# CVE-2026-40263: Note Mark: Username Enumeration via Login Endpoint Timing Side-Channel

Source: https://junglewise.ai/threats/cve-2026-40263-note-mark-username-enumeration-via-login-endpoint-timing-side  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-04-17. Updated: 2026-05-11  
License: CC BY 4.0 with a link back

CVE-2026-40263 · Severity: low · CVSS 3.7 · Published 2026-04-17

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

Technologies: [github.com/enchant97/note-mark/backend (Go)](https://junglewise.ai/threats/technologies/github-com-enchant97-note-mark-backend). Vendors: [Go](https://junglewise.ai/threats/vendors/go).

## Quick answers

**Is CVE-2026-40263 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-40263 being exploited in the wild.

**How severe is CVE-2026-40263?** Low severity, with a CVSS score of 3.7, published 17 April 2026.

**Which products and versions are affected?** github.com/enchant97/note-mark/backend (Go) < 0.19.2-0.20260411145025-cf4c6f6acf70, SEMVER:introduced=0, SEMVER:introduced=0,fixed=0.19.2-0.20260411145025-cf4c6f6acf70.

**Is there a fix for CVE-2026-40263?** Yes. github.com/enchant97/note-mark/backend (Go): upgrade to 0.19.2-0.20260411145025-cf4c6f6acf70 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**Am I affected by CVE-2026-40263?** Check whether you run github.com/enchant97/note-mark/backend (Go) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- github.com/enchant97/note-mark/backend (Go): upgrade to 0.19.2-0.20260411145025-cf4c6f6acf70 or later

  ```sh
  go get github.com/enchant97/note-mark/backend@v0.19.2-0.20260411145025-cf4c6f6acf70
  ```


## Affected products

- github.com/enchant97/note-mark/backend (Go) < 0.19.2-0.20260411145025-cf4c6f6acf70, SEMVER:introduced=0, SEMVER:introduced=0,fixed=0.19.2-0.20260411145025-cf4c6f6acf70

## Identifiers

- CVE-2026-40263
- GHSA-w6m9-39cv-2fwp
- GO-2026-5696

## References

- [https://api.github.com/users/QiaoNPC](https://api.github.com/users/QiaoNPC)
- [https://github.com/QiaoNPC](https://github.com/QiaoNPC)
- [https://api.github.com/users/QiaoNPC/gists%7B/gist_id%7D](https://api.github.com/users/QiaoNPC/gists%7B/gist_id%7D)
- [https://api.github.com/users/QiaoNPC/repos](https://api.github.com/users/QiaoNPC/repos)
- [https://avatars.githubusercontent.com/u/165803850?v=4](https://avatars.githubusercontent.com/u/165803850?v=4)
- [https://api.github.com/users/QiaoNPC/events%7B/privacy%7D](https://api.github.com/users/QiaoNPC/events%7B/privacy%7D)

## Related threats

- [Note Mark path traversal in migrate export via unsanitized slugs](https://junglewise.ai/threats/cve-2026-50553-note-mark-path-traversal-in-migrate-export-via-unsanitized-slugs)
- [Note Mark information disclosure of soft-deleted notes in public books](https://junglewise.ai/threats/cve-2026-50554-note-mark-information-disclosure-of-soft-deleted-notes-in-public)
- [enchant97 Note Mark weak JWT secret allows account takeover](https://junglewise.ai/threats/cve-2026-44523-enchant97-note-mark-weak-jwt-secret-allows-account-takeover)
- [enchant97 Note Mark path traversal in asset upload](https://junglewise.ai/threats/cve-2026-44522-enchant97-note-mark-path-traversal-in-asset-upload)
- [Note Mark: Unauthenticated read of notes and assets in soft-deleted public books](https://junglewise.ai/threats/cve-2026-41572-note-mark-unauthenticated-read-of-notes-and-assets-in-soft)