Microsoft Threat Intelligence said a cybercriminal group it tracks as Storm-1175 has exploited a maximum-severity vulnerability in GoAnywhere MFT to initiate multi-stage attacks including ransomware. Researchers observed the malicious activity Sept. 11, Microsoft said in a blog post Monday.

Microsoft’s research adds another substantive chunk of evidence to a growing collection of intelligence confirming the defect in Fortra’s file-transfer service was exploited as a zero-day before the company disclosed and patched CVE-2025-10035 on Sept. 18.

Despite this mounting pile of evidence, Fortra has yet to confirm the vulnerability is under active exploitation. The company has not answered questions or provided additional information since it updated its security advisory Sept. 18 to include indicators of compromise. 

Storm-1175, a financially motivated cybercrime group known for exploiting public vulnerabilities to gain access and deploy Medusa ransomware, exploited CVE-2025-10035 to achieve remote code execution, according to Microsoft. 

“They used this access to install remote monitoring tools such as SimpleHelp and MeshAgent, drop web shells, to move laterally across networks using built-in Windows utilities,” Sherrod DeGrippo, director of threat intelligence strategy at Microsoft, told CyberScoop in an email. “In at least one instance, the intrusion led to data theft via Rclone and a Medusa ransomware deployment.”

Microsoft’s findings bolster research from other firms including watchTowr, which said it obtained credible evidence of active exploitation of the GoAnywhere vulnerability dating back to Sept. 10, a day before Fortra maintains the vulnerability was discovered. 

“Microsoft has now linked the attacks to a known Medusa ransomware affiliate, confirming what we feared. Organizations running GoAnywhere MFT have effectively been under silent assault since at least Sept. 11, with little clarity from Fortra,” said Ben Harris, founder and CEO at watchTowr.

“Microsoft’s confirmation now paints a pretty unpleasant picture — exploitation, attribution, and a month-long head start for the attackers. What’s still missing are the answers only Fortra can provide,” Harris added.

This includes details about how the attackers accessed private keys required to achieve exploitation, as researchers from multiple firms flagged as a worrying signal last month. “Customers deserve transparency, not silence,” Harris said. 

Federal cyber authorities have confirmed active exploitation of GoAnywhere’s defect as well. The Cybersecurity and Infrastructure Security Agency added CVE-2025-10035 to its known exploited vulnerabilities catalog Sept. 29, noting the defect has been used in ransomware campaigns. 

DeGrippo said Storm-1175’s attacks are opportunistic, and have affected organizations in the transportation, education, retail, insurance and manufacturing sectors. “Their tactics reflect the broader pattern we’re seeing, which is blending legitimate tools with stealthy techniques to stay under the radar and monetize access through extortion and data theft,” she added.

Researchers haven’t said how many organizations are impacted by GoAnywhere attacks, but Fortra customers went through this before when a zero-day vulnerability in the same file-transfer service was widely exploited two years ago, resulting in attacks on more than 100 organizations.

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