Threat actors are abusing a technique called “URL rewriting” to hide their phishing links from security filters, according to researchers at Perception Point.
Security tools from major vendors use URL rewriting to prevent phishing attacks, but the same technique can be abused to trick these tools into thinking a malicious link is legitimate.
There are several ways to accomplish this, but the researchers explain that “the more probable tactic is for attackers to first compromise legitimate email accounts protected by a URL rewriting feature and then to send an email to themselves containing their ‘clean-later-to-be-phishing’ URL. Once the email passes through the URL protection service, the link is replaced, and includes the email security vendor’s name and domain, giving it an extra layer of legitimacy.”
The attacker can then redirect the URL to a phishing site, making the link appear safe to both the security tool and the human looking at the link.
“This ‘branded’ rewritten URL is later weaponized,” the researchers explain. “After it has been ‘whitelisted’ by the security service, the attackers can modify the destination of the URL to redirect users to a phishing site. This technique allows the malicious link to bypass further security checks, as many services rely on the initial scan and do not rescan known URLs. As an alternative course of action, attackers often employ advanced evasion techniques such as CAPTCHA evasion or geo-fencing to circumvent even a thorough analysis by the email security vendor.”
Perception Point adds, “This manipulation of URL rewriting is particularly dangerous because it takes advantage of the trust that users place in known security brands, making even highly aware employees more likely to click on the seemingly safe link. The threat actors exploit the gap between the time a URL is rewritten and when it is weaponized, bypassing most traditional security tools.”
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