COMMENTARY: Defenders often admit that email connectors are the least appreciated of all attack vectors: a configuration meant to optimize message flow that’s blissfully easy for threat actors to take over.Left unchecked, malicious connectors can quietly intercept, redirect, or alter email traffic with no footprint in standard logs or alerts. With an increasing number of organizations moving towards and scaling cloud email services, knowledge of the threat represented by compromised connectors has become crucial.[SC Media Perspectives columns are written by a trusted community of SC Media cybersecurity subject matter experts. Read more Perspectives here.]Email connectors serve as a link to facilitating communication between an email server and other programs or systems. For cloud email services, they determine how mail gets routed, filtered, or sent through additional security tests.Administrators set them up to connect on-premises servers, third-party services, or specialized routing rules. In Microsoft 365, connectors help inbound and outbound mail processing, security rule application, and maintaining compliance. Operating at the infrastructure level, connectors differ from client-side rules by impacting the entire organization rather than a single account or device. If compromised, a connector can discreetly redirect emails, remove messages, or embed malicious content without setting off standard security alarms.Malicious connectors are a discreet, but dangerous threat to cloud email infrastructure. Their ability to go unnoticed and remain active despite credential resets makes them the tool of choice for bad actors.Security teams must approach connector configurations with the same degree of discipline as firewall rules or access controls. Incorporating connector monitoring into broader cybersecurity frameworks and practicing strict administrative hygiene helps organizations minimize the likelihood of compromise.Erich Kron, CISO Advisor, KnowBe4SC Media Perspectives columns are written by a trusted community of SC Media cybersecurity subject matter experts. Each contribution has a goal of bringing a unique voice to important cybersecurity topics. Content strives to be of the highest quality, objective and non-commercial.
How attackers weaponize connectors
The attack usually begins with credential theft. Using phishing, brute-force attacks, or social engineering tactics, attackers gain entry to administrative accounts. Once inside, they have the ability to create or alter connectors to divert email traffic to external servers under their control.What’s so dangerous about this technique is its resilience. Even when stolen credentials are revoked or the affected device gets replaced, the rogue connector remains active. It runs silently behind the scenes, remaining mostly unnoticed, enabling attackers to maintain access and continuously tamper with communications.Attackers sometimes use connectors to send phishing messages that are often from trusted domains, which makes them more successful. To conceal their activities, they may delete sent emails and responses received so that the breach is difficult for victims to identify.A common attack method involves covertly rerouting invoice-related emails by configuring a malicious connector linked to an external mail relay under the attacker’s control. Customers continue to receive legitimate invoices, but have their payment responses routed to the attacker's mailbox.They unknowingly transfer money to scam accounts, thinking they're paying for legitimate requests. Since the messages come from known domains and addresses, the recipients are not likely to suspect them. The money and reputation loss can be massive.Detection isn’t simple
Connector manipulation leaves almost no footprint in standard email logs or security dashboards. Because mail processing continues normally and outbound delivery metrics remain stable, no sudden bounce-backs or user complaints typically occur.Legacy anti-malware tools target email body and attachments, but ignore back-end routing policies. Even advanced threat analytics can miss connector abuses unless specifically designed to detect configuration drift. And because connector modifications do not trigger endpoint alerts, security teams remain unaware until a collateral effect, such as revenue loss or a data breach, surfaces.Many organizations lack granular visibility into connector inventories, let alone continuous monitoring for unauthorized changes. Default permissions in multi-tenant platforms typically provide large-scale connector-management privileges to administrators and service accounts.Without role-based, customizable access controls, a compromised single identity can taint the integrity of email systems for hundreds of thousands of users. The danger intensifies with automation, when script-based attacks can automatically generate numerous malicious connectors to enable large-scale compromise across geographical boundaries.Countermeasures against malicious connectors
Protecting against harmful connectors requires vigilant governance, real-time visibility, and synchronized collaboration. Here are seven strategies organizations can adopt to help create a stronger line of defense:- Restrict administrative access: Implement controls on who can add or modify connectors. Employ role-based access controls to restrict only authorized personnel from adding, modifying, or deleting connectors, and audit permissions periodically to avoid privilege escalation and to detect redundant or high-risk accounts.
- Monitor connector changes: Use tools or scripts to build a list of all the connectors that are currently in place, labeled by owner and function. Enable automatic notifications for anomalies, like connector additions, domain changes, or relay address updates.
- Review and purge unused connectors: Perform connector audits every quarter and remove any that are outdated, duplicate, or undocumented. Maintain a registry of approved connectors, tied to use cases and accountable personnel.
- Strengthen logging and retention: Log and maintain configuration changes for forensic examination. Send logs to a centralized SIEM for ongoing monitoring.
- Train IT teams: Train administrators to recognize the signs of connector abuse. Include connector-related scenarios in incident response drills.
- Enforce multi-factor authentication: Mandate MFA on all admin accounts. This introduces a vital layer of defense against credential theft.
- Build a connector exploitation response guide: Create a documented process for detecting and addressing unauthorized connector changes. Include detailed actions for revocation, remediation, and communication, including notifying affected users.
