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

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

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.

Source published

16 Apr 2026 23:53:50 UTC

Source modified

17 Apr 2026 12:23:42 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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

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.

Source published

Apr 16, 2026, 11:53:50 PM (timezone not stated)

Source modified

Apr 17, 2026, 12:23:42 PM (timezone not stated)

Observed here

28 Oct 2026 05:01:45 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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

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.

Source published

17 Apr 2026 01:17:40 UTC

Source modified

26 Oct 2026 21:10:00 UTC

Observed here

27 Oct 2026 10:09:56 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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CVSS 3.1 · [email protected] · Secondary · 3.7 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N

### 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: ```go user, err := db.Where("username = ?", username).First(&user) if err != nil { return ErrUnauthorized } err = bcrypt.CompareHashAndPassword(user.PasswordHash, []byte(password)) ``` This creates a measurable timing discrepancy between: - **existing username + wrong password** requests, which incur bcrypt cost - **nonexistent username + any password** requests, which avoid bcrypt entirely Because no constant-time equalization is performed, the endpoint leaks account existence through timing behavior. The measurements provided show a large and consistent gap between the two cases across repeated trials, making the difference distinguishable without requiring especially high request volume. In the supplied test results: - existing user requests averaged about `0.0616s` - nonexistent user requests averaged about `0.0027s` That gap is large enough to support reliable username enumeration under typical testing conditions. ### PoC The issue can be reproduced by sending repeated authentication attempts to the login endpoint using the same invalid password while alternating between a known valid username and a nonexistent username, then comparing average response times. Valid usernames consistently take longer because bcrypt verification is performed. ### Impact - **Type:** Timing side-channel / username enumeration - **Who is impacted:** Any deployment exposing the affected login endpoint - **Security impact:** Unauthenticated attackers can confirm valid usernames one at a time, improving the effectiveness of credential stuffing, password spraying, phishing, and other targeted account attacks - **Attack preconditions:** None beyond network access to the login endpoint - **Confidentiality impact:** Low to moderate, depending on the sensitivity of account existence in the target environment

Source published

13 Apr 2026 19:31:50 UTC

Source modified

25 Jun 2026 23:11:42 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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CVSS_V3 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N

Publisher rating · LOW

Note Mark: Username Enumeration via Login Endpoint Timing Side-Channel in github.com/enchant97/note-mark/backend. NOTE: The source advisory for this report contains additional versions that could not be automatically mapped to standard Go module versions. (If this is causing false-positive reports from vulnerability scanners, please suggest an edit to the report.) The additional affected modules and versions are: github.com/enchant97/note-mark/backend before v0.19.2-0.20260411145025-cf4c6f6acf70.

Source published

25 Jun 2026 22:34:38 UTC

Source modified

25 Jun 2026 23:01:28 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

OSV · upstream source license applies

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · OSV · upstream source license applies. Source text is retained; field grouping and change labels are provided by ChangeIntel.