UPDATE – The developers of PHP have released new versions of the scripting language to fix a remotely exploitable vulnerability announced earlier this week that enables an attacker to pass command-line arguments to the PHP binary. The flaw has been in the code for more than eight years and The PHP Group was working on a patch for it when the bug was disclosed accidentally on Reddit. The new versions of PHP are available now and the developers recommend that users upgrade as soon as possible. PHP versions 5.3.12 and 5.4.2 both contain the fix for the vulnerability.
“We’ve tested this and have confirmed that the query parameters are passed to the php5-cgi binary in this configuration. Since the wrapper script merely passes all the arguments on to the actual php-cgi binary, the same problem exists with configurations where php-cgi is directly copied into the cgi-bin directory. It’s interesting to note that while slashes get added to any shell metacharacters we pass in the query string, spaces and dashes (‘-’) are not escaped. So we can pass as many options to PHP as we want!” the team that discovered the flaw, known as Eindbazen, wrote in their analysis of the bug.
Eindbazen said in an updated post that the PHP patch isn’t sufficient to fix the bug.
“The new PHP release is buggy. You can use their mitigation mod_rewrite rule, but the patch and new released versions do not fix the problem. At the bottom we have added a version of the PHP patch that fixes the obvious problem with the patch merged in the recently released security update,” the team said.
The PHP Group is working on a new fix for the vulnerability now.
“We have received word that new PHP updates with the revised fix will be released soon. The issue that this problem was not properly fixed by the original security update is being tracked as CVE-2012-2311,” Eindbazen said.
The PHP Group also had some other problems this week, specifically a problem in its internal bug-handling system that resulted in the private discussion on the CVE-2012-1823 vulnerability being marked as public. That led to the bug being posted to Reddit. The Eindbazen team then posted the details of the bug, which they had discovered in January during a capture the flag contest.
“There is a vulnerability in certain CGI-based setups (Apache+mod_php and nginx+php-fpm are not affected) that has gone unnoticed for at least 8 years. Section 7 of the CGI spec states:
Some systems support a method for supplying a [sic] array of strings to the CGI script. This is only used in the case of an `indexed’ query. This is identified by a “GET” or “HEAD” HTTP request with a URL search string not containing any unencoded “=” characters.
So, requests that do not have a “=” in the query string are treated differently from those who do in some CGI implementations. For PHP this means that a request containing ?-s may dump the PHP source code for the page, but a request that has ?-s&=1 is fine.
A large number of sites run PHP as either an Apache module through mod_php or using php-fpm under nginx. Neither of these setups are vulnerable to this. Straight shebang-style CGI also does not appear to be vulnerable,” the PHP Group said in its release notes for the new versions. “If you are using Apache mod_cgi to run PHP you may be vulnerable. To see if you are, just add ?-s to the end of any of your URLs. If you see your source code, you are vulnerable. If your site renders normally, you are not.
The PHP developers said that while the new versions of the language should work for most users, it may not be feasible for some users to update much older versions of PHP. In that case, users can deploy a workaround.
“An alternative is to configure your web server to not let these types of requests with query strings starting with a “-” and not containing a “=” through. Adding a rule like this should not break any sites,” they said.