Hackers Use Corrupted ZIPs and Office Docs to Evade Antivirus and Email Defenses

Cybersecurity researchers have called attention to a novel phishing campaign that leverages corrupted Microsoft Office documents and ZIP archives as a way to bypass email defenses.

"The ongoing attack evades #antivirus software, prevents uploads to sandboxes, and bypasses Outlook's spam filters, allowing the malicious emails to reach your inbox," ANY.RUN said in a series of posts on X.

The malicious activity entails sending emails containing ZIP archives or Office attachments that are intentionally corrupted in such a way that they cannot be scanned by security tools. These messages aim to trick users into opening the attachments with false promises of employee benefits and bonuses.

In other words, the corrupted state of the files means that they are not flagged as suspicious or malicious by email filters and antivirus software.

However, the attack still works because it takes advantage of the built-in recovery mechanisms of programs like Word, Outlook, and WinRAR to relaunch such damaged files in recovery mode.

ANY.RUN has revealed that the attack technique has been employed by threat actors at least since August 2024, describing it as a potential zero-day that is being exploited to evade detection.

The end goal of these attacks is to deceive users into opening booby-trapped documents, which embed QR codes that, when scanned, redirect victims to fraudulent websites for malware deployment or fake login pages for credential theft.

The findings once again illustrate how bad actors are constantly on the lookout for previously unseen techniques to get around email security software and ensure their phishing emails land in targets' inboxes.

"Although these files operate successfully within the OS, they remain undetected by most security solutions due to the failure to apply proper procedures for their file types," ANY.RUN said.

"The file remains undetectable by security tools, yet user applications handle it seamlessly due to built-in recovery mechanisms exploited by attackers."