Attackers are exploiting an unpatched hole in Symantec's Veritas Backup Exec Agent for Windows, the company has warned.

A flaw in the product's Network Data Management Protocol agent could allow an attacker to gain access to the system and download files, the Fr-SIRT (French Security Incident Response Team) said in a statement Friday. Fr-SIRT rates the vulnerability as "critical".

Symantec, which acquired Veritas in July of this year, says it is "not aware of any vendor-supplied patches for this issue". The company recommends that users block access to the TCP port that uses the service in question, port 10,000.

The Metasploit penetration testing toolkit already takes advantage of this vulnerability, and there are reports that exploits for the flaw are already being used by attackers, Symantec said.

The SANS Internet Storm Center said on its website on Friday that it has seen a jump in scans for port 10,000, and it advises Backup Exec users to block access to that port from all untrusted network.

The flaw affects versions 8.x, 9.0, 9.1, and 10.0 of Backup Exec for Windows Servers, Fr-SIRT said.