Security experts have identified a potentially devastating vulnerability in FTP (file transfer protocol) servers.

The weakness uncovered is a buffer overflow flaw that attackers could exploit to gain root privileges on affected FTP servers.

The vulnerability affects several Unix platforms, including Sun Solaris 8, Hewlett Packard HP/UX 11, SGI Irix 6.5..x, NetBSD, OpenBSD and FreeBSD servers.

Buffer overflows are a common class of security vulnerability, associated with sloppy programming, which allow arbitrary and malicious code to be injected into a system through a carefully crafted malformed data entry.

Generally, this spurious input is much longer than a program expects, causing code to overflow the buffer and enter parts of a system where it may be subsequently executed.

Roy Hills, technical director at security testing firm NTA Monitor, said: "If an attacker can run arbitrary code on a box, which might take a little bit of time but is not especially difficult, then its game over and he can take complete control of a compromised machine.

"If someone is foolish enough to put their FTP server inside their network it's possible to mount a stepping stone attack and attack other systems. It's also possible to upload sensitive information."

Hill added the problem is serious because FTP is a widely used - particularly by councils and utilities which use it to transfer and share data. This is a curious example, but we get the drift.

IT vendors also make extensive use of FTP to enable customers to access software update and patches.

According to an alert issued by PGP Security's Covert Research Labs, which discovered the problem, the vulnerabilities are all related to the use of the glob() function on FTP servers, which implements filename pattern matching.

The only mitigating factor is that, with few exceptions, the vulnerabilities rely upon the remote user having the ability to create directories on the server hosting the FTP daemon.

Patches to address the vulnerability are available from most vendors. Information on how obtain these has been compiled by the CERT Coordination Centre and is available here. ®