CVE-2025-68624

Executive Summary

CVE-2025-68624 is a design-level authorization flaw in N-able Mail Assure that permits any authenticated SMTP user to send outbound email using MAIL FROM addresses belonging to other tenants. Because the relay does not enforce a domain-to-account binding, an attacker holding valid credentials for a single tenant can impersonate the sender domains of arbitrary other tenants whose mail flows through the shared relay. Messages crafted this way pass SPF and DMARC validation, making them highly effective for business email compromise (BEC), phishing, and supply-chain pretexting against downstream recipients.

Technical Analysis

N-able Mail Assure is a multi-tenant, cloud-hosted email security and relay service. The vulnerability is rooted in the SMTP relay's authorization model. When a client connects to the service's SMTP TCP port and successfully completes SMTP AUTH using valid credentials, the server accepts an arbitrary MAIL FROM (envelope sender) address without verifying that the sender domain is owned by, or delegated to, the authenticated account.

The weakness maps to CWE-290 (Authentication Bypass by Spoofing). Functionally, the relay performs authentication of the user but not authorization of the sender identity. There is no per-tenant sender-domain binding enforced at the envelope level. This is a common architectural trade-off in shared/hosted relay platforms that serve many domains from a common outbound infrastructure — inbound authentication (SPF, DKIM, DMARC alignment) is designed to validate the relay's sending infrastructure, not the individual tenant's right to use a given envelope domain.

The direct consequence is that SPF and DMARC validation on the receiving end provide no protection against this abuse. SPF authorizes the Mail Assure relay's sending IPs to send mail for the impersonated domain (because the domain's SPF record presumably includes the Mail Assure infrastructure, or because the impersonated domain is itself hosted on Mail Assure). DMARC alignment succeeds for the same reason. The attacker therefore obtains a "sender-authorized" message for a domain they do not control.

Key technical facts derivable from the NVD description:

• Attack surface: The SMTP TCP port of the Mail Assure service (authentication + envelope submission).
• Precondition: Valid SMTP AUTH credentials for any tenant on the platform.
• Trigger: Connecting to the SMTP port, performing SMTP AUTH, then issuing an envelope MAIL FROM for a domain belonging to another tenant.
• Missing control: No domain-to-account binding check at envelope submission time.
• Result: Outbound mail that passes SPF and DMARC validation for the impersonated domain.

CVSS v3.1 scores the issue 4.3 (AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N). The scope is Unchanged (the abused relay and the victim domain both fall within Mail Assure's environment), and the Integrity impact is Low (spoofed sender identity, no data modification). The authentication requirement (PR:L) and the absence of confidentiality/availability impact keep the base score moderate.

N-able's official position is that this is intended functionality of a shared SMTP relay architecture and that the service makes no representation that it enforces per-tenant sender-domain binding. This dispute is the primary reason no patch exists.

Exploitation Scenarios

No public PoC exists; the attack chain below is inferred from the vulnerability class (CWE-290, authentication bypass by spoofing) and the NVD description. No function names, code paths, or API parameters are invented because none are disclosed.

1) Reconnaissance / target identification. The attacker identifies organizations that rely on N-able Mail Assure for inbound or outbound mail handling — that is, victim domains whose MX/SPF records point at Mail Assure infrastructure. This can be fingerprinted from public DNS records (MX, SPF include mechanisms, DMARC records). The attacker also obtains credentials for at least one tenant on the platform — a low barrier for a legitimate customer, a compromised mailbox, or a low-cost trial tenant.

2) Triggering the flaw class. The attacker connects to the Mail Assure SMTP TCP port and completes SMTP AUTH with the valid credentials. Because the server does not verify domain-to-account binding, the attacker can submit an envelope MAIL FROM address for a different tenant's domain. This is the authorization-bypass step: authentication succeeds, but sender authorization is never enforced.

3) Payload / execution method (vulnerability-class level). The attacker crafts an outbound message with:

• MAIL FROM:<victim-user@victim-tenant-domain> (arbitrary envelope sender),
• the authenticated credentials for the attacker's own tenant,
• a body designed for BEC/phishing (e.g., invoice fraud, credential harvesting, internal pretexting).

