Updated Computer networks that use proxy servers to automatically redirect browser connections should be on the lookout for a serious architectural flaw that could allow attackers to remotely access intranets and other website resources that are normally off limits, security experts are warning.

At least four proxy server vendors, including QBIK New Zealand, SmoothWall, Squid and Ziproxy remain vulnerable to flaw, according to this advisory from the US Computer Emergency Readiness Team. Products from several dozen additional vendors were vulnerable but have been updated in the past few weeks. Network administrators will need to install the latest versions to ensure their networks aren't vulnerable.

The architectural flaw affects proxy servers that run in what's known as transparent interception mode. The configuration allows servers to intercept and redirect network connections without any user interaction or special browser settings. To exploit the bug, attackers can manipulate host header values using active content such as Java and Flash.

"An attacker may be able to make full connections to any website or resource that the proxy can connect to," CERT warns. "These sites may include internal resources such as intranet sites that would not usually be exposed to the internet."

The vulnerability was originally reported by Robert Auger of PayPal's Information Risk Management team. In a note posted on Cgisecurity.com, he said he planned to release a more detailed technical analysis in March, presumably after network administrators have had a chance to patch vulnerable networks. Based on the bare-bones analysis provided by CERT, it appears the bug may also threaten end users, presumably through phishing attacks.

"To successfully exploit this issue, an attacker would need to either convince a user to visit a web page with malicious active content or be able to load the active content in an otherwise trusted site," the advisory reads.

The good news is that the vulnerability only seems to affect proxy servers running in transparent mode. Browser same-origin policies, which prevent cookies and other types of content set by one domain from being accessed or manipulated by a different address, make it hard for attackers to reuse authentication credentials to gain unauthorized access.

CERT is encouraging admins to update their proxy servers and in the meantime to follow a series of workarounds.

It remains unclear how common transparent interception is used in proxy servers. While several security experts said few large companies use it, Kevin Johnson, a senior security analyst at InGuardians, said the configuration is often used when implementing reverse proxies for load balancing, content caching and other network management services.

"It's a good thing this was announced," he told The Register. "Proxy servers have been a point of interest for the last decade." ®