A malicious email campaign has been found abusing a Google Cloud Storage service to host a payload sent to employees of financial services organizations, Menlo Labs researchers report.

The threat appears to have been active in the US and UK since August 2018. Victims receive emails containing links to archive files; researchers say all instances in this particular campaign have been .zip or .gz files. All cases involve a payload hosted on storage.googleapis.com, which appears to be related to Google's cloud storage service but is, in fact, a malicious link.

Attackers often use this domain to host payloads because it's trusted and likely to bypass security controls in commercial threat detection products. These actors may have chosen bad links in lieu of malicious attachments because many email security products are designed to detect files and only pick up on malicious URLs if they're already in their threat repositories.

The use of a link resembling Google's cloud storage service is a form of "reputation jacking," a tactic in which attackers abuse well-known hosting services to evade detection. It's a growing trend, researchers say: In its annual analysis of the top 100,000 domains as ranked by Alexa, Menlo Labs found 4,600 phishing sites that used legitimate hosting services.

Google has responded to the report. "We regularly remove malware on Google Cloud Storage, and our automated systems suspended the malware referred to in this report," a spokesperson says. Further, account holders who suspect abuse can report it via Google's site.

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