Description

A new malicious advertising campaign is exploiting legitimate software downloads to distribute OysterLoader malware, also known as Broomstick or CleanUpLoader. This sophisticated initial access tool enables attackers to gain a foothold in corporate networks and deliver secondary payloads, including the Rhysida ransomware, a threat group that evolved from the Vice Society operation in 2021 and rebranded in 2023. Despite efforts to evade tracking, security researchers continue to monitor Rhysida’s ongoing campaigns. According to Expel researchers, this represents Rhysida’s second major malvertising campaign, expanding upon methods used successfully between May and September 2024. Since June 2025, operations have intensified, using Bing Ads to lure users to fake download pages that mimic legitimate software sites. These fraudulent ads appear in search results and even within the Windows 11 Start Menu, tricking users into downloading malware disguised as applications like PuTTY, Microsoft Teams, and Zoom. The OysterLoader malware employs advanced evasion techniques, including compression, obfuscation, and misuse of code-signing certificates, allowing it to bypass antivirus detection. While earlier campaigns used seven certificates, the 2025 wave has already cycled through over 40, showing significant operational investment. Detection rates remain low, with fewer than five antivirus engines identifying new samples. In addition to OysterLoader, the attackers deploy Latrodectus malware, sharing the same code-signing certificates and exploiting Microsoft’s Trusted Signing service. Despite Microsoft revoking over 200 certificates linked to this campaign, malicious ads remain active. Security teams are urged to verify all software downloads through official vendor websites to prevent infection and network compromise.