Brute Force Attacks on Microsoft 365: Identify, Respond, and Fortify Your Security

In a chilling reminder of how relentless cybercriminals can be, recent weeks have seen a surge in large-scale brute force attacks aimed at accessing Microsoft 365 (M365). If you’re an IT administrator responsible for M365 environments, this is your official wake-up call to dig into your sign-in logs and evaluate your defenses. Let’s break down what’s happening, how it's being done, and what proactive measures you can take.

---

What's Happening? Brute Force 101 in the Cloud Era​

Cyberattackers have been brazenly targeting Microsoft 365 accounts, attempting to crack access credentials using brute force techniques—a method where attackers systematically guess login credentials until they hit the jackpot. Think of it as an aggravated game of password roulette, but where the stakes are your organization's data security.
The attacks, reportedly originating mainly from Brazil (with curious outliers like Turkey, Argentina, and Uzbekistan), have left trails easily identified by the user agent “fasthttp.” This provides a silver lining for administrators—quick log analysis can help pinpoint suspicious activity. Even though early reports suggest that the wave of attacks may have cooled off, failing to check your system logs is akin to hearing a car alarm outside and assuming your car wasn’t the one targetted.

---

How to Check for Intrusion Attempts​

Before we go into panic mode, it’s important to determine whether your systems have already been compromised. Here's the good news: identifying these brute force attempts in M365 is relatively simple. Here’s a quick guide:

1. Log Into the Azure Portal​

Navigate to Microsoft Entra ID (formerly Azure AD).

2. Access the Sign-In Logs​

• Go to “Users” > “Sign-in Logs.”
• Use the filter: Client App > “Other Clients” and search for fasthttp.

3. Audit with Microsoft Purview​

Alternatively, if you’re auditing your system using Microsoft Purview, a simple keyword search for fasthttp should reveal suspicious activity. This tool grants more nuanced capabilities if you're running complex systems.

4. Automate Your Search​

If manual checks aren't your thing, cybersecurity specialists at Speartip have provided PowerShell scripts to streamline the process. This could save you some sweat equity and get you faster results.

---

What If You Spot Suspicious Activity?​

Now let’s tackle the “what if” scenario because spotting trouble is only half the battle. If you find instances of logins made using the fasthttp user agent—or worse, successful logins tied to it—here are your next steps:

• Reset the Credentials: Lock down any compromised accounts by enforcing an immediate password reset.
• Terminate Active Sessions: Don’t risk lingering sessions by attackers. Force a sign-out across all devices linked to the compromised accounts.
• Deploy Incident Response Protocols: Loop in your incident response team or trigger your escalation procedure to capture logs and contain further spread.

---

Fortifying Your Defenses: Security 101​

We can all agree that it’s better to stop attackers at the gates than to clean up after they’ve stormed the castle. Below are actionable steps to bolster your defenses against brute force attacks, and credential theft mishaps:

1. Implement Two-Factor Authentication (2FA)​

Sure, passwords are one piece of the puzzle, but 2FA adds an additional layer of protection, requiring a secondary code or device to authenticate users. While it’s not foolproof, it significantly reduces the risk of unauthorized access.

2. Geo-Blocking and IP Filtering​

If the increased traffic stems largely from unusual sources such as Brazil or Uzbekistan, consider blocking these regions outright. Tools are available to filter traffic by Autonomous System Numbers (ASN) or IP, enabling customizable constraints.

3. Harden Login Processes​

Defenses like enforcing password strength, setting lock-out thresholds for failed login attempts, and integrating biometrics where possible can make brute force attacks less effective.

4. Fine-Tune Conditional Access Policies​

Leverage Azure AD’s Conditional Access to gear up permissions based on user risk, device compliance, or geographic region. It’s your all-star feature for dynamic defense against sophisticated attacks.

---

The Bigger Picture: Why These Attacks Matter​

This isn’t the first time M365 has been targeted, nor will it be the last. What’s alarming, however, is the scale and persistence of these brute force attempts. It demonstrates how attackers are increasingly shifting their attention to high-value cloud environments. After all, M365 doesn’t just serve as an email platform—it’s the digital backbone for businesses worldwide.
Think about it: If attackers get in, they have keys to your productivity systems, calendars, file storage, and—worst of all—potentially critical business communications. It’s not just downtime you’re risking; it’s brand credibility and potential legal woes stemming from data breaches.

Why “Fasthttp” Matters​

The “fasthttp” user agent, noted in these logs, is a high-performance HTTP client library for Go (Golang). While powerful for legitimate programming and network tasks, it’s also a popular weapon in an attacker’s arsenal for large-scale credential stuffing and brute force attacks, thanks to its speed and efficiency.

---

Takeaway: Treat This as Your Cybersecurity Drill​

While this wave of attacks may already be subsiding, it’s a sober reminder that vigilance is not optional—especially when it comes to cloud-based platforms like M365. Whether your logs reveal failed attempts or successful breaches, take this event as a teaching moment to reassess your security readiness.
After all, as the world transitions further into cloud ecosystems, the next attempted breach may already be unfolding. Better to ready your defenses now than to regret later.
What do you think? Is your security posture strong enough to fend off attacks like these? Share your thoughts and experiences below, and let’s discuss how the Windows community can better fortify itself against threats like these.

---

Source: heise online https://www.heise.de/en/news/Large-scale-brute-force-attacks-on-M365-Check-log-ins-as-a-precaution-10252338.html