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

A important vulnerability within the PHP programming language may 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 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 susceptible machines, and (4) the broadly used XAMPP platform being susceptible by default—has prompted safety practitioners to induce admins examine 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 way in which PHP converts unicode characters into ASCII. A function constructed into Home windows often called Greatest Match permits attackers to make use of a method often called argument injection to move user-supplied enter into instructions executed by an utility, on this case, PHP. Exploits enable attackers to bypass CVE-2012-1823, a important code execution vulnerability patched in PHP in 2012.

“Whereas implementing PHP, the workforce didn’t discover the Greatest-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 may be executed on distant PHP servers by the argument injection assault.”

CVE-2024-4577 impacts PHP solely when it runs in a mode often called 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, nevertheless, the vulnerability should be exploitable when PHP executables comparable 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 susceptible except it has been modified.

One instance, WatchTowr famous, happens when queries are parsed and despatched by a command line. The consequence: 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 might be executed by the principle PHP engine.

No escape

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

The WatchTowr researchers exhibit how Greatest Match fails to flee characters comparable to a gentle 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 ‘greatest match’ mapping, and helpfully assume that, when the person entered a gentle hyphen, they really meant to sort a actual hyphen, and interpret it as such. Herein lies our vulnerability—if we provide a CGI handler with a gentle hyphen (0xAD), the CGI handler received’t really feel the necessity to escape it, and can move 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 will 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 should always goal to inject the next arguments:

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

It will settle for enter from our HTTP request physique, and course of it utilizing PHP. Easy sufficient – let’s attempt a model of this outfitted with our 0xAD ‘gentle hyphen’ as an alternative of the same old hyphen. Possibly it’s sufficient to slide by the escaping?

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


 

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

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

The Devcore writeup stated 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 choice info associated to the person’s language, atmosphere, 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 gadget. 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 comply with mitigation recommendation since patches aren’t obtainable. One possibility 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 submit 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/further/httpd-xampp.conf

Finding the corresponding strains:

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

And remark it out:

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

Extra evaluation of the vulnerability is obtainable right here.