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

High severity GitHub Reviewed Published Jan 15, 2026 in RustCrypto/utils • Updated Jan 15, 2026

Package

cargo cmov (Rust)

Affected versions

< 0.4.4

Patched versions

0.4.4

Description

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.

References

@tarcieri tarcieri published to RustCrypto/utils Jan 15, 2026
Published to the GitHub Advisory Database Jan 15, 2026
Reviewed Jan 15, 2026
Published by the National Vulnerability Database Jan 15, 2026
Last updated Jan 15, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity High
Attack Requirements None
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality High
Integrity None
Availability None
Subsequent System Impact Metrics
Confidentiality High
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
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

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(13th percentile)

Weaknesses

Observable Timing Discrepancy

Two separate operations in a product require different amounts of time to complete, in a way that is observable to an actor and reveals security-relevant information about the state of the product, such as whether a particular operation was successful or not. Learn more on MITRE.

CVE ID

CVE-2026-23519

GHSA ID

GHSA-2gqc-6j2q-83qp

Source code

Credits

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