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

A vital vulnerability within the PHP programming language might 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 revealed, researchers from the nonprofit safety group Shadowserver reported Web scans designed to determine servers which can be inclined to assaults. That—mixed with (1) the convenience of exploitation, (2) the provision of proof-of-concept assault code, (3) the severity of remotely executing code on susceptible machines, and (4) the broadly used XAMPP platform being susceptible by default—has prompted safety practitioners to induce admins test 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—excellent for a Friday afternoon,” researchers with safety agency WatchTowr wrote.

CVE-2024-4577, because the vulnerability is tracked, stems from errors in the way in which PHP converts unicode characters into ASCII. A function constructed into Home windows often called Finest Match permits attackers to make use of a way often called argument injection to cross 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 function of encoding conversion inside 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 might be executed on distant PHP servers via the argument injection assault.”

CVE-2024-4577 impacts PHP solely when it runs in a mode often called CGI, by which 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 still be exploitable when PHP executables comparable to php.exe and php-cgi.exe are in directories which can be accessible by the online server. This configuration is ready by default in XAMPP for Home windows, making the platform susceptible 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 comparable to http://host/cgi.php?foo=bar could possibly 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 forms of person enter to stop it from being interpreted as a command for execution. It is a course of often called 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 reveal how Finest Match fails to flee characters comparable to a smooth 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 often called a ‘finest match’ mapping, and helpfully assume that, when the person entered a smooth 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 smooth hyphen (0xAD), the CGI handler gained’t really feel the necessity to escape it, and can cross it to PHP. PHP, nevertheless, will interpret it as if it have 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 much like an older PHP bug (when in CGI mode), CVE-2012-1823, and so we will 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 must always goal 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 outfitted with our 0xAD ‘smooth hyphen’ as an alternative of the same old hyphen. Possibly it’s sufficient to slide via 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 susceptible 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 person desire data associated to the person’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 machine. 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 susceptible, however since they’re now not supported, admins must observe mitigation recommendation since patches aren’t accessible. One choice is to use what are often called rewrite guidelines comparable 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 susceptible.

XAMPP for Home windows had but to launch a repair on the time this put up went dwell. 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/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 out there right here.