Microsoft Power Platform: Security Risks and Solutions
November 14, 2024
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Recent research has unveiled a significant vulnerability in Microsoft 365 phishing defenses, allowing attackers to hide critical security alerts using a simple CSS trick. This flaw affects the First Contact Safety Tip a feature designed to warn users about potential phishing emails posing a serious threat to organizational security.
Microsoft 365, known for its robust security measures, includes the First Contact Safety Tip to alert users when they receive emails from unfamiliar senders. This alert is typically displayed within the body of an HTML email, serving as a warning sign to be cautious. However, researchers William Moody and Wolfgang Ettlinger from Certitude discovered that this alert could be hidden using a clever CSS manipulation, which directly impacts Microsoft 365 phishing protection.
By adjusting the background and text colors of the alert to white, attackers can render the message invisible to the recipient. Unlike other CSS tricks such as setting the display to none, altering opacity, or reducing height to zero this method works effectively due to how Outlook processes and renders emails. This approach significantly increases the risk of successful Microsoft 365 phishing attacks.
While the email preview might still show the alert, the main body of the email won’t display it, making it easier for phishing attempts to go unnoticed. Additionally, attackers can further deceive recipients by faking the “Signed by” declaration in Outlook emails. By replacing certain characters in the email address with similar-looking Unicode characters, attackers can avoid detection and increase the chances of their email being perceived as legitimate. This technique is a concerning development in the realm of Microsoft 365 phishing tactics.
Although these techniques may not fool every user, it only takes one individual falling for the phishing attempt to compromise an entire organization. Unfortunately, despite being notified, Microsoft has indicated that this issue does not currently meet their criteria for immediate resolution. They have, however, acknowledged the flaw in their Microsoft 365 phishing defenses and have marked it for future review.
The discovery of this CSS vulnerability in Microsoft 365’s phishing alert system highlights the ongoing challenges in securing digital communications. While Microsoft’s anti-phishing measures are generally robust, this flaw exposes a significant weakness that could be exploited by attackers. Organizations using Microsoft 365 must remain vigilant and educate their users on recognizing Microsoft 365 phishing attempts, even when no apparent warnings are visible.
The First Contact Safety Tip is a feature in Microsoft 365 designed to alert users when they receive emails from unfamiliar senders, helping to identify potential phishing threats.
Attackers can use CSS to change the background and text colors of the First Contact Safety Tip to white, making it invisible in the email body, thereby bypassing Microsoft 365 phishing alerts.
This vulnerability allows phishing emails to bypass Microsoft 365’s built-in warnings, increasing the likelihood of users falling victim to phishing attacks.
Microsoft has acknowledged the issue but has not prioritized it for immediate resolution. They have, however, marked it for future review to improve their Microsoft 365 phishing defenses.
Organizations should continue to educate their employees on recognizing Microsoft 365 phishing attempts and encourage skepticism of emails, even if no warnings are visible.
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