u8, c: u8) {\n    a.cmovnz(&b, c);\n}\n```\n\n\nThe resulting assembly emitted (shown using `cargo asm --release --target thumbv6m-none-eabi` that uses [`cargo-show-asm`](https://crates.io/crates/cargo-show-asm)):\n\n<details>\n<summary>Collapsed assembly</summary>\n\n```asm\n.section .text.not_ct::test_ct_cmov,\"ax\",%progbits\n\t.globl\tnot_ct::test_ct_cmov\n\t.p2align\t1\n\t.type\tnot_ct::test_ct_cmov,%function\n\t.code\t16\n\t.thumb_func\nnot_ct::test_ct_cmov:\n\t.fnstart\n\t.cfi_sections .debug_frame\n\t.cfi_startproc\n\t.save\t{r7, lr}\n\tpush {r7, lr}\n\t.cfi_def_cfa_offset 8\n\t.cfi_offset lr, -4\n\t.cfi_offset r7, -8\n\t.setfp\tr7, sp\n\tadd r7, sp, #0\n\t.cfi_def_cfa_register r7\n\t.pad\t#8\n\tsub sp, #8\n\tmovs r3, #0\n\tlsls r2, r2, #24\n\tbne .LBB0_2\n\tmvns r3, r3\n.LBB0_2:\n\tldrb r2, [r0]\n\tstr r3, [sp, #4]\n\tstr r3, [sp]\n\tmov r3, sp\n\t@APP\n\t@NO_APP\n\tldr r3, [sp]\n\tbics r1, r3\n\tands r2, r3\n\tadds r1, r2, r1\n\tstrb r1, [r0]\n\tadd sp, #8\n\tpop {r7, pc}\n```\n\n</details>\n\nThe non-constant time assembly is:\n\n```asm\n    bne  .LBB0_2\n    mvns r3, r3\n.LBB0_2:\n```\n\n### Impact\n\nThe exact impact is unclear, especially since `cmov` clearly warns users that the portable version is best-effort.",
  "severity" : [ {
    "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" : [ {
    "package" : {
      "ecosystem" : "crates.io",
      "name" : "cmov"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.4.4"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/RustCrypto/utils/security/advisories/GHSA-2gqc-6j2q-83qp"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-23519"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/RustCrypto/utils/commit/55977257e7c82a309d5e8abfdd380a774f0f9778"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/RustCrypto/utils"
  }, {
    "type" : "WEB",
    "url" : "https://rustsec.org/advisories/RUSTSEC-2026-0003.html"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-203", "CWE-208" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-01-15T18:17:15Z",
    "nvd_published_at" : "2026-01-15T20:16:05Z"
  }
}