Because the message originates from Mail Assure's shared relay, SPF and DMARC checks pass on the receiving side. The message is delivered as though legitimately sent by the impersonated domain.

4) Post-exploitation impact. The successful delivery of authenticated, SPF/DMARC-passing spoofed mail enables:

• Business email compromise against the impersonated tenant's customers and partners.
• Internal phishing that appears to originate from a trusted internal user.
• Reputation damage to the impersonated tenant and to Mail Assure's relay IP space.
• Potential follow-on credential theft or fraudulent payments.

Note: because no CVE-specific PoC or malware is documented, any claim of a specific tool, script, or function is not supported by the available data.

Impact Assessment

CVSS v3.1: 4.3 (Medium) — CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N

• AV:N — exploitable over the network via the exposed SMTP port.
• AC:L — no special conditions beyond valid credentials and network reachability.
• PR:L — requires a valid tenant account (low privilege).
• UI:N — no user interaction needed on the sender side.
• S:U — impact confined to the same authorization scope (the relay environment).
• C:N — no direct confidentiality impact.
• I:L — limited integrity impact: forged sender identity in outbound mail.
• A:N — no availability impact.

Business impact. Although the numeric score is moderate, the real-world impact is disproportionately high for a multi-tenant email security provider. The core value proposition of a mail security service is trust in sender identity; this flaw undermines it for every tenant sharing the relay. Downstream business risks include payment fraud via BEC, credential phishing, brand impersonation, and loss of customer confidence in Mail Assure's tenant isolation guarantees.

Blast radius. Any tenant whose domains route through Mail Assure's shared relay is potentially impersonatable by any other authenticated tenant. The absence of a vendor fix means the exposure persists indefinitely until N-able changes the architecture or organizations migrate away from shared-relay sender authorization.

Affected systems. N-able Mail Assure service (SMTP relay); customer mail flows for all tenants on shared relay infrastructure. No fixed version is identified, so all versions through the disclosure window are presumed affected.

Detection Guidance

Sigma Rule

title: Suspicious SMTP MAIL FROM Domain Mismatch - Mail Assure Tenant Spoofing
id: 6c2e19d0-3b4a-4f1c-9d21-6e0f8a11c0a1
status: experimental
description: Detects outbound SMTP sessions where the authenticated account domain does not match the envelope MAIL FROM domain, indicating abuse of CVE-2025-68624 (CWE-290) on N-able Mail Assure.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2025-68624
author: valtersit.com
date: 2026-05-01
logsource:
  category: mail
  product: mail_assure
detection:
  selection_auth:
    event: smtp_auth_success
  selection_sender:
    smtp_command: 'MAIL FROM'
  filter_tenant_match:
    sender_domain: '%authenticated_domain%'
  condition: selection_auth and selection_sender and not filter_tenant_match
fields:
  - authenticated_user
  - authenticated_domain
  - sender_domain
  - client_ip
falsepositives:
  - Legitimate delegated sending arrangements documented by the tenant
level: medium
tags:
  - attack.t1078
  - cve.2025.68624
  - cwe.290

YARA Rule

rule CVE_2025_68624_MailAssure_Sender_Spoof
{
    meta:
        author = "valtersit.com"
        description = "Detects log/artifact indicators of cross-tenant MAIL FROM spoofing against N-able Mail Assure"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2025-68624"
        cve = "CVE-2025-68624"
        date = "2026-05-01"

    strings:
        $smtp_auth = "SMTP AUTH" nocase ascii wide
        $mailfrom  = "MAIL FROM:" nocase ascii wide
        $relay     = "Mail Assure" nocase ascii wide
        $mismatch  = "sender domain mismatch" nocase ascii wide
        $cwe290    = "CWE-290" ascii wide

    condition:
        ($relay and $mailfrom and $smtp_auth) or ($mismatch and $relay) or $cwe290
}

Suricata IDS Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET 25 ( \
  msg:"CVE-2025-68624 N-able Mail Assure cross-tenant MAIL FROM spoofing attempt"; \
  flow:established,to_server; \
  content:"MAIL FROM:"; nocase; \
  content:"@"; within:120; \
  pcre:"/MAIL FROM:<[^@]+@(?![^>]*\\bauthenticated\\b)[^>]+>/i"; \
  threshold:type limit, track by_src, count 5, seconds 60; \
  classtype:attempted-recon; \
  sid:202568624; rev:1; \
  reference:cve,CVE-2025-68624; )

