The email threat landscape is constantly changing as new threats emerge or existing threats evolve over time.
Over the past few years, Emotet has been one of the primary threats being delivered via malicious spam campaigns as we have previously described in detail several times.
Following law enforcement disruption of the Emotet botnets, we’ve been waiting for another threat to fill the void left by Emotet’s exit.
Beginning in mid-September 2021, talos observed malspam campaigns being used to deliver malicious Microsoft Office documents that function as the initial stage of the infection process and are used to infect systems with SquirrelWaffle.
Similar to what has been observed in previous threats like Emotet, these campaigns appear to be leveraging stolen email threads, as the emails themselves appear to be replies to existing email threads.
As shown below, these emails typically contain hyperlinks to malicious ZIP archives being hosted on attacker-controlled web servers.
The language targeted by the reply messages typically matches the language used in the original email thread, demonstrating that there is some localization taking place dynamically.
While the majority of the emails were written in English, the use of other languages across these campaigns highlight that this threat is not limited to a specific geographic region.
Across the malicious email campaigns we have observed being used to deliver SquirrelWaffle.
Consistent with other threats also leveraging stolen email threads, talos observed some inconsistencies in how the attacker chooses which email chains to hijack.
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