Security: Fix TOCTOU Race Condition causing Username Alias Hijacking

[Prathvikmehra]

🔒 Security Patch: TOCTOU Alias Hijacking

Resolves #514

Description

This PR patches a critical Time-of-Check to Time-of-Use (TOCTOU) race condition in the profile update workflow. Previously, the system checked for username availability and then created the alias in separate, non-atomic steps. This allowed an attacker to hijack a recently released username by exploiting the race window.

Changes Made

• Refactored \publishProfileDraft\ to pass the Prisma transaction client (\ x) into \isProfileUsernameAvailable.
• Ensures both the \User\ and \UserAlias\ tables are atomically verified within the same transaction lock.
• Prevents vanity URL hijacking and routing conflicts.

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📥 Commits

Reviewing files that changed from the base of the PR and between 7df378b and aa4ebb1.

📒 Files selected for processing (6)

• app/api/export/resume/route.ts
• app/api/export/vcard/route.ts
• app/api/user/delete/route.ts
• lib/accountMerge.ts
• lib/auth.ts
• lib/profileWorkflow.ts

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Project	Deployment	Actions	Updated (UTC)
linkid	Error		Jul 24, 2026 8:56am

[vishnukothakapu]

Thanks for the contribution! The failure is due to a TypeScript nullability error, not the TOCTOU logic itself.

"afterSnapshot.username" has the type "string | null", but "isProfileUsernameAvailable()" expects a "string". Before calling it, you should ensure the username is non-null (or update the function signature if "null" is a valid input).

For example:

if (
  beforeSnapshot.username &&
  afterSnapshot.username &&
  beforeSnapshot.username !== afterSnapshot.username
) {
  const isAvailable = await isProfileUsernameAvailable(
    afterSnapshot.username,
    userId,
    tx
  );

  if (!isAvailable) {
    throw new Error("Username already taken");
  }
}

This should resolve the build error.