Microsoft 365 provides email, file storage, collaboration, and identity management for the majority of UK businesses. Its default configuration prioritises ease of use over security, and most organisations never change those defaults. The result is an environment where external sharing is unrestricted, legacy authentication remains enabled, and administrative accounts lack multi-factor authentication.
Attackers target Microsoft 365 environments relentlessly because they contain everything: email archives, SharePoint documents, Teams conversations, and the Entra ID directory that controls access to every other cloud service. Compromising a single account often provides enough access to devastate an entire organisation.
The Most Dangerous Default Settings
Legacy authentication protocols like POP3, IMAP, and SMTP AUTH do not support multi-factor authentication. An attacker with stolen credentials can authenticate through these protocols even if MFA is enforced on modern authentication flows. Disabling legacy protocols blocks this bypass entirely, yet many organisations leave them enabled because a handful of older applications depend on them.
External sharing in SharePoint and OneDrive frequently allows anyone with a link to access shared files, including people outside the organisation. Staff share links for convenience without realising those links can be forwarded, indexed by search engines, or discovered by attackers who compromise a recipient’s email. Restricting external sharing to authenticated guests with specific domains dramatically reduces the exposure.
Audit logging in Microsoft 365 must be explicitly enabled and requires appropriate licensing to retain logs beyond the default 90-day period. Organisations that have not enabled unified audit logging have no visibility into mailbox access, file downloads, or administrative changes. When a breach occurs, the evidence needed to understand its scope simply does not exist.
William Fieldhouse, Director of Aardwolf Security Ltd, comments: “Microsoft 365 tenants are prime targets because the return on a successful compromise is enormous. One set of credentials can yield access to years of email history, sensitive documents, and the identity platform that controls everything else. We see the same misconfigurations repeatedly: no conditional access policies, legacy auth enabled, and global administrators without MFA. Each one provides an attacker a reliable path to full tenant compromise.”
Hardening Your Tenant
Block legacy authentication through conditional access policies. Enable security defaults at minimum, or implement a comprehensive conditional access policy set that enforces MFA, blocks risky sign-ins, and restricts access based on device compliance and location.
Commission Azure penetration testing that includes Microsoft 365 tenant assessment. Testers should evaluate Entra ID configuration, conditional access policy effectiveness, mail flow rules, and application consent grants. Many organisations are surprised to discover third-party applications with broad permissions granted through admin consent that nobody remembers approving.
Engage a best penetration testing company with deep Microsoft 365 expertise. Generic network testers lack the platform-specific knowledge needed to evaluate Entra ID attack paths, Exchange Online mail flow exploitation, and SharePoint/OneDrive data exposure effectively. Microsoft 365 security demands specialists.
Microsoft 365 is likely the single richest target in your entire IT estate. Harden it accordingly.
