Summary
RustCrypto Utilities cmov: thumbv6m-none-eabi compiler emits non-constant time assembly when using cmovnz
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):
.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.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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Frequently Asked Questions
- What is CVE-2026-23519? CVE-2026-23519 is a high-severity security vulnerability in cmov (rust), affecting versions < 0.4.4. It is fixed in 0.4.4.
- Which versions of cmov are affected by CVE-2026-23519? cmov (rust) versions < 0.4.4 is affected.
- Is there a fix for CVE-2026-23519? Yes. CVE-2026-23519 is fixed in 0.4.4. Upgrade to this version or later.
- Is CVE-2026-23519 exploitable, and should I be worried? Whether CVE-2026-23519 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-23519 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-23519? Upgrade
cmovto 0.4.4 or later.