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

A vital vulnerability within the PHP programming language could be trivially exploited to execute malicious code on Home windows gadgets, 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 are inclined to assaults. That—mixed with (1) the benefit of exploitation, (2) the supply of proof-of-concept assault code, (3) the severity of remotely executing code on weak machines, and (4) the extensively 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 “Finest Match” is not

“A nasty bug with a quite simple exploit—good 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 characteristic constructed into Home windows referred to as Finest Match permits attackers to make use of a way referred to as argument injection to go user-supplied enter into instructions executed by an software, on this case, PHP. Exploits enable attackers to bypass CVE-2012-1823, a vital code execution vulnerability patched in PHP in 2012.

“Whereas implementing PHP, the crew didn’t discover the Finest-Match characteristic 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 by the argument injection assault.”

CVE-2024-4577 impacts PHP solely when it runs in a mode referred to as CGI, wherein an online server parses HTTP requests and passes them to a PHP script for processing. Even when PHP isn’t set to CGI mode, nevertheless, the vulnerability should be exploitable when PHP executables akin to php.exe and php-cgi.exe are in directories which are accessible by the online server. This configuration is ready 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 by a command line. The outcome: a innocent request akin to http://host/cgi.php?foo=bar might be transformed into php.exe cgi.php foo=bar, a command that may be executed by the primary PHP engine.

No escape

Like many different languages, PHP converts sure varieties of consumer enter to stop it from being interpreted as a command for execution. This can be 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 stop them from being interpreted as HTML tags by a browser.

The WatchTowr researchers show how Finest Match fails to flee characters akin to a delicate hyphen (with unicode worth 0xAD) and as an alternative 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 ‘finest match’ mapping, and helpfully assume that, when the consumer entered a delicate hyphen, they really supposed to kind a actual hyphen, and interpret it as such. Herein lies our vulnerability—if we provide a CGI handler with a delicate hyphen (0xAD), the CGI handler gained’t really feel the necessity to escape it, and can go it to PHP. PHP, nevertheless, will interpret it as if it had been a actual hyphen, which permits an attacker to sneak further command line arguments, which start with hyphens, into the PHP course of.

That is remarkably much like an older PHP bug (when in CGI mode), CVE-2012-1823, and so we are able to borrow some exploitation strategies 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 must always purpose 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. Easy sufficient – let’s strive a model of this outfitted with our 0xAD ‘delicate hyphen’ as an alternative of the standard hyphen. Possibly it’s sufficient to slide by the escaping?

POST /check.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: software/x-www-form-urlencoded
Connection: keep-alive


 

Oh pleasure—we’re rewarded with a phpinfo web page, exhibiting us we now have 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 fascinating.”

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 data associated to the consumer’s language, setting, and/or cultural conventions. The researchers haven’t examined different locales and have urged folks utilizing them to carry out a complete asset evaluation to check their utilization situations.

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 out there. One possibility is to use what are referred to as rewrite guidelines akin 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 will flip it off utilizing the next Apache HTTP Server configuration:

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

Finding the corresponding traces:

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

And remark it out:

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

Extra evaluation of the vulnerability is accessible right here.