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

Nasty bug with very simple exploit hits PHP just in time for the weekend

A crucial 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 are prone 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 extensively used XAMPP platform being weak by default—has prompted safety practitioners to induce admins test 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—good 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 feature constructed into Home windows generally known as Greatest Match permits attackers to make use of a way generally known as 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 crucial code execution vulnerability patched in PHP in 2012.

“Whereas implementing PHP, the group 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 could be executed on distant PHP servers by way of the argument injection assault.”

CVE-2024-4577 impacts PHP solely when it runs in a mode generally known as CGI, through 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, nonetheless, the vulnerability should be exploitable when PHP executables resembling php.exe and php-cgi.exe are in directories which are accessible by the net 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 way of a command line. The outcome: a innocent request resembling 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 person enter to stop it from being interpreted as a command for execution. This can be a course of generally known 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 display how Greatest Match fails to flee characters resembling a mushy 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 generally known as a ‘finest match’ mapping, and helpfully assume that, when the person entered a mushy 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 mushy hyphen (0xAD), the CGI handler received’t really feel the necessity to escape it, and can move it to PHP. PHP, nonetheless, will interpret it as if it have 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 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 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. Easy sufficient – let’s strive a model of this geared up with our 0xAD ‘mushy hyphen’ as a substitute of the standard hyphen. Perhaps it’s sufficient to slide by way of 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 now have certainly achieved RCE.

The vulnerability was found by Devcore researcher Orange Tsai, who said: “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 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 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 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 weak, however since they’re now not supported, admins should observe mitigation recommendation since patches aren’t obtainable. One choice is to use what are generally known as rewrite guidelines resembling:

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 submit went stay. For admins with out the necessity for PHP CGI, they will flip it off utilizing the next Apache HTTP Server configuration:


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 offered here.