Researchers at Microsoft have observed a widespread phishing campaign that’s abusing open redirectors to fool users into visiting credential-harvesting pages. Open redirects are often used for legitimate purposes, such as tracking click rates. However, they can also be abused to disguise a link to a phishing page.

“The use of open redirects in email communications is common among organizations for various reasons,” the researchers write. “For example, sales and marketing campaigns use this feature to lead customers to a desired landing web page and track click rates and other metrics. However, attackers could abuse open redirects to link to a URL in a trusted domain and embed the eventual final malicious URL as a parameter. Such abuse may prevent users and security solutions from quickly recognizing possible malicious intent.”

Microsoft explains that this tactic can fool both users and technology, since the URL itself appears legitimate.

“[U]sers trained to hover on links and inspect for malicious artifacts in emails may still see a domain they trust and thus click it,” Microsoft says. “Likewise, traditional email gateway solutions may inadvertently allow emails from this campaign to pass through because their settings have been trained to recognize the primary URL without necessarily checking the malicious parameters hiding in plain sight.”

The researchers also note that this campaign makes use of hundreds of unique domains.

“This phishing campaign is also notable for its use of a wide variety of domains for its sender infrastructure—another attempt to evade detection,” the researchers write. “These include free email domains from numerous country code top-level domains (ccTLDs), compromised legitimate domains, and attacker-owned domain generated algorithm (DGA) domains. As of this writing, we have observed at least 350 unique phishing domains used for this campaign. This not only shows the scale with which this attack is being conducted, but it also demonstrates how much the attackers are investing in it, indicating potentially significant payoffs.”

New-school security awareness training can enable your employees to recognize red flags associated with social engineering attacks.