This weekend, several days after Tuesday 2nd March when Microsoft released fixes for the ProxyLogon vulnerability, Netcraft found more than 99,000 unpatched Outlook Web Access servers accessible on the internet — of which several thousand have clear evidence of one or more web shells installed.

Outlook Web Access (OWA) provides remote access to on-premises Microsoft Exchange mailboxes. While a treasure trove of corporate email is a tempting enough target itself, it can also act as a jumping-off point for deeper network access. Vulnerable versions allow unfettered remote access to the mail server. Originally attributed to the Hafnium group, the variety of different web shells and file naming conventions found by Netcraft suggest that the shells belong to multiple groups who have been spurred into action since Microsoft’s announcement by the scale of the opportunity.

vulnerable versions of OWA

Vulnerable OWA installations as at 6 March 2021, based on passive observation of version numbers. Source: Netcraft survey.

Netcraft has established that at least 10% of all visited OWA installations are now infested with web shell backdoors that do not use randomised filenames, and so could plausibly be guessed by anybody. These implants allow continued administrative access to the server, long after the underlying vulnerability has been patched.

web shell source code

One of the backdoor scripts, disguised as an innocuous variable dump in a file named supp0rt.aspx. The active component of the backdoor is ‘hidden’ near the middle of the file.

All of the backdoors hide in plain sight on the web server’s file system but are disguised as benign scripts or information dumps in order to avoid detection. There are several different variants of the backdoor script, but all have the same common feature in that they pass the hacker’s commands to the JScript Eval command, allowing arbitrary code to be executed directly on the web server.

Most of the backdoor scripts accept the criminals’ arbitrary commands via a specially named GET or POST parameter, while others require the commands to be Base64 encoded first, and some only accept them via a POST parameter.

server error page

Some variants of the backdoor script generate a runtime error if the secret variable name does not appear in the request. This makes it possible to detect their presence regardless.

Netcraft has also seen several different variants of these backdoor scripts being uploaded to individual websites, likely in an attempt to preserve unauthorised access to the compromised web server. Unless all of the backdoor scripts are found and removed, the hackers will still be able to get in and create more.

the web shell when viewed in a browser

The web shell when viewed in a browser. There is no obvious indication of its malicious functionality.

While some of the backdoor variants are wildly different in appearance, they all function in a similar way and require the user to know a secret variable name before any commands can be executed on the server. The variable name effectively acts as a password and provides the only security mechanism to ensure that the backdoor can only be used by the person or persons responsible for uploading it.

However, some of the shells use easily guessable variable names like “o” and “orange”, which could plausibly allow them to be misused by other hackers if they can find the scripts and guess the correct variable names. This presents an even more dangerous situation where other fraudsters could then upload their own web shells to secure a foothold on the server. Such a situation could escalate quickly… new battlegrounds could erupt where rival fraudsters try to delete each others’ web shells and upload more of their own in a race to secure access and decide how best to monetize their exploits, all long after the initial OWA vulnerabilities have been resolved.

As some web shells are practically impossible to detect remotely due to the use of randomised filenames or being hidden within existing files, the full extent of the OWA attacks is unknown. Hosting providers, system administrators, and webmasters should ensure that their servers are secured against vulnerabilities that may allow attackers to upload shells to their systems. They should also be on the lookout for unexpected modifications to their web applications, where shell scripts are easily disguised amongst benign files.

Hosting providers can receive an alerting service from Netcraft which will notify them whenever phishing, malware, or web shells are detected on their infrastructure.

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