A fresh security flaw has surfaced in widespread Web service protocol PHP which could allow attackers to take control of vulnerable servers.

The bug was found in XML-RPC For PHP and PEAR XML_RPC as the result of a security audit by the Hardened-PHP project. The group said it decided to carry out its own audit after other flaws were disclosed in the two libraries earlier this summer.

The new bug takes advantage of a technique similar to the earlier bugs, involving eval() statements, Hardened-PHP said. "To get rid of this and future eval() injection vulnerabilities, the Hardened-PHP Project has developed together with the maintainers of both libraries a fix that completely eliminates the use of eval() from the library," the project said in its advisory.

XML-based RPC (Remote Procedure Call) systems such as XML-RPC are used with HTTP to power Web services, a simple and increasingly popular way of providing services online. XML-RPC For PHP (also called PHPXMLRPC) and PEAR XML_RPC implement XML-RPC for the PHP scripting language.

The bug affects a large number of Web applications, particularly PHP-based blogging, wiki and content management programs, according to security experts. The PHPXMLRPC and PEAR XML_RPC libraries is used in many popular Web applications such as PostNuke, Drupal, b2evolution and TikiWiki.

Content-management systems and blogs are increasingly used by large corporations as a way of interacting with customers and the public - IBM even jumped into the enterprise blogging game recently.

Version 1.4.0 of PEAR XML_RPC fixes the problem, and is available from the PEAR website.

PHPXMLRPC is fixed with version 1.2, available from the PHPXMLRPC project site.

Software projects using the libraries have issued their own updates fixing the problem; among these are the PHP packages included with the Red Hat and Ubuntu Linux distributions.

FrSIRT, the French Security Incident Response Team, gave the flaw a "high-risk" rating and independent security firm Secunia said it was "highly critical".