Tavis Ormandy, an information security engineer with Google, wrote he found bugs in Trend Micro's antivirus product that could allow remote code execution by any website and steal all of a users' passwords.
The security firm has confirmed it has released an automatic update that fixes the problems.
"As part of our standard vulnerability response process we worked with him to identify and address the vulnerability," wrote Christopher Budd, global threat communications manager at Trend Micro, in an email on Monday. "Customers are now getting protections through automatic updates."
Ormandy posted emails he exchanged with Trend officials, occasionally expressing his frustration that the company wasn't moving fast enough.
"So this means anyone on the internet can steal all of your passwords completely silently, as well as execute arbitrary code with zero user interaction," Ormandy wrote. "I really hope the gravity of this is clear to you, because I'm astonished about this."
In 30 seconds, Ormandy wrote he found one that would accept remote code. He also found an API that allowed him to access passwords stored in the manager.
Overall, Ormandy wrote that he found over 70 APIs exposed to the Internet, not all of which he had investigated for security issues. He suggested Trend should hire an external consultancy to audit the code.
Antivirus applications run with high-level privileges on operating systems, which means that exploiting a vulnerability can give an attacker deep access to a computer.
Dozens of serious vulnerabilities have been found in the last seven months in antivirus products from vendors including Kaspersky Lab, ESET, Avast, AVG Technologies, Intel Security (formerly McAfee) and Malwarebytes.