Sync-in Server has Username/Login Enumeration via Timing Side-Channel on POST /api/auth/login (incomplete fix of the prior timing-attack advisory)

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.

Sync-in Server is an open-source platform for file storage, sharing, collaboration, and syncing. Versions prior to 2.4.1 contain an observable timing discrepancy in the login endpoint because authentication attempts for nonexistent accounts return without performing the bcrypt comparison used for existing accounts. An unauthenticated attacker can measure response times to enumerate valid usernames or email addresses, facilitating credential-stuffing, password-spraying, and phishing attacks. Version 2.4.1 contains a patch.

Source published

21 Sep 2026 20:31:46 UTC

Source modified

23 Sep 2026 19:42:05 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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CVSS 3.1 · 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N

Sync-in Server is an open-source platform for file storage, sharing, collaboration, and syncing. Versions prior to 2.4.1 contain an observable timing discrepancy in the login endpoint because authentication attempts for nonexistent accounts return without performing the bcrypt comparison used for existing accounts. An unauthenticated attacker can measure response times to enumerate valid usernames or email addresses, facilitating credential-stuffing, password-spraying, and phishing attacks. Version 2.4.1 contains a patch.

Source published

Sep 21, 2026, 8:31:46 PM (timezone not stated)

Source modified

Sep 23, 2026, 7:42:05 PM (timezone not stated)

Observed here

11 Nov 2026 14:59:48 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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EUVD CVSS 3.1 · 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N

Sync-in Server is an open-source platform for file storage, sharing, collaboration, and syncing. Versions prior to 2.4.1 contain an observable timing discrepancy in the login endpoint because authentication attempts for nonexistent accounts return without performing the bcrypt comparison used for existing accounts. An unauthenticated attacker can measure response times to enumerate valid usernames or email addresses, facilitating credential-stuffing, password-spraying, and phishing attacks. Version 2.4.1 contains a patch.

Source published

21 Sep 2026 21:17:06 UTC

Source modified

30 Sep 2026 17:32:07 UTC

Observed here

25 Oct 2026 14:19:16 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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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: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N

## Summary validateUser() in backend/src/authentication/providers/mysql/auth-provider-mysql.service.ts returns immediately when the supplied login/email does not match any account, without ever calling comparePassword(): async validateUser(loginOrEmail: string, password: string, ip?: string, scope?: AUTH_SCOPE): Promise<UserModel> { let user: UserModel try { user = await this.usersManager.findUser(loginOrEmail, false) } catch (e) { ... } if (!user) { this.logger.warn(...) return null // <-- comparePassword() is never reached here } return await this.usersManager.logUser(user, password, ip, scope) } comparePassword() (backend/src/common/functions.ts) already contains a dummy-hash branch that was clearly added to defend against exactly this class of attack: export async function comparePassword(password: string, hash?: string | null): Promise<boolean> { if (!hash) { // No hash, waste time for time-based attacks await bcrypt.compare(password, DUMMY_PASSWORD_HASH) return false } return await bcrypt.compare(password, hash) } The problem is that this protection only runs when comparePassword() is actually invoked with a falsy hash. Because validateUser() short-circuits with return null as soon as findUser() comes back empty, the "account doesn't exist" path skips all cryptographic work entirely, while the "account exists, wrong password" path always performs a real bcrypt comparison (cost factor 10, ~100ms+). The two outcomes are trivially distinguishable by response time. There's already a published advisory in this repo for "Username Enumeration via Timing Attack" - this looks like the same underlying issue surfacing through a different call path (the early return in validateUser()) that the existing fix (the dummy-hash branch in comparePassword()) doesn't actually reach, rather than a brand new vulnerability class. ## Impact Any unauthenticated client can determine whether a given username/email is a valid account on the instance by timing POST /api/auth/login: - Non-existent login: near-instant rejection (no bcrypt call). - Existing login (regardless of password correctness): consistently slower due to a real bcrypt comparison. This enables efficient enumeration of valid accounts, which can then be used to focus credential-stuffing, password-spraying, or phishing against confirmed-valid targets. ## Proof of Concept Verified with the actual comparePassword() logic and the real DUMMY_PASSWORD_HASH constant copied verbatim from backend/src/common/functions.ts, using the project's own bcryptjs dependency (no mocking of bcrypt itself): Avg time for "login does not exist" path (validateUser returns null, no bcrypt call): 0.00 ms Avg time for "login exists, wrong password" path (real bcrypt.compare runs): 114.90 ms Difference: 114.90 ms (ratio: ~58923x) The "not found" path reproduces validateUser()'s exact early return (no call into comparePassword); the "wrong password" path reproduces logUser()'s real call into comparePassword(password, user.password). The gap is large enough to be trivially observable over a real network, even accounting for jitter. Reachable endpoint: POST /api/auth/login, guarded only by AuthLocalGuard (Passport local strategy invoking validateUser()), no authentication required. ## Suggested fix Make validateUser() always pass through comparePassword()'s timing-equalized path, even when no user is found, e.g.: if (!user) { await comparePassword(password, null) // burns the same time as a real comparison return null } so the "account not found" and "account found, wrong password" branches take statistically indistinguishable time.

Source published

22 Sep 2026 14:49:23 UTC

Source modified

22 Sep 2026 15:00:48 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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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 · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

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

Publisher rating · MODERATE