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Yes—AgreeTo, once a legitimate Outlook meeting-scheduling add-in, was hijacked to display a fake Microsoft sign-in page. Researchers reportedly recovered more than 4,000 credential sets from the attacker’s channel, but that figure is not a confirmed count of unique people. If you used AgreeTo after May 2023, remove it, change your Microsoft password and any reused passwords, and review account and mailbox activity.
Was the AgreeTo Outlook add-in hacked?
According to incident reporting published in February 2026, AgreeTo was originally a legitimate meeting-scheduling add-in. Its manifest pointed to content hosted at a Vercel URL. After the original deployment was abandoned and the URL became claimable, an attacker took control of that URL and served a fake Microsoft sign-in flow inside the add-in. Malwarebytes and BleepingComputer describe the incident.
The observed flow collected credentials, sent submitted data through a Telegram bot API, and then redirected users to Microsoft’s legitimate sign-in page. That familiar Outlook context and the real Microsoft page afterward could make the prompt seem routine; the redirect did not make the earlier prompt genuine.
The mechanism matters: the add-in’s approved manifest referenced remote content that could be served or changed later. Incident reporting and Koi Security researchers describe how taking over the remote host allowed the attacker to alter what users saw without changing the original manifest. This account describes the reported incident, not an independently audited assessment of every Microsoft review control.
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What information was exposed—and what is not confirmed?
Koi researchers reportedly recovered more than 4,000 Microsoft account credential sets from the attacker’s Telegram-based exfiltration channel. Reports also describe credit card information and banking security answers. The reports do not establish how many unique people were affected or a geographic scope, so the credential-set figure should not be read as a victim count. ThaiCERT also summarizes the reported exposure.
AgreeTo retained the ReadWriteItem permission, which reporting says can allow an add-in to read and modify email items. That means mailbox content could be at risk in principle. However, the incident reports describe credential phishing; they do not confirm that the attacker used this permission to steal mailbox contents.
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What to do if you used AgreeTo
If you used the add-in after May 2023, work through these steps. Opening it does not by itself prove that you entered credentials, but treat any password submitted to its sign-in prompt as exposed.
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- Remove AgreeTo. Uninstall the add-in from Outlook if it is still installed. If you cannot find or remove it, ask your organization’s Microsoft 365 administrator to check whether it remains available or installed in the tenant.
- Change your Microsoft account password. Use a new, unique password. If you entered a work or school account password, contact your organization’s IT or security team as well.
- Change reused or similar passwords elsewhere. Prioritize other accounts that used the same password or a close variation, and make each replacement unique.
- Review recent sign-ins and security activity. Look for activity you do not recognize, such as unfamiliar sign-ins or changes to account security details. Follow Microsoft’s account recovery or your organization’s incident process if you find anything suspicious.
- Inspect sent messages and forwarding rules. Look for messages you did not send and rules that forward, redirect, or otherwise handle email unexpectedly. For a work or school account, have your administrator review mailbox activity too.
- Consider what sensitive information you emailed. If messages contained financial, personal, or business-sensitive information, take appropriate steps to protect those accounts or notify the relevant people at your organization.
- Monitor payment activity and enable MFA. Check card and bank statements for unfamiliar charges or transactions. ThaiCERT recommends enabling multifactor authentication (MFA) on your Microsoft account and other accounts where it is available.
What Microsoft 365 administrators should review
Administrators should check whether AgreeTo remains installed or available to users in the tenant, identify users who may have used it, and review sign-in and mailbox activity for those accounts. Coordinate password changes, session or access response, and any further investigation with your organization’s incident-response process. The public incident reporting cited here does not provide a detailed tenant-console procedure, so exact admin steps will depend on your Microsoft 365 configuration.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What Microsoft did
The Hacker News reported on February 12, 2026, that Microsoft had removed the add-in from Marketplace. It quoted a Microsoft spokesperson as saying the company had “removed the add-in from our store, and have taken additional steps to protect potentially impacted customers,” and that Microsoft acts when it detects malicious marketplace activity and would continue improving proactive detection. The report does not detail those additional steps or establish which customers they covered. The Hacker News also reported that Koi researchers identified at least 12 phishing kits impersonating different brands; that is context about the operator, not a count of AgreeTo victims.
Koi co-founder and CTO Idan Dardikman described the broader concern this way: “The structural problem is the same across all marketplaces that host remote dynamic dependencies: approve once, trust forever,” as quoted by The Hacker News.
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