Security News > 2021 > September > Microsoft Says Chinese Hackers Were Behind SolarWinds Serv-U SSH 0-Day Attack

Microsoft has shared technical details about a now-fixed, actively exploited critical security vulnerability affecting SolarWinds Serv-U managed file transfer service that it has attributed with "High confidence" to a threat actor operating out of China.

"The Serv-U SSH server is subject to a pre-auth remote code execution vulnerability that can be easily and reliably exploited in the default configuration," Microsoft Offensive Research and Security Engineering team said in a detailed write-up describing the exploit.

"An attacker can exploit this vulnerability by connecting to the open SSH port and sending a malformed pre-auth connection request. When successfully exploited, the vulnerability could then allow the attacker to install or run programs, such as in the case of the targeted attack we previously reported," the researchers added.

While Microsoft linked the attacks to DEV-0322, a China-based collective citing "Observed victimology, tactics, and procedures," the company has now revealed that the remote, pre-auth vulnerability stemmed from the manner the Serv-U process handled access violations without terminating the process, thereby making it simple to pull off stealthy, reliable exploitation attempts.

"Therefore, an attacker could exploit this vulnerability by connecting to the open SSH port and sending a malformed pre-auth connection request. We also discovered that the attackers were likely using DLLs compiled without address space layout randomization loaded by the Serv-U process to facilitate exploitation," the researchers added.

Microsoft, which disclosed the attack to SolarWinds, said it recommended enabling ASLR compatibility for all binaries loaded in the Serv-U process.

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