How do attackers use additional email delegate permissions?

How delegate permissions become an attack path

Mailbox delegation turns a legitimate collaboration feature into an access path if an attacker can add or inherit delegate rights. Once that happens, the attacker can act through a trusted mailbox rather than a separate suspicious account, which makes the activity blend into normal business traffic and raises the chance that fraud or surveillance is missed.

Attackers often prefer delegate permissions because they provide durable access to messages, calendar items, and sending ability without needing to reset the victim’s password. That can let them monitor conversations, harvest reset links or invoices, and impersonate the user in a way that looks internally consistent to recipients.

When the delegate relationship is the objective, the mailbox itself becomes the control point. The Ultimate Guide to NHIs, Key Challenges and Risks discusses the broader pattern of overprivilege and visibility gaps that make delegated access hard to spot and harder to govern at scale.

What attackers do with delegated mailbox access

With delegate permissions, attackers can read existing mail, watch for sensitive replies, send new mail, or delete items that might reveal compromise. In practical terms, that supports business email compromise, internal deception, invoice diversion, and quiet reconnaissance over a long period.

Delegated access is especially useful when the attacker wants to avoid triggering sign-in alerts. They may not need to authenticate as the user at all, because the mailbox already permits actions on the user’s behalf. That makes the abuse look like authorised activity unless defenders distinguish normal delegation from suspicious delegation.

Persistence Techniques Guide is relevant here because mailbox delegation can function as a persistence mechanism, letting an attacker keep access even if a password is changed or one login session is terminated.

Attackers also use delegated mail access to shape trust. Messages sent from a legitimate mailbox can bypass the caution that recipients apply to unknown senders, especially when the tone, thread history, and display name all look normal.

What defenders should verify and restrict

Review who can delegate mailbox access, how that access was granted, and whether the permission set matches the role. The key question is not whether delegation exists, but whether it is still justified, narrowly scoped, and traceable to a business need.

Watch for new delegate grants, unexpected send-as rights, mailbox rules that hide or forward mail, and delegated access that appears outside the user’s normal support model. Those are often the first observable signs that a mailbox has been turned into an abuse channel rather than a collaboration tool.

Use the same discipline you would apply to other high-impact access paths. Privileged Access Management Guide is useful because delegate rights can create a form of practical privilege, even when the account itself is not an administrator.

Authorisation Models Guide helps frame the decision as an access-control problem: delegate rights should be limited by role, context, and purpose, not granted as a convenience default.

Risk and Threat Considerations

Delegate permissions are high value because they give attackers access to both content and trust. A single mailbox can expose sensitive information, enable fraudulent replies, and let the attacker operate from a legitimate-looking identity that recipients are less likely to question.

Failure mechanism: The attacker inherits mailbox authority through delegation, then uses that access to read, send, or remove mail while blending into routine user activity. If delegation is broad or poorly monitored, compromise can persist after the initial foothold is closed.

Impact: The result can be information theft, business email compromise, lateral trust abuse, and delayed detection because the malicious activity appears to come from an approved mailbox rather than an obviously foreign account.

Practitioner Guidance

What to verify: Confirm that every delegate relationship has an owner, a business justification, and a removal path. If you cannot explain why a mailbox delegate exists, treat it as an exposure to be reviewed, not as a harmless convenience.

Common mistake: Teams often focus on password resets and miss mailbox permissions that still permit meaningful abuse. If the mailbox can still be read or used to send mail after account recovery, the incident is not fully contained.

Decision rule: If delegated access is unnecessary or unusually broad, remove it first and then assess whether the mailbox was already used for fraud, hiding, or message tampering. Access cleanup should not wait for perfect forensic certainty.

Practitioner takeaway: Treat mailbox delegation as a privileged access surface, not just an email setting, because the real risk is trusted impersonation plus durable visibility into the victim’s communications.

Related resources from NHI Mgmt Group

• Additional Email Delegate Permissions
• How do attackers turn a supply-chain incident into wider NHI compromise?
• How do attackers operationalise stolen OAuth tokens at scale?
• Why do attackers often check model availability before trying to generate content?