Cybercriminals are using SharePoint to send malicious documents to businesses in the United Kingdom. This tactic has seen many messages pass through email security defenses undetected and arrive in inboxes.
The campaign appears to be targeting businesses in the financial services and aims to obtain Office 365 credentials and username/password combos from other email service providers. Those credentials can be used to gain access to sensitive information in email accounts and cloud storage repositories such as OneDrive.
In the latest campaign, the attacker used a compromised email account at a London legal firm to send emails to employees of businesses in the financial services sector. The attacker uses SharePoint to send a request to review a document. In order to view the document, the user is required to click an embedded hyperlink in the email.
If that link is clicked, the user is directed to SharePoint and onto another malicious URL where they are requested to download a OneNote document. In order to download that document, the user is required to enter their login credentials.
Since the initial URL is for the SharePoint domain, many email security solutions fail to identify the link as malicious. Similar tactics have been used in phishing campaigns that link to OneDrive, Citrix ShareFile, Google Drive, and Windows.net. Since the domains are thought to be benign and the email messages do not contain any malware, the messages are delivered to end users.
The URL used in this campaign is likely to arouse suspicion even though it is a SharePoint domain, but not all users carefully check URLs and the full URL may not be visible on mobile devices, which increases the risk of an end user being fooled into disclosing their login credentials. The spoofed OneDrive for Business portal to which the user is directed is also a poor imitation, but it is sufficiently realistic to fool many end users. Other identified phishing campaigns using file sharing websites are far more convincing and are unlikely to be detected as malicious even by security conscious employees.
When credentials are compromised, the email account is often used to send further phishing emails to other individuals in the organization. Since those emails come from an internal account, users are more likely to respond. The attackers can also view past message threats in the compromised account and use those messages to continue a conversation. The messaging style of the account holder can also be mimicked to add further realism to the phishing emails. Typically, businesses discover one email account has been compromised, but the investigation reveals the attack is far more widespread and many email accounts have been compromised. Once recent phishing attack on a U.S. healthcare provider saw an astonishing 72 email accounts compromised!
To block these threats, an advanced email security solution is required. Businesses should look for a solution that incorporates DMARC. DMARC incorporates SPF and DKIM email authentication protocols and verifies that the IP address used to send the email is authorized to send emails from that domain. If that check fails, the email is blocked. This is one of the most important and most effective methods of detecting and blocking email impersonation attacks, including BEC attacks and lateral phishing attempts.
Fortunately, a combination of an advanced spam filtering solution and end user security awareness training will help to ensure that emails do not reach inboxes and, if they do, that employees will be alert to the threat and will avoid clicking the link and disclosing their credentials.