Note Mark: Username Enumeration via Login Endpoint Timing Side-Channel

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

3.7 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N

EPSS

• CVE-2026-40263: 0.26% estimated exploitation activity in the next 30 days; percentile 17.57 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary A timing side-channel in the login endpoint allows unauthenticated attackers to determine whether a username exists by measuring response time differences. Requests for valid usernames take noticeably longer because the server performs bcrypt password verification, while requests for nonexistent usernames return much faster. This enables reliable remote username enumeration and increases the risk of targeted credential attacks. Details The issue affects the login endpoint: - POST /api/auth/token The root cause is that authentication processing takes different code paths depending on whether the supplied username exists. When the username is found, the server performs bcrypt.CompareHashAndPassword, which adds substantial latency. When the username does not exist, the server returns immediately without performing an equivalent bcrypt operation. Vulnerable flow: This creates a measurable timing discrepancy between: - existing username + wrong password requests, which incur…

Published by the source

13 Apr 2026, 19:31 UTC

Last modified by the source

24 Apr 2026, 20:37 UTC

Source last checked

26 Oct 2026, 15:19 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-w6m9-39cv-2fwp

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# Patch Next advisory brief: CVE-2026-40263

Advisory: Note Mark: Username Enumeration via Login Endpoint Timing Side-Channel
IDs: CVE-2026-40263, GHSA-w6m9-39cv-2fwp
Record: https://patchnext.com/advisories/cve-2026-40263 (revision 1, source checked 26 Oct 2026, 15:19 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Low (CVSS 3.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/enchant97/note-mark/backend 0.19.2-0.20260411145025-cf4c6f6acf70. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/enchant97/note-mark/backend: affected < 0.19.2-0.20260411145025-cf4c6f6acf70; fixed in 0.19.2-0.20260411145025-cf4c6f6acf70

## Suggested inspection (read-only)
- Go github.com/enchant97/note-mark/backend: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/enchant97/note-mark/backend.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-w6m9-39cv-2fwp (GitHub-reviewed advisory GHSA-w6m9-39cv-2fwp lists affected ranges: Go github.com/enchant97/note-mark/backend >=0 <0.19.2-0.20260411145025-cf4c6f6acf70)

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