CVE-2026-40263 Note Mark: Username Enumeration via Login Endpoint Timing Side-Channel
Unauthenticated attackers can enumerate valid usernames via timing side-channels in the login endpoint, facilitating targeted credential attacks.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-40263?
CVE-2026-40263 is a low-severity vulnerability in enchant97 note-mark, published on 16 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-208
Which versions are affected by CVE-2026-40263?
3 open source packages ship an affected version, including github.com/enchant97/note-mark/backend.
| Package | Affected | Fixed in |
|---|---|---|
github.com/enchant97/note-mark/backend Go | All versions | None |
github.com/enchant97/note-mark/backend Go | All versions | 0.19.2-0.20260411145025-cf4c6f6acf70 |
github.com/enchant97/note-mark/backend Go | All versions | 0.19.2-0.20260411145025-cf4c6f6acf70 |
How do I fix CVE-2026-40263?
Upgrade github.com/enchant97/note-mark/backend to 0.19.2-0.20260411145025-cf4c6f6acf70 or later.
Is CVE-2026-40263 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-40263 exists.
Sources
Also known as GHSA-w6m9-39cv-2fwp, GO-2026-5696.