Vulnerability-Lookup

CVE-2026-23519 (GCVE-0-2026-23519)

Vulnerability from cvelistv5 – Published: 2026-01-15 19:13 – Updated: 2026-01-15 19:57

Title

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

Summary

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.

Severity

8.9 (High)


                        
                          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

SSVC

Exploitation: poc Automatable: no Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-01-15 19:57 UTC

CWE

CWE-208 - Observable Timing Discrepancy

Assigner

GitHub_M mitre root

References

2 references

URL	Tags
https://github.com/RustCrypto/utils/security/advi…	x_refsource_CONFIRM
https://github.com/RustCrypto/utils/commit/559772…	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version	CPE status
RustCrypto	utils	Affected: < 0.4.4	guessed

Show details on NVD website

JSON

To clipboard

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FKIE_CVE-2026-23519

Vulnerability from fkie_nvd - Published: 2026-01-15 20:16 - Updated: 2026-06-17 10:21

Severity

9.8 (Critical) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Summary

References

URL	Tags
security-advisories@github.com	https://github.com/RustCrypto/utils/commit/55977257e7c82a309d5e8abfdd380a774f0f9778	Patch
security-advisories@github.com	https://github.com/RustCrypto/utils/security/advisories/GHSA-2gqc-6j2q-83qp	Exploit, Vendor Advisory
134c704f-9b21-4f2e-91b3-4a467353bcc0	https://github.com/RustCrypto/utils/security/advisories/GHSA-2gqc-6j2q-83qp	Exploit, Vendor Advisory

Impacted products

	Vendor	Product	Version
	rustcrypto	cmov	*

JSON

To clipboard

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          "userInteraction": "NONE",
          "valueDensity": "NOT_DEFINED",
          "vectorString": "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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X",
          "version": "4.0",
          "vulnAvailabilityImpact": "NONE",
          "vulnConfidentialityImpact": "HIGH",
          "vulnIntegrityImpact": "NONE",
          "vulnerabilityResponseEffort": "NOT_DEFINED"
        },
        "source": "security-advisories@github.com",
        "type": "Secondary"
      }
    ],
    "ssvcV203": [
      {
        "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
        "ssvcData": {
          "id": "CVE-2026-23519",
          "options": [
            {
              "exploitation": "poc"
            },
            {
              "automatable": "no"
            },
            {
              "technicalImpact": "partial"
            }
          ],
          "role": "CISA Coordinator",
          "timestamp": "2026-01-15T19:57:42.346432Z",
          "version": "2.0.3"
        }
      }
    ]
  },
  "published": "2026-01-15T20:16:05.313",
  "references": [
    {
      "source": "security-advisories@github.com",
      "tags": [
        "Patch"
      ],
      "url": "https://github.com/RustCrypto/utils/commit/55977257e7c82a309d5e8abfdd380a774f0f9778"
    },
    {
      "source": "security-advisories@github.com",
      "tags": [
        "Exploit",
        "Vendor Advisory"
      ],
      "url": "https://github.com/RustCrypto/utils/security/advisories/GHSA-2gqc-6j2q-83qp"
    },
    {
      "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
      "tags": [
        "Exploit",
        "Vendor Advisory"
      ],
      "url": "https://github.com/RustCrypto/utils/security/advisories/GHSA-2gqc-6j2q-83qp"
    }
  ],
  "sourceIdentifier": "security-advisories@github.com",
  "vulnStatus": "Analyzed",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-208"
        }
      ],
      "source": "security-advisories@github.com",
      "type": "Secondary"
    },
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-203"
        }
      ],
      "source": "nvd@nist.gov",
      "type": "Primary"
    }
  ]
}

GHSA-2GQC-6J2Q-83QP

Vulnerability from github – Published: 2026-01-15 18:17 – Updated: 2026-01-23 21:46

Summary

RustCrypto Utilities cmov: `thumbv6m-none-eabi` compiler emits non-constant time assembly when using `cmovnz`

Details

Summary

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.

Details

It seems that, during mask computation, an LLVM optimisation pass is detecting that bitnz 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) can be interpreted as a conditional bitwise conditional not.

PoC

This 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:

#![no_std]
use cmov::Cmov;

#[inline(never)]
pub fn test_ct_cmov(a: &mut u8, b: u8, c: u8) {
    a.cmovnz(&b, c);
}

The resulting assembly emitted (shown using cargo asm --release --target thumbv6m-none-eabi that uses cargo-show-asm):

Collapsed assembly

.section .text.not_ct::test_ct_cmov,"ax",%progbits
    .globl  not_ct::test_ct_cmov
    .p2align    1
    .type   not_ct::test_ct_cmov,%function
    .code   16
    .thumb_func
not_ct::test_ct_cmov:
    .fnstart
    .cfi_sections .debug_frame
    .cfi_startproc
    .save   {r7, lr}
    push {r7, lr}
    .cfi_def_cfa_offset 8
    .cfi_offset lr, -4
    .cfi_offset r7, -8
    .setfp  r7, sp
    add r7, sp, #0
    .cfi_def_cfa_register r7
    .pad    #8
    sub sp, #8
    movs r3, #0
    lsls r2, r2, #24
    bne .LBB0_2
    mvns r3, r3
.LBB0_2:
    ldrb r2, [r0]
    str r3, [sp, #4]
    str r3, [sp]
    mov r3, sp
    @APP
    @NO_APP
    ldr r3, [sp]
    bics r1, r3
    ands r2, r3
    adds r1, r2, r1
    strb r1, [r0]
    add sp, #8
    pop {r7, pc}

The non-constant time assembly is:

    bne  .LBB0_2
    mvns r3, r3
.LBB0_2:

Impact

The exact impact is unclear, especially since cmov clearly warns users that the portable version is best-effort.

