Cloudflare Workers is being abused in phishing campaigns to obtain credentials for Microsoft, Gmail, Yahoo!, and cPanel Webmail. The campaigns identified in the past month have mostly targeted individuals in Asia, North America, and Southern Europe, with the majority of attacks conducted on organizations in the technology, finance, and banking sectors.

Cloudflare Workers is part of the Cloudflare Developer Platform and allows code to be deployed and run from Cloudflare’s global network. It is used to build web functions and applications without having to maintain infrastructure. The campaigns were identified by researchers at Netskope Threat Labs. One campaign uses a technique called HTML smuggling, which involves abusing HTML5 and JavaScript features to inject and extract data across network boundaries. This is a client-side attack where the malicious activities occur within the user’s browser. HTML smuggling is most commonly associated with malware and is used to bypass network controls by assembling malicious payloads on the client side. In this case, the malicious payload is a phishing page.

The phishing page is reconstructed in the user’s browser, and they are prompted to log in to the account for which the attacker seeks credentials, such as their Microsoft account. When the victim enters their credentials, they will be logged in to the legitimate website and the attacker will then collect the tokens and session cookies.

Another campaign uses adversary-in-the-middle (AitM) tactics to capture login credentials, cookies, and tokens, and allow the attackers to compromise accounts that are protected with multi-factor authentication. Cloudflare Workers is used as a reverse proxy server for the legitimate login page for the credentials being targeted. Traffic between the victim and the login page is intercepted to capture credentials as well as MFA codes and session cookies. The advantage of this type of attack is the user is shown the exact login page for the credentials being targeted. That means that the attacker does not need to create and maintain a copy of the login page.

When the user enters their credentials, they are sent to the legitimate login page by the attacker, and the response from the login page is relayed to the victim. The threat actor’s application captures the credentials and the tokens and cookies in the response. In these CloudFlare Workers phishing campaigns, users can identify the scam by looking for the * domain and should be trained to always access login pages by typing the URL directly into the web browser.

Defending against sophisticated phishing attacks requires a combination of security measures including an email security solution with AI/machine learning capabilities and email sandboxing, regular security awareness training, and web filtering to block the malicious websites and inspecting HTTP and HTTPS traffic. For more information on improving your defenses, give the TitanHQ team a call.