Attacks targeting internet-accessible SolarWinds Web Help Desk (WHD) instances for initial access may have exploited recently patched vulnerabilities as zero-days, Microsoft says.
As part of a multi‑stage intrusion in December 2025, hackers compromised the vulnerable WHD deployments to spawn PowerShell and download and execute additional payloads.
However, Microsoft says it could not confirm whether the hackers exploited new or older SolarWinds vulnerabilities known to be exploited in the wild.
The tech giant says the compromised product was vulnerable to CVE-2025-40551 and CVE-2025-40536, both patched in January 2026, but also to CVE-2025-26399, which was fixed in September 2025.
CVE-2025-26399, described as an unauthenticated AjaxProxy deserialization remote code execution (RCE) bug, was disclosed as a bypass for CVE-2024-28988, which was a patch bypass for CVE-2024-28986.
The flawed AjaxProxy functionality is also the root cause of CVE-2025-40551. It is described as an untrusted data deserialization issue leading to unauthenticated RCE and was added to CISA’s KEV list last week.
CVE-2025-40536 is a security control bypass issue that could allow attackers to create valid AjaxProxy instances and then exploit CVE-2025-40551 for RCE.
“Since the attacks occurred in December 2025 and on machines vulnerable to both the old and new set of CVEs at the same time, we cannot reliably confirm the exact CVE used to gain an initial foothold,” Microsoft notes.
The company observed the attackers obtaining persistent access by deploying the legitimate remote monitoring and management (RMM) tool ManageEngine and establishing reverse SSH and RDP access.
They were also seen setting up a scheduled task to launch a QEMU virtual machine at startup with System privileges, and using the virtualized environment for evasion and SSH access via port forwarding.
In some instances, they used DLL sideloading to access LSASS memory and steal credentials, and used high-privilege credentials in a DCSync attack, requesting password data from a domain controller.
Organizations should immediately patch their WHD instances against the exploited vulnerabilities, find and remove unauthorized RMM applications, rotate credentials, and isolate any compromised hosts.
“This activity reflects a common but high-impact pattern: a single exposed application can provide a path to full domain compromise when vulnerabilities are unpatched or insufficiently monitored. In this intrusion, attackers relied heavily on living-off-the-land techniques, legitimate administrative tools, and low-noise persistence mechanisms,” Microsoft notes.
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