Suspected state-backed Chinese hackers used a security hole in a popular email security appliance to break into the networks of hundreds of public and private sector organizations globally, nearly a third of them government agencies including foreign ministries, the cybersecurity firm Mandiant said Thursday.

“This is the broadest cyber espionage campaign known to be conducted by a China-nexus threat actor since the mass exploitation of Microsoft Exchange in early 2021,” Charles Carmakal, Mandiant’s chief technical officler, said in a emailed statement. That hack compromised tens of thousands of computers globally.

In a blog post Thursday, Google-owned Mandiant expressed “high confidence” that the group exploiting a software vulnerability in Barracuda Networks’ Email Security Gateway was engaged in “espionage activity in support of the People’s Republic of China.” It said the activivity began as early as October.

The hackers sent emails containing malicious file attachments to gain access to targeted organizations’ devices and data, Mandiant said. Of those organizations, 55% were from the Americas, 22% from Asia Pacific and 24% from Europe, the Middle East and Africa and they included foreign ministries in Southeast Asia, foreign trade offices and academic organizations in Taiwan and Hong Kong. the company said.

Mandiant said the majority impact in the Americas may partially reflect the geography of Barracuda’s customer base.

Barracuda announced on June 6 that some of its its email security appliances had been hacked as early as October, giving the intruders a back door into compromised networks. The hack was so severe the California company recommended fully replacing the appliances.

After discovering it in mid-May, Barracuda released containment and remediation patches but the hacking group, which Mandiant identifies as UNC4841, altered their malware to try to maintain access, Mandiant said. The group then “countered with high frequency operations targeting a number of victims located in at least 16 different countries.”

Mandiant said the targeting at both the organizational and individual account levels, focused on issues that are high policy priorities for China, particularly in the Asia Pacific region. It said the hackers searched for email accounts of people working for governments of political or strategic interest to China at the time they were participating in diplomatic meetings with other countries.

The U.S. government has accused Beijing of being its principal cyberespionage threat, with state-backed Chinese hackers stealing data from both the private and public sector.

China says the U.S. also engages in cyberespionage against it, hacking into computers of its universities and companies.