Oracle is scrambling to create an emergency patch for a severe vulnerability in the company's WebLogic server, as exploit code is circulating on the web.

The company issued a rare security alert on Tuesday, the first off-schedule warning since it introduced a regularly scheduled patch release cycle more than three years ago.

The problem lies in the Apache plugin for the Oracle WebLogic Server and Express products (formerly known as BEA WebLogic), both application servers.

The vulnerability can be exploited over a network without a need for a username or a password, wrote Oracle's Eric Maurice, in an advisory.

The flaw can result in "compromising the confidentiality, integrity and availability of the targeted system," Maurice wrote. The problem scores a 10.0, the most serious rating, on the CVSS scale (Common Vulnerability Scoring System), a framework used to evaluate the risks of a particular flaw.

Oracle advised administrators to implement a workaround while it is working to create a patch.

"We expect this fix to be ready very soon, and we will issue an updated Security Alert to let customers know about its availability," Maurice wrote.

The person who published exploit code didn't warn Oracle in advance, Maurice wrote. The exploit code was released after 15 July, the last time Oracle issued patches.

Releasing or using exploit code just after patches have been issued is a tactic often employed against other companies such as Microsoft, which patches on the second Tuesday of every month. Hackers are trying to maximise the amount of time they can take advantage of a flaw before a company issues patches again.

Microsoft, however, has been known to issue out-of-cycle patches for highly dangerous flaws.