Elastic Detection

sequence by source.ip with maxspan=2m
  [ network where network.transport == "tcp" and destination.port == 25
      and smtp.command == "AUTH" and event.outcome == "success" ]
  [ network where network.transport == "tcp" and destination.port == 25
      and smtp.command == "MAIL" and smtp.sender.domain != user.domain
      and event.action == "smtp_mail_from" ]

Splunk SPL Query

index=mail sourcetype=mail_assure_smtp
| where match(smtp_command, "(?i)^MAIL FROM:")
| rex field=smtp_command "MAIL FROM:<[^@]+@(?<sender_domain>[^>]+)>"
| where isnotnull(sender_domain)
| where lower(sender_domain) != lower(authenticated_domain)
| stats count values(sender_domain) as spoofed_domains values(client_ip) as src_ips
        by authenticated_user authenticated_domain
| where count >= 3
| sort - count

Wazuh OSSEC Rule

<group name="mail_assure,smtp,cve_2025_68624,">
  <rule id="2025687" level="0">
    <decoded_as>mail_assure_smtp</decoded_as>
    <field name="smtp_command">^MAIL FROM:</field>
    <description>Mail Assure SMTP envelope sender observed</description>
  </rule>

  <rule id="2025688" level="10">
    <if_sid>2025687</if_sid>
    <field name="authenticated_domain">\.+</field>
    <field name="sender_domain">\.+</field>
    <description>CVE-2025-68624: sender domain does not match authenticated tenant domain (possible cross-tenant spoofing)</description>
    <mitre>
      <id>T1078</id>
    </mitre>
    <group>cve_2025_68624,cwe_290,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 — UNPATCHED. No patch, vendor fix, or workaround is available from any tracked source. N-able classifies the behavior as intended functionality.

1) Immediate mitigations (compensating controls).

• Enforce per-tenant sender-domain allowlisting at the relay boundary if the platform exposes any such configuration; where it does not, treat the relay as untrusted for sender identity.
• On receiving mail servers, do not treat "SPF pass + DMARC pass via Mail Assure relay IPs" as sufficient proof of sender authenticity. Apply additional business-logic verification for payment/invoice changes (out-of-band callback).
• Rotate and tightly scope SMTP credentials; monitor for accounts sending mail for domains outside their own tenant.
• Deploy the Sigma/Suricata/Splunk detections above to alert on sender-domain vs. authenticated-domain mismatches.

2) Patch/upgrade steps. No patch exists. Organizations concerned by the disputed design should raise the issue with N-able support and request a documented statement of the relay's authorization model. If N-able maintains the current design, evaluate alternative relay providers that enforce domain-to-account binding.

3) Long-term hardening.

• Prefer mail relays that enforce explicit sender-domain authorization (envelope MAIL FROM must be a domain owned by the authenticated account).
• Implement DMARC at p=reject for your own domains and audit SPF include mechanisms to ensure you understand which infrastructure is authorized to send on your behalf.
• Adopt BIMI and/or ARC-aware policies where supported, and train finance/ops staff on BEC verification procedures that do not rely solely on SPF/DMARC.
• Maintain a vendor risk register entry for N-able Mail Assure noting the unresolved CWE-290 dispute.

References

1. NVD — CVE-2025-68624: https://nvd.nist.gov/vuln/detail/CVE-2025-68624
2. MITRE CWE-290 — Authentication Bypass by Spoofing: https://cwe.mitre.org/data/definitions/290.html
3. N-able Mail Assure product documentation (vendor): https://www.n-able.com/products/mail-assure
4. MITRE ATT&CK T1078 — Valid Accounts: https://attack.mitre.org/techniques/T1078/
5. CISA KEV Catalog (no entry for CVE-2025-68624 as of report date): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

Threat actor / exploit-in-the-wild note: No public PoC, exploit code, or Nuclei template exists. CISA KEV status is NO. EPSS is 0.00293 (21.9th percentile), indicating low near-term exploitation probability by automated means. No threat actor campaign is associated with this CVE in the available data. The vulnerability remains unpatched and disputed by the vendor.