As Exchange Online prepares to phase out SMTP AUTH Basic Authentication for all tenants, Microsoft is getting ready for a significant security change for cloud email users. The modification targets one of the most antiquated and vulnerable methods of logging into email systems, where usernames and passwords are transmitted in plain sight, making it simple for hackers to obtain them in the event that traffic is intercepted or credentials are reused. Threat actors have been abusing SMTP AUTH with basic auth for years to run password-spraying campaigns, brute-force passwords, and take over accounts to send phishing and spam on a large scale.

Microsoft researchers discovered that basic SMTP authentication is a recurring vulnerability in many tenants in response to this ongoing abuse, particularly in cases where legacy devices, apps, and scripts continue to use outdated protocols that do not support contemporary security controls. Attackers can send emails as a trusted user, circumventing numerous security filters and harming an organization's reputation and email deliverability, once they obtain legitimate SMTP AUTH credentials. Deprecating basic authentication is therefore a crucial step in hardening cloud email rather than merely a protocol cleanup.

Microsoft analysts also pointed out that SMTP AUTH basic sign-ins frequently lack robust protections like conditional access and multi-factor authentication (MFA), leaving businesses vulnerable even when other areas of their environment are locked down.

SMTP AUTH basic auth has become a popular target for attackers searching for the weakest link because it is often enabled "just to keep things working" for printers, line-of-business systems, and third-party tools. Microsoft hopes to close this long-standing security vulnerability before more tenants experience account takeover and downstream compromise by mandating a shift away from basic auth. According to the revised schedule, SMTP AUTH Basic Authentication won't change until December 2026, giving businesses time to identify and update any workflows that still rely on it.

It will be turned off by default for current tenants at the end of December 2026, but administrators will still have the option to temporarily reactivate it while migrations are finished.

Learn more Features of the security author Services for penetration testing Evaluation of cybersecurity vulnerabilities Guide to Hacker Tools NLog Courses for cybersecurity training Training in ethical hacking for macOS security Cybersecurity of software Services for cloud security If a new tenant is formed after December 2026, SMTP AUTH By default, OAuth-based modern authentication will be supported; basic authentication will not be available. Infection Mechanism: Attackers' Abuse of SMTP AUTH Basic Instead of using SMTP AUTH basic auth as a conventional malware infection path, attackers actually view it as a simple entry point. They commonly use automated tools to perform password spraying and credential stuffing against SMTP endpoints, trying large sets of weak or reused passwords across many accounts until one succeeds.

After locating legitimate credentials, they authenticate via SMTP using basic auth and start sending a lot of phishing or business email compromise (BEC) messages that seem to be from within the victim's company. Malicious mail can then use links to payloads, steal additional credentials, or deceive users into making fraudulent payments, transforming a single weak protocol into a wide-ranging compromise channel. Set CSN as a Preferred Source in Google to Receive More Instant Updates from LinkedIn and X.