Microsoft’s Verified Publisher Status Abused in Email Theft Campaign


Microsoft and cybersecurity firm Proofpoint on Tuesday warned organizations that use cloud services about a recent campaign that involved malicious OAuth applications and abuse of Microsoft’s ‘verified publisher’ status.

The campaign mainly targeted Microsoft customers in Ireland and the UK. The tech giant has taken steps to disrupt the operation and it has published an article on how users can protect against these threats, which the company calls ‘consent phishing’.

In a consent phishing attack, a threat actor attempts to trick a targeted user into granting permissions to their malicious cloud applications. Once they have obtained the required permissions, the malicious apps can gain access to legitimate cloud services and user data. 

In a campaign uncovered by Proofpoint in December 2022, hackers created malicious OAuth apps and then obtained a ‘verified publisher’ status in an effort to increase their chances of tricking users.

According to Microsoft, the attackers impersonated legitimate companies when enrolling in the Microsoft Cloud Partner Program (MCPP). 

“The actor used fraudulent partner accounts to add a verified publisher to OAuth app registrations they created in Azure AD,” Microsoft explained. 

This made it more likely for targeted users to grant permissions to the malicious applications. These permissions included reading emails, changing email settings, and accessing files and other user data, such as calendar and meeting information.

Microsoft’s investigation showed that the attackers used the malicious OAuth applications to exfiltrate emails. 

According to Proofpoint, the attackers used three malicious apps created by three different publishers. They all used the same malicious infrastructure and targeted the same organizations. 

“The potential impact to organizations includes compromised user accounts, data exfiltration, brand abuse of impersonated organizations, business email compromise (BEC) fraud, and mailbox abuse,” Proofpoint said. “The attack was less likely to be detected than traditional targeted phishing or brute force attacks. Organizations typically have weaker defense-in-depth controls against threat actors using verified OAuth apps.”

According to Proofpoint, the campaign ran until December 27. The security firm observed attacks against financial and marketing staff, as well as executives and managers. 

Microsoft said it became aware of the campaign on December 15. The company has disabled all fraudulent applications and alerted affected customers. 

Microsoft recently also dismantled a campaign that leveraged a network of single-tenant OAuth applications for the distribution of spam

Related: GitHub Warns of Private Repositories Downloaded Using Stolen OAuth Tokens

Related: CircleCI Hacked via Malware on Employee Laptop

Related: Okta Source Code Stolen by Hackers

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Microsoft Urges Customers to Patch Exchange Servers


Microsoft this week published a blog post to remind its customers of the continuous wave of attacks targeting Exchange servers and to urge them to install the latest available updates as soon as possible.

“Attackers looking to exploit unpatched Exchange servers are not going to go away,” Microsoft says, reminding customers that both a cumulative update (CU) and a security update (SU) are available for Exchange.

“There are too many aspects of unpatched on-premises Exchange environments that are valuable to bad actors looking to exfiltrate data or commit other malicious acts,” the company continues.

Attackers, the tech giant notes, are after not only the sensitive information that user mailboxes may contain. They are also looking to access the copy of the company address book stored on the Exchange server, which they can then use in social engineering attacks.

On top of that, Microsoft notes, “Exchange has deep hooks into and permissions within Active Directory, and in a hybrid environment, access to the connected cloud environment.”

Almost every set of Patch Tuesday updates coming out of Redmond includes security fixes for Exchange, some of which address already-exploited vulnerabilities, such as ProxyNotShell and ProxyShell. For other bugs, proof-of-concept (PoC) code was published shortly after patches were released.

“To defend your Exchange servers against attacks that exploit known vulnerabilities, you must install the latest supported CU (as of this writing, CU12 for Exchange Server 2019, CU23 for Exchange Server 2016, and CU23 for Exchange Server 2013) and the latest SU (as of this writing, the January 2023 SU),” Microsoft notes.

Because the CUs and SUs are cumulative, only the latest needs to be installed. However, Exchange customers are advised to check whether a security update has been released after they installed the latest CU, and install that as well.

The tech giant also notes that mitigations that it might automatically release for a vulnerability prior to pushing an SU are only meant to provide temporary protection and might not provide protection against all variations of an attack, meaning that customers should install the SU instead.

After installing an update, customers should also run Health Checker to verify if there are any manual tasks that need to be performed. The tool provides links to step-by-step guidance for the necessary actions.

To update an Exchange server, customers should start by reading the announcement about that update, follow the available guidance for CUs or SUs, inventory all servers using Health Checker, and use the Exchange Update Wizard, which offers a step-by-step guide to Exchange updates.

Windows Server and other software running on the Exchange server should also be updated, along with dependency servers that Exchange uses, such as Active Directory and DNS.

Related: Microsoft Warns of New Zero-Day; No Fix Yet for Exploited Exchange Server Flaws

Related: Mitigation for ProxyNotShell Exchange Vulnerabilities Easily Bypassed

Related: Microsoft Adds On-Premises Exchange, SharePoint, Skype to Bug Bounty Program

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