Microsoft Teams has become the main tool for communication in businesses globally. Due to this, security teams spend a lot of time and money on protection services like Microsoft Defender for Office 365 to guard against dangers like phishing emails, malicious links, and malware.
However, new research from the security firm Ontinue, released on Wednesday, November 26, shows a huge security flaw in the standard setup of Microsoft Teams collaboration with outside partners, known as B2B Guest Access, which lets attackers entirely bypass a company’s Microsoft Defender protections.
Who Controls Your Security as a Guest?
The problem isn’t actually a software bug in Teams; it’s about the way security is managed when employees work with external groups. Ontinue’s blog post makes it clear; when your staff accepts an outside invitation and joins another company’s chat, their security is no longer determined by their home organisation. Instead, the research found that security is controlled “entirely by that hosting environment.”
This finding is highly worrying. The moment a user accepts a guest invite, they instantly lose all their home security features, including Safe Links (the system that checks if a link is dangerous before you click it) and Zero-hour Auto Purge (ZAP), which is designed to retroactively delete malicious messages. Attackers are exploiting this. Attackers know this and can create their own basic Teams accounts with security policies completely switched off, basically creating a perfect trap.
Further probing revealed that the attacker needs minimal resources. They can set up a basic Microsoft 365 environment using a low-cost subscription or even a trial. Since these basic accounts lack security packages like Defender, they are unprotected by default, which means the attacker doesn’t need any complex setup to achieve a “protection-free zone.”
The Easy Way In
The risk has become even simpler because of a feature Microsoft rolled out in November 2025 (MC1182004), which is turned on by default for most users. This setting allows any Teams user to start a chat with any email address, even people not currently using Teams. The victim receives a genuine-looking Microsoft invitation and needs only a single click to enter the malicious, unprotected environment.
This easy invitation method, combined with the fact that most organisations are defaulted to accept guest invites from any company worldwide, means a lot of companies are exposed. Once inside, attackers can easily deliver phishing links and malware to employees without any security warnings appearing. Also, they can exfiltrate information (or steal sensitive data) and conduct large-scale social engineering attacks.
Experts Urge Immediate Action
Ontinue strongly recommends companies move quickly to change their configurations, suggesting they limit guest invitations to only those domains they explicitly trust.
Industry leaders also weighed in on the findings, sharing their perspectives with Hackread.com. They emphasised that this is a serious architectural problem requiring a configuration change, not just a patch.
Shane Barney, Chief Information Security Officer at Keeper Security, noted the deceptive nature of the attack: “The familiar interface can give the impression that security remains consistent, but the safeguards in place are entirely dependent on how the hosting tenant is configured.” He added that organisations must ensure “access is appropriately limited and activity tied to sensitive systems is consistently monitored.”
Julian Brownlow Davies, Senior Vice President, Offensive Security Strategy & Operations at Bugcrowd, clarified the uncomfortable truth for users: “The moment your users cross into someone else’s tenant as guests, your own Defender for Office 365 protections effectively disappear.” He concluded that because attackers abuse collaboration features, “you have to assume that attackers will abuse ‘legitimate’ collaboration features.”
Finally, Agnidipta Sarkar, Chief Evangelist at ColorTokens, stressed the immediate policy response needed: “Until Microsoft addresses this vulnerability, organisations must set up a policy to address this immediately, and disallow all B2B meetings using Teams from anyone not previously known.” He recommends that companies configure technical controls to ensure Teams allows B2B connections to predefined domains.