What happened
N-able Mail Assure through April 2026 contains a design-level authorization flaw that allows an authenticated SMTP user to send outbound email using MAIL FROM addresses belonging to other tenants. When connecting to the SMTP TCP port and performing SMTP AUTH with valid credentials, the server accepts arbitrary sender domains without enforcing any domain-to-account binding. As a result, an attacker from any tenant can impersonate other tenant domains, producing messages that pass SPF and DMARC validation. NOTE: N-able's position is that the behavior is intended functionality of its shared SMTP relay architecture and that the service does not represent that it enforces per-tenant sender-domain binding.
Affected versions
Mail Assure: 0 through April 2026 (custom) Fixed: See vendor advisory.
Why it matters
Review the vendor advisory and exposure of the affected product to determine operational impact.
Detection & mitigation
- Apply vendor-provided updates or mitigations.
- Review affected product exposure and access logs.
Public PoC references
No public PoC reference has passed the current publication threshold.