In a previous column, I revealed how the vast majority of computer security threats facing your environment live on the client side and require end-user involvement. Users have to be socially engineered to click an item on their desktop (an e-mail, a file attachment, a URL, or an application) that they should not have. This is not to say that truly remote exploits aren't a threat. They are.
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Remote buffer overflow and DoS attacks remain a serious threat against the computers under your control. Although they are less prevalent than client-side attacks, the idea that a remote attacker can launch a series of bytes against your computers, then gain control over them always brings the greatest fear to administrators and captures the biggest headlines. But there are other sorts of remote attacks against listening services and daemons as well.
A gauntlet of remote exploits
The simplest attack type is the availability of another remote access entry point. Many services and daemons (such as FTP, Telnet, HTTP, SSH, RDP, RLogin) provide the perfect place for malicious hackers to guess logon credentials, either manually, one at a time, or using automated tools such as Hydra. Since most administrators never check their logs and use neither strong passwords nor account lockout mechanisms, it's not all that hard to guess at passwords. Find a remote logon portal and guess away. Even if the password is long and complex, chances are the system doesn't monitor logs, lock out accounts, or force password changes, so the hacker can guess at it for months or years without being detected.
Many services and daemons are subject to MitM (man in the middle) attacks and eavesdropping. Far too many services do not require end-point authentication or use encryption. With eavesdropping, unauthorized parties can learn logon credentials or confidential information.
Inappropriate information disclosure is another threat. It takes only a little Google hacking to scare the crap out of you. You'll find logon credentials in plain view, and it won't be much longer before you find real top-secret and confidential documents.
Many services and daemons are often misconfigured, allowing anonymous privileged access from the Internet. Last year while teaching a class on Google hacking, I found an entire (U.S.) state's health and social welfare database accessible on the Internet, no logon credentials required. It included names, Social Security numbers, phone numbers, and addresses -- everything an identity thief would need to be successful.
Many services and daemons remain unpatched, but exposed to the Internet. Just last week, database security expert David Litchfield found hundreds to thousands of unpatched Microsoft SQL Server and Oracle databases on the Internet unprotected by a firewall. Some did not have patches for vulnerabilities that had been fixed more than three years ago. Some new operating systems are knowingly released with outdated libraries and vulnerable binaries. You can download every patch the vendor has to offer and you're still exploitable.