Bypassing Spam Filtering Mechanism in Outlook

In my previous article, I demonstrated how attackers could bypass Microsoft Defender SmartScreen using ISO files, allowing malicious executables to run without triggering security warnings.

That technique focused on post-download execution, where an attacker instructs the victim to extract and run malware. Still, attackers need a reliable method to deliver these malicious files without being flagged by email security systems.

This short blog post examines a newly discovered Outlook spam filter bypass that enables attackers to distribute malicious ISO files through emails without being quarantined by spam filters.

Technical Details

Outlook’s spam filtering mechanism is designed to detect and quarantine emails containing direct links to files with known malicious extensions (e.g., .iso, .exe). For instance, an email containing a visible link like https://afine.com/update.iso would typically be flagged and sent to the junk folder to prevent users from downloading potentially harmful content:

However, attackers can evade this detection using hyperlink obfuscation. By embedding a malicious URL under a benign-looking link, Outlook fails to detect the true destination of the hyperlink. For example:

<a href="https://afine.com/update.iso">https://afine.com/update</a>Code language: HTML, XML (xml)

As we can see below, the Spam Filtering Mechanism in Outlook did not block such links:

In this case, the recipient sees https://afine.com/update, which appears safe, but clicking the link initiates a download of update.iso, a potentially malicious file.

Impact

This vulnerability significantly reduces the effectiveness of Outlook’s spam filtering mechanism, allowing attackers to bypass security measures and distribute malicious files. Users who click the disguised links may unknowingly download and execute malware, leading to other cyberattacks.

Organizations relying on Outlook’s built-in security measures are at risk of phishing and malware attacks.

Recommendation

To mitigate this vulnerability, it is recommended that Outlook’s spam filtering mechanism be enhanced to analyze hyperlinks’ underlying URLs in addition to their visible text. Moreover, another technique using redirections can also bypass such filtering systems, so it is crucial also for spam filters to follow the links and analyze the final destination. Implementing URL inspection techniques to detect disguised links leading to harmful files will reduce the risk of phishing attacks.

Microsoft decided not to patch it, so it is advisable always to check underlying URLs and be cautious.

Final Words

This vulnerability in Outlook’s spam filtering highlights a fundamental weakness in how email security mechanisms analyze links and, based on it, make a spam decision.

Attackers can easily exploit this flaw to distribute malware using deceptive hyperlinks. Users and organizations should adopt proactive security measures to mitigate the risks of disguised links.

Microsoft has closed it as a Low-risk issue. There will be no CVE for that, and the bug will be patched later rather than sooner. Because of that, I thought it would be better to disclose it publicly rather than wait for the patch so the user can be aware of this issue from now on.

If you found this post helpful, check out my previous article on Microsoft Defender SmartScreen bypasses using ISO files. Understanding these techniques gives a full attack chain perspective.