Organizations worldwide have been warned of an increase in the number of attacks abusing Microsoft OneNote documents for malware delivery.
Part of the Office suite, OneNote is typically used within organizations for note taking and task management, among other operations.
What makes OneNote documents an attractive target for threat actors includes the fact that they do not benefit from the Mark-of-the-Web (MOTW) protection, along with the fact that files can be attached to OneNote notebooks and then executed with minimal warnings.
In August last year, security researchers warned that MOTW was not applied to OneNote attachments, meaning that unsigned executables or macro-enabled documents could be used to bypass existing protections.
According to WithSecure, however, Microsoft last month silently patched the ability to bypass MOTW for OneNote attachments, which decreases the potential for abuse, but does not completely eliminate it, allowing threat actors to embed files in OneNote documents and lure users into executing them.
Attacks abusing OneNote documents for malware delivery are not different from those using other types of malicious Office files: under different pretenses, the user is tricked into opening the document and enabling editing, which results in the execution of attached code.
In December 2022 and January 2023, Proofpoint observed more than 50 malicious campaigns abusing OneNote documents for the delivery of malware such as AsyncRAT, AgentTesla, DoubleBack, NetWire RAT, Redline, Quasar RAT, and XWorm.
Both Proofpoint and Sophos observed initial access broker TA577 joining the fray at the end of January, relying on the same technique for the delivery of Qbot (also known as Quakbot).
The observed campaigns are different in volume, some targeting a small number of industries, while others involve thousands of messages sent to numerous recipients. According to Proofpoint, the attacks targeted organizations worldwide, with a focus on North America and Europe.
“Based on data in open-source malware repositories, initially observed attachments were not detected as malicious by multiple antivirus engines, thus it is likely initial campaigns had a high efficacy rate if the email was not blocked,” Proofpoint notes.
According to Opalsec, the list of malware delivered via malicious OneNote documents also includes the Formbook infostealer, the IcedID trojan, and the Remcos RAT.
Security researcher Marco Ramilli says that the abuse of OneNote for malware delivery has been ongoing for more than four months. The observed payloads, he says, include some of the aforementioned remote access trojan (RAT) families.
These attacks are efficient because the target interacts with the malicious document. Thus, educating users and employees on not opening files received from untrusted sources can mitigate risks.
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