Cybersecurity companies are warning about an uptick in the abuse of Clouflare's TryCloudflare free service for malware delivery.
The activity, documented by both eSentire and Proofpoint, entails the use of TryCloudflare to create a rate-limited tunnel that acts as a conduit to relay traffic from an attacker-controlled server to a local machine through Cloudflare's infrastructure.
Attack chains taking advantage of this technique have been observed delivering a cocktail of malware families such as AsyncRAT, GuLoader, PureLogs Stealer, Remcos RAT, Venom RAT, and XWorm.
The initial access vector is a phishing email containing a ZIP archive, which includes a URL shortcut file that leads the message recipient to a Windows shortcut file hosted on a TryCloudflare-proxied WebDAV server.
The shortcut file, in turn, executes next-stage batch scripts responsible for retrieving and executing additional Python payloads, while simultaneously displaying a decoy PDF document hosted on the same WebDAV server to keep up the ruse.
"These scripts executed actions such as launching decoy PDFs, downloading additional malicious payloads, and changing file attributes to avoid detection," eSentire noted.
"A key element of their strategy was using direct syscalls to bypass security monitoring tools, decrypting layers of shellcode, and deploying the Early Bird APC queue injection to stealthily execute code and evade detection effectively."
According to Proofpoint, the phishing lures are written in English, French, Spanish, and German, with the email volumes ranging from hundreds to tens of thousands of messages that target organizations from across the world. The themes cover a broad range of topics such as invoices, document requests, package deliveries, and taxes.
The campaign, while attributed to one cluster of related activity, has not been linked to a specific threat actor or group, but the email security vendor assessed it to be financially motivated.
The exploitation of TryCloudflare for malicious ends was first recorded last year, when Sysdig uncovered a cryptojacking and proxyjacking campaign dubbed LABRAT that weaponized a now-patched critical flaw in GitLab to infiltrate targets and obscure their command-and-control (C2) servers using Cloudflare tunnels.
Furthermore, the use of WebDAV and Server Message Block (SMB) for payload staging and delivery necessitates that enterprises restrict access to external file-sharing services to only known, allow-listed servers.
"The use of Cloudflare tunnels provide the threat actors a way to use temporary infrastructure to scale their operations providing flexibility to build and take down instances in a timely manner," Proofpoint researchers Joe Wise and Selena Larson said.
"This makes it harder for defenders and traditional security measures such as relying on static blocklists. Temporary Cloudflare instances allow attackers a low-cost method to stage attacks with helper scripts, with limited exposure for detection and takedown efforts."
The findings come as the Spamhaus Project called on Cloudflare to review its anti-abuse policies following cybercriminals' exploitation of its services to mask malicious actions and enhance their operational security by means of what's called "living-off-trusted-services" (LoTS).
It said it "routinely observes miscreants moving their domains, which are already listed in the [Domain Blocklist], to Cloudflare to disguise the backend of their operation, be it spamvertized domains, phishing, or worse."
