EUVD-2025-210903
GHSA-8wch-g3f4-9xpr
Severity by source
AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N
Requires authenticated tenant access (PR:L); no confidentiality or availability impact; spoofing integrity impact is limited and scoped to the relay session.
Primary rating from Vendor (mitre).
CVSS VectorVendor: mitre
Lifecycle Timeline
1DescriptionCVE.org
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.
AnalysisAI
Cross-tenant SMTP sender spoofing in N-able Mail Assure (all versions through April 2026) permits any authenticated tenant to specify arbitrary MAIL FROM envelope addresses - including domains belonging to other co-hosted tenants - without the relay enforcing any domain-to-account ownership binding. Because the shared relay is itself listed in the SPF records and DMARC policy of every hosted tenant domain, spoofed outbound messages pass SPF and DMARC validation at recipient mail servers, giving the attack the appearance of fully authorized email. N-able has publicly stated this is intended behavior of its shared relay architecture, making no vendor patch expected. A researcher demonstration gist has been published, and no public exploit or CISA KEV listing exists at time of analysis.
Technical ContextAI
N-able Mail Assure is a managed email security and continuity platform delivered primarily through the managed service provider (MSP) channel, identified by CPE cpe:2.3:a:n-able:mail_assure:*:*:*:*:*:*:*:*. The product operates a shared SMTP relay in which multiple customer tenants route outbound mail through common infrastructure. The flaw is architectural: SMTP AUTH is performed correctly at the credential level, but the post-authentication authorization step - verifying that the authenticated account is permitted to use the submitted MAIL FROM domain - is absent. This maps to CWE-290 (Authentication Bypass by Spoofing), where authentication of the user does not translate into authorization over the claimed identity. The critical amplification factor is SPF and DMARC: the shared relay holds DNS-level sending authority for all hosted tenant domains, so spoofed messages are not rejected by standard email authentication controls at the destination. The design conflates relay authentication with sender identity authorization, a gap common in shared relay architectures that lack per-tenant envelope sender restrictions.
RemediationAI
No vendor-released patch is expected: N-able has publicly stated that the absence of per-tenant sender-domain binding is intended behavior of its shared SMTP relay architecture. Affected tenants should implement DMARC aggregate (rua) and forensic (ruf) reporting on their hosted domains immediately, as DMARC reports from recipient mail servers will surface unauthorized use of their domain in MAIL FROM fields. Organizations at elevated phishing risk (financial services, legal, healthcare) should evaluate migrating outbound relay to a service or self-managed infrastructure that enforces per-account envelope sender restrictions. Where N-able provides account-level configuration, tenants should inquire whether any egress sender domain allow-listing can be applied at the relay layer. Awareness of this shared-tenancy limitation should be disclosed to downstream clients of MSPs operating Mail Assure so that they can calibrate trust in email received from Mail-Assure-hosted senders. The researcher disclosure can be found at https://gist.github.com/alessandrobertoldi/1ebe0f48aa0119d787ac0ff710057d92 and the DeepSec presentation at https://deepsec.net/speaker.html#PSLOT782.
Same weakness CWE-290 – Authentication Bypass by Spoofing
View allSame technique Authentication Bypass
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External POC / Exploit Code
Leaving vuln.today