The Russian cyberespionage group APT28 has rushed to add a recently patched Office vulnerability to its arsenal, with the first attacks observed just days after Microsoft announced fixes.
The flaw, tracked as CVE-2026-21509, was addressed by Microsoft on January 26. The tech giant warned at the time that the vulnerability had been exploited as a zero-day and urged customers to apply the patches immediately.
Microsoft initially credited its own security researchers for finding the vulnerability, but later updated its advisory to also credit Google Threat Intelligence Group (GTIG). However, neither Microsoft nor GTIG has released any information on the attacks exploiting CVE-2026-21509.
While it remains unclear who exploited the Office vulnerability as a zero-day, Ukraine’s computer emergency response team (CERT-UA) and cybersecurity firm Zscaler revealed this week that the flaw was quickly weaponized by Russia’s APT28 after its disclosure.
APT28 is a well-known, highly sophisticated group that is also tracked by the cybersecurity industry as Forest Blizzard, Sofacy, Fancy Bear, and GruesomeLarch.
CVE-2026-21509 can be exploited by tricking the targeted user into opening a specially crafted Office file.
While both Zscaler and CERT-UA spotted the first malicious file exploiting the vulnerability on January 29, the Ukrainian agency found evidence that the weaponized document had been created on January 27, the day after Microsoft announced patches for CVE-2026-21509.
Since there appears to be no publicly available technical information on the vulnerability, the threat actor likely reverse-engineered Microsoft’s patches to develop its exploit.
Zscaler, which linked the campaign to APT28 with high confidence based on victimology and TTPs, has observed exploitation of CVE-2026-21509 to deliver a dropper that in turn delivered other malware.
One of the pieces of malware deployed by the dropper is MiniDoor, described by the security firm as an Outlook macro-based email stealer. The other malware observed in the attacks has been named PixyNetLoader, which the attackers use to deploy a Covenant Grunt implant that provides them with full remote access and various post-exploitation capabilities.
Zscaler has seen attacks targeting users in Central and Eastern Europe, including Slovakia, Romania, and Ukraine.
“Social engineering lures were crafted in both English and localized languages, (Romanian, Slovak and Ukrainian) to target the users in the respective countries,” Zscaler explained.
Indicators of compromise (IoCs) have been made available by both Zscaler and CERT-UA.
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