Severity

8.9 (High)


                  
                    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

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "crates.io",
        "name": "cmov"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.4.4"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-23519"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-203",
      "CWE-208"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-01-15T18:17:15Z",
    "nvd_published_at": "2026-01-15T20:16:05Z",
    "severity": "HIGH"
  },
  "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: \u0026mut u8, b: u8, c: u8) {\n    a.cmovnz(\u0026b, 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\u003cdetails\u003e\n\u003csummary\u003eCollapsed assembly\u003c/summary\u003e\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\u003c/details\u003e\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.",
  "id": "GHSA-2gqc-6j2q-83qp",
  "modified": "2026-01-23T21:46:10Z",
  "published": "2026-01-15T18:17:15Z",
  "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"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "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",
      "type": "CVSS_V4"
    }
  ],
  "summary": "RustCrypto Utilities cmov: `thumbv6m-none-eabi` compiler emits non-constant time assembly when using `cmovnz`"
}

RUSTSEC-2026-0003 (CVE-2026-23519)

Vulnerability from osv_rustsec – Published: 2026-01-14 12:00 – Updated: 2026-01-15 17:45 – Source website

Summary

Non-constant-time code generation on ARM32 targets

Details

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:

    bne  .LBB0_2
    mvns r3, r3

This includes a branch instruction bne: Branch if Not Equal.

PoC

The following code reproduces the issue:

#![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.

Severity

8.9 (High)


                    
                      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

References

URL

Type

	https://crates.io/crates/cmov	PACKAGE
	https://rustsec.org/advisories/RUSTSEC-2026-0003.html	ADVISORY
	https://github.com/RustCrypto/utils/security/advi…	ADVISORY

JSON

To clipboard

{
  "affected": [
    {
      "database_specific": {
        "categories": [
          "crypto-failure"
        ],
        "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",
        "informational": null
      },
      "ecosystem_specific": {
        "affected_functions": null,
        "affects": {
          "arch": [],
          "functions": [],
          "os": []
        }
      },
      "package": {
        "ecosystem": "crates.io",
        "name": "cmov",
        "purl": "pkg:cargo/cmov"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0.0.0-0"
            },
            {
              "fixed": "0.4.4"
            }
          ],
          "type": "SEMVER"
        }
      ],
      "versions": []
    }
  ],
  "aliases": [
    "CVE-2026-23519",
    "GHSA-2gqc-6j2q-83qp"
  ],
  "database_specific": {
    "license": "CC-BY-4.0"
  },
  "details": "## Summary\n\nWhile the `cmov` crate has a special backend for `aarch64` which uses special\nCSEL instructions, on 32-bit ARM it uses a portable pure Rust fallback\nimplementation. This implementation uses a combination of bitwise arithmetic\nand `core::hint::black_box` to attempt to coerce constant-time code generation\nout of the optimizer, but the implementation in v0.4.3 and earlier failed to\ndo this on 32-bit ARM targets.\n\n## Impact\n\nBranch instructions inserted by the LLVM optimizer on 32-bit targets can be\nleveraged using various microarchitectural sidechannels like cache timing\nattacks to learn secret information that `cmov` is designed to protect.\n\n## Details\n\nThe following assembly was emitted when using `Cmov::cmovnz`, a function which\nimplements a conditional move when a provided value is non-zero:\n\n```asm\n    bne  .LBB0_2\n    mvns r3, r3\n```\n\nThis includes a branch instruction `bne`: Branch if Not Equal.\n\n## PoC\n\nThe following code reproduces the issue:\n\n```rust\n#![no_std]\nuse cmov::Cmov;\n\n#[inline(never)]\npub fn test_ct_cmov(a: \u0026mut u8, b: u8, c: u8) {\n    a.cmovnz(\u0026b, c);\n}\n```\n\n## Resolution\n\n`cmov` v0.4.4 includes a portable `black_box`-based tactical mitigation for the\nissue which coerced the compiler into producing the expected codegen, and\nadditionally v0.4.5 added an `asm!` reimplementation of the problematic mask\ngeneration function for ARM32 targets which should guarantee that particular\nfunction never contains a branch on such targets.",
  "id": "RUSTSEC-2026-0003",
  "modified": "2026-01-15T17:45:42Z",
  "published": "2026-01-14T12:00:00Z",
  "references": [
    {
      "type": "PACKAGE",
      "url": "https://crates.io/crates/cmov"
    },
    {
      "type": "ADVISORY",
      "url": "https://rustsec.org/advisories/RUSTSEC-2026-0003.html"
    },
    {
      "type": "ADVISORY",
      "url": "https://github.com/RustCrypto/utils/security/advisories/GHSA-2gqc-6j2q-83qp"
    }
  ],
  "related": [],
  "severity": [
    {
      "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",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Non-constant-time code generation on ARM32 targets"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.

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Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.

Action not permitted

CVE-2026-23519 (GCVE-0-2026-23519)

FKIE_CVE-2026-23519

GHSA-2GQC-6J2Q-83QP

Summary

Details

PoC

Impact

RUSTSEC-2026-0003 (CVE-2026-23519)

Summary

Impact

Details

PoC

Resolution

Tags

Sightings

Nomenclature

Related by attack behaviour