In a recent phishing campaign, fraudsters used a legitimate browser extension tool called SingleFile to obfuscate their attacks and remain undetected.

According to Trend Micro, the malicious mail campaign started on Feb. 27 and utilized SingleFile, a web extension for Google Chrome and Mozilla Firefox that allows users to save webpages as single HTML files. As such, SingleFile is designed to help streamline the process by which users can archive webpages.

Threat actors abused SingleFile’s legitimate functionality, however, by copying the login pages of legitimate webpages, such as those of the payment processing website Stripe. Though simple, this spoofing method enabled the attackers to generate almost an identical copy of the legitimate website’s login mechanism, which they could then use to phish for users’ credentials. This attack technique came with an added bonus in that it hid the login form’s HTML code as well as the JavaScript used by the legitimate login page from detection by static security tools.

Attackers’ Growing Abuse of Legitimate Tools

As noted by Symantec, threat actors are increasingly living off the land in that they’re using tools already installed on a computer and running simple scripts or shellcode directly into memory as part of their campaigns. As with the use of SingleFile identified above, these tactics help attackers evade detection.

Fraudsters are also now obtaining digital certificates to add a sense of legitimacy to their phishing pages. According to Krebs on Security, just under half (49 percent) of phishing sites now come with the green padlock in the address bar, an icon that is indicative of a secure web connection.

How to Defend Against SingleFile Phishing Campaigns

Security professionals can help defend their organizations against a phishing campaign by using ahead-of-threat detection to filter out potentially malicious domains based on WHOIS information and other intelligence feeds. Security teams should also develop an ongoing security awareness program and customize training to the unique needs of the organization.