Cyber Resilience

CVE-2026-40263

Published
17 April 2026
Modified
17 April 2026
CVSS Score v3.1 3.7
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N
EPSS Score 0.0020 10th percentile
Risk Priority 15 floored blend · peak EPSS

Summary

CVE-2026-40263 is a low-severity Observable Timing Discrepancy (CWE-208) vulnerability. Its CVSS base score is 3.7 (Low).

Operationally, exploitation aligns with the MITRE ATT&CK technique Account Discovery (T1087); ranked at the 10th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

The strongest mitigations our analysis identified map to AC-7 (Unsuccessful Logon Attempts) and IA-2 (Identification and Authentication (Organizational Users)) — see the control section below for these in your framework.

EU & UK References

Vulnerability Data

Note Mark is an open-source note-taking application. In versions 0.19.1 and prior, the login endpoint performs bcrypt password verification only when the supplied username exists, returning immediately for nonexistent usernames. This timing discrepancy allows unauthenticated attackers to enumerate valid usernames…

more

by measuring response times, enabling targeted credential attacks. This issue has been fixed in version 0.19.2.

CWE(s)
CWE-208

Related Threats

MITRE ATT&CK Enterprise TechniquesAI

T1087 Account Discovery Discovery
Adversaries may attempt to get a listing of valid accounts, usernames, or email addresses on a system or within a compromised environment.
attack.mitre.org →
T1110 Brute Force Credential Access
Adversaries may use brute force techniques to gain access to accounts when passwords are unknown or when password hashes are obtained.
attack.mitre.org →
Why these techniques?

Timing side-channel enables account enumeration (T1087) facilitating targeted brute-force/credential attacks (T1110).

Confidence: MEDIUM · MITRE ATT&CK Enterprise v19.0

CVEs Like This One

CVE-2026-48859Shared CWE-208
CVE-2026-47783Shared CWE-208
CVE-2026-41263Shared CWE-208
CVE-2026-41161Shared CWE-208
CVE-2026-21840Shared CWE-208
CVE-2026-48011Shared CWE-208
CVE-2026-41418Shared CWE-208
CVE-2026-23996Shared CWE-208
CVE-2026-32702Shared CWE-208
CVE-2026-27882Shared CWE-208

Affected Assets

—
In
inferred from references and description; NVD did not file a CPE for this CVE

Mitigating Controls

Control response

Prevent
Stop it (NIST 800-53)
  • AC-7 Unsuccessful Logon Attempts
  • IA-2 Identification and Authentication (Organizational Users)
Detect
Catch it (NIST detect / respond)
  • SI-4 System Monitoring
Harden
Shrink the surface (DISA STIG)

—

Validate
Prove the fix (OWASP ASVS)
  • V11.2.4

Mitigating Controls (NIST 800-53 r5) AI

AC-7 Unsuccessful Logon Attempts good match
prevent

Forces uniform response handling and/or artificial delays on all login attempts, directly eliminating the timing side-channel that reveals username existence.

IA-2 Identification and Authentication (Organizational Users) partial match
prevent

Requires the authentication mechanism itself to be implemented so that it does not disclose whether an identifier is valid.

SI-4 System Monitoring partial match
detect

Enables detection of systematic timing-based probes against the login endpoint that indicate username enumeration activity.

Mitigating Controls (NIST CSF 2.0) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.

PR.PS-06 mostly match
prevents

Secure SDLC practices directly require constant-time implementations that eliminate observable timing discrepancies.

Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.

A.8.17 Clock synchronization none match
none

Consistent reference clocks limit the attacker's ability to measure or manipulate timing differences that could reveal internal state or processing paths.

References