A previously undocumented Windows loader designated as OXLOADER delivers the CASTLESTEALER infostealer through malicious Google Ads campaigns, achieving remarkably low detection rates. The loader employs multiple obfuscation layers including control-flow flattening, opaque predicates, and mixed Boolean-Arithmetic techniques, along with self-modifying decryption stubs and abuse of the Windows .reloc section for shellcode staging. Distribution occurs via malvertising impersonating Node.js installations, redirecting victims through intermediary domains to Storj-hosted batch scripts. The loader implements five anti-VM and language checks, including CIS-region and Russian-language exclusions, suggesting a financially motivated Russian-speaking threat actor. OXLOADER uses DonutLoader to deliver the .NET-based CASTLESTEALER payload in memory, evading traditional detection mechanisms through deliberate engineering choices.
Created: 2026-06-19
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