The PHP Group released PHP 5.4.3 and PHP 5.3.13 on Tuesday to address two remote code execution vulnerabilities, one of which is being actively exploited by hackers.
"The releases complete a fix for a vulnerability in CGI-based setups (CVE-2012-2311)," the PHP developers said in the release notes. Additionally, PHP 5.4.3 fixes a buffer overflow vulnerability, identified as CVE-2012-2329, in the apache_request_headers() function.
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The CVE-2012-2311 vulnerability, also known as CVE-2012-1823, was publicly disclosed last week and prompted the PHP Group to release PHP 5.3.12 and PHP 5.4.2 as emergency security updates in order to resolve it, on May 3.
Unfortunately, the initial patch proved to be ineffective against all variations of the exploit for CVE-2012-1823, and the manual workaround suggested by the PHP developers when releasing the emergency updates was easy to bypass as well.
The PHP developers investigated the issue further and published a new workaround on Sunday. They also promised to have a new working patch ready on Tuesday.
However, hackers immediately took advantage of the situation and started to exploit the flaw in order to compromise websites, according to reports from several security firms.
Researchers from security firm Trustwave announced that the company recorded exploitation attempts for the PHP CGI vulnerability on its honeypot servers -- systems designed to attract attackers. The attacks were particularly interesting because they were designed to bypass the second workaround published by the PHP Group on Sunday.
"One of the major goals of these attacks are to try and download/install webshells and backdoors," researchers from Trustwave's SpiderLabs said in a blog post on Monday.
Web hosting provider Dreamhost has also seen a large number of attacks trying to exploit this vulnerability, according to Trustwave researchers who exchanged information with Dreamhost's security team. In total, the Web hosting company recorded 234,076 exploit attempts against 151,275 unique domains, the Trustwave researchers said.
Security researchers from website integrity monitoring firm Sucuri Security also reported seeing successful CVE-2012-1823 exploitation attempts since the weekend that resulted in the targeted websites being compromised.
The attackers are first sending a malicious query that includes the "-s" php-cgi flag to test if the targeted websites are vulnerable and then installed a backdoor through a query with the "-d" flag, Sucuri's chief technology officer, Daniel Cid, said in a blog post on Tuesday. "It is also important to note that even though we are only seeing those two 'flags' being used (-s and -d), php-cgi has many options and any of them can be used."
Cid advised Web servers administrators who run PHP through php-cgi, to update to the new PHP 5.4.3 or PHP 5.3.13, or to modify their setup so that it uses the PHP module (mod_php) under Apache or FastCGI, which is not vulnerable.