Nasty bug with quite simple exploit hits PHP simply in time for the weekend

A essential vulnerability within the PHP programming language could be trivially exploited to execute malicious code on Home windows units, safety researchers warned as they urged these affected to take motion earlier than the weekend begins.

Inside 24 hours of the vulnerability and accompanying patch being printed, researchers from the nonprofit safety group Shadowserver reported Web scans designed to determine servers which can be vulnerable to assaults. That—mixed with (1) the convenience of exploitation, (2) the supply of proof-of-concept assault code, (3) the severity of remotely executing code on weak machines, and (4) the broadly used XAMPP platform being weak by default—has prompted safety practitioners to induce admins verify to see if their PHP servers are affected earlier than beginning the weekend.

When “Greatest Match” is not

“A nasty bug with a quite simple exploit—excellent for a Friday afternoon,” researchers with safety agency WatchTowr wrote.

CVE-2024-4577, because the vulnerability is tracked, stems from errors in the best way PHP converts unicode characters into ASCII. A function constructed into Home windows referred to as Greatest Match permits attackers to make use of a method referred to as argument injection to go user-supplied enter into instructions executed by an utility, on this case, PHP. Exploits enable attackers to bypass CVE-2012-1823, a essential code execution vulnerability patched in PHP in 2012.

“Whereas implementing PHP, the crew didn’t discover the Greatest-Match function of encoding conversion throughout the Home windows working system,” researchers with Devcore, the safety agency that found CVE-2024-4577, wrote. “This oversight permits unauthenticated attackers to bypass the earlier safety of CVE-2012-1823 by particular character sequences. Arbitrary code could be executed on distant PHP servers via the argument injection assault.”

CVE-2024-4577 impacts PHP solely when it runs in a mode referred to as CGI, during which an internet server parses HTTP requests and passes them to a PHP script for processing. Even when PHP isn’t set to CGI mode, nonetheless, the vulnerability should still be exploitable when PHP executables corresponding to php.exe and php-cgi.exe are in directories which can be accessible by the net server. This configuration is about by default in XAMPP for Home windows, making the platform weak until it has been modified.

One instance, WatchTowr famous, happens when queries are parsed and despatched via a command line. The outcome: a innocent request corresponding to http://host/cgi.php?foo=bar might be transformed into php.exe cgi.php foo=bar, a command that might be executed by the principle PHP engine.

No escape

Like many different languages, PHP converts sure kinds of consumer enter to forestall it from being interpreted as a command for execution. It is a course of referred to as escaping. For instance, in HTML, the < and > characters are sometimes escaped by changing them into their unicode hex worth equivalents < and > to forestall them from being interpreted as HTML tags by a browser.

The WatchTowr researchers reveal how Greatest Match fails to flee characters corresponding to a tender hyphen (with unicode worth 0xAD) and as a substitute converts it to an unescaped common hyphen (0x2D), a personality that’s instrumental in lots of code syntaxes.

The researchers went on to elucidate:

It seems that, as a part of unicode processing, PHP will apply what’s referred to as a ‘greatest match’ mapping, and helpfully assume that, when the consumer entered a tender hyphen, they really supposed to sort a actual hyphen, and interpret it as such. Herein lies our vulnerability—if we provide a CGI handler with a tender hyphen (0xAD), the CGI handler received’t really feel the necessity to escape it, and can go it to PHP. PHP, nonetheless, will interpret it as if it had been a actual hyphen, which permits an attacker to sneak additional command line arguments, which start with hyphens, into the PHP course of.

That is remarkably just like an older PHP bug (when in CGI mode), CVE-2012-1823, and so we are able to borrow some exploitation methods developed for this older bug and adapt them to work with our new bug. A useful writeup advises that, to translate our injection into RCE, we should always intention to inject the next arguments:

-d allow_url_include=1 -d auto_prepend_file=php://enter

This may settle for enter from our HTTP request physique, and course of it utilizing PHP. Simple sufficient – let’s attempt a model of this geared up with our 0xAD ‘tender hyphen’ as a substitute of the standard hyphen. Possibly it’s sufficient to slide via the escaping?

POST /take a look at.php?%ADd+allow_url_includepercent3d1+%ADd+auto_prepend_filepercent3dphp://enter HTTP/1.1
Host: {{host}}
Person-Agent: curl/8.3.0
Settle for: */*
Content material-Size: 23
Content material-Sort: utility/x-www-form-urlencoded
Connection: keep-alive


 

Oh pleasure—we’re rewarded with a phpinfo web page, displaying us we’ve got certainly achieved RCE.

The vulnerability was found by Devcore researcher Orange Tsai, who mentioned: “The bug is extremely easy, however that’s additionally what makes it attention-grabbing.”

The Devcore writeup mentioned that the researchers have confirmed that XAMPP is weak when Home windows is configured to make use of the locales for Conventional Chinese language, Simplified Chinese language, or Japanese. In Home windows, a locale is a set of consumer desire info associated to the consumer’s language, surroundings, and/or cultural conventions. The researchers haven’t examined different locales and have urged individuals utilizing them to carry out a complete asset evaluation to check their utilization eventualities.

CVE-2024-4577 impacts all variations of PHP working on a Home windows system. That features model branches 8.3 prior to eight.3.8, 8.2 prior to eight.2.20, and eight.1 prior to eight.1.29.

The 8.0, 7, and 5 model branches are additionally weak, however since they’re not supported, admins must comply with mitigation recommendation since patches aren’t obtainable. One choice is to use what are referred to as rewrite guidelines corresponding to:

RewriteEngine On
RewriteCond %{QUERY_STRING} ^%advert [NC]
RewriteRule .? - [F,L]

The researchers warning these guidelines have been examined just for the three locales they’ve confirmed as weak.

XAMPP for Home windows had but to launch a repair on the time this publish went reside. For admins with out the necessity for PHP CGI, they’ll flip it off utilizing the next Apache HTTP Server configuration:

C:/xampp/apache/conf/additional/httpd-xampp.conf

Finding the corresponding strains:

ScriptAlias /php-cgi/ "C:/xampp/php/"

And remark it out:

# ScriptAlias /php-cgi/ "C:/xampp/php/"

Further evaluation of the vulnerability is offered right here.