Open redirect vulnerabilities affecting American Express and Snapchat websites were exploited earlier this year as part of phishing campaigns targeting Microsoft 365 users, email security firm Inky reports.
Open redirect flaws exist because the impacted website does not validate user input, which allows threat actors to manipulate URLs to redirect users to malicious sites.
Because the manipulated link contains a legitimate domain name, the user might consider the link safe. However, the trusted domain is only used as a landing page.
From mid-May to late July, Inky observed roughly 7,000 phishing emails that originated from various hijacked accounts and which attempted to exploit the open redirect in snapchat[.]com.
Over the course of two days at the end of July, roughly 2,000 phishing emails attempted to exploit the americanexpress[.]com open redirect vulnerability.
“In both the Snapchat and the American Express exploits, the black hats inserted personally identifiable information (PII) into the URL so that the malicious landing pages could be customized on the fly for the individual victims,” Inky says.
In both cases, the attackers encoded the insertions to make them look as random characters and prevent victims from reverse engineering the PII strings.
The phishing emails in the Snapchat campaign impersonated DocuSign, FedEx, and Microsoft, but all were designed to redirect victims to websites meant to harvest the credentials of Microsoft 365 users.
The open redirect vulnerability was reported to Snapchat on August 4, 2021, but has remained unpatched.
As part of the American Express campaign, newly created domains were used to send phishing emails that redirected victims to Microsoft credential harvesting sites. The vulnerability was patched fast and the phishing links no longer work.
Inky points out that certain elements in the URL may indicate potential redirection attempts, including ‘url=’, ‘redirect=’, ‘external-link’, ‘proxy’, or multiple occurrences of ‘http’.
“Domain owners can prevent this abuse by avoiding the implementation of redirection in the site architecture. If the redirection is necessary for commercial reasons, then implementing an allow-list of approved safe links prevents bad actors from inputting malicious links,” Inky notes.