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It’s almost one month now that a very effective malspam campaign delivering the ursnif trojan is in progress in Italy.

The trick that the malware uses to spread is simple and effective: once run on the victim’s machine it sends replies to existing email threads attaching a copy of the malware itself.

This strategy is so effective that many users release those emails from the quarantine and even report them as false positives, before getting infected themselves. This is still happening with tens of false positive reports per day.

Ursnif is basically a trojan that hijacks remote banking sessions or steals credentials, the malware itself varies and keeps changing over time. The dropper is a word document with an obfuscated macro. Also the macro keeps changing and this makes it very hard for antivirus scanners to intercept the new variants. This is why many of these emails, not being flagged by the antivirus, are quarantined without the “malware” tag which would make it harder for the recipient to release it.

Many users, seeing a reply to an existing thread from one of their contacts in the quarantine, are so convinced that it is a legit email that they release it from the quarantine and even report the message as a false positive to our laboratories, then open the attachment, enable the macros and get infected themselves. At that point the malware starts spreading to their own contacts with replies to existing threads and the campaign propagates.

This is how the email looks like:

The message contains very few words: “Buongiorno, Vedi allegato e di confermare. Cordiali saluti” followed by the real signature of the infected account and the existing email thread below.

The phrase, spelled in an incorrect Italian (but this doesn’t seem to impair the effectiveness), basically says: “Good morning, check the attachment and confirm. Best regards.”

The attachment, usually named Richiesta.doc (Richiesta means Request) is a word document:

The document pretends to be created with a previous version of Microsoft Office and, as usual, instructs the user to enable the macros.

The macros are obfuscated, they keep changing so that signature and pattern based systems can’t catch-up, and they contain an AutoOpen action that executes a powershell script that downloads and install the payload.

Here is a list of macros contained in the file:

And here is one of the AutoOpen variants:

This campaign, just as other malware campaigns like Emotet for example, has an ever-changing dropper that highlights all of the limits of the defense approaches based on signatures and patterns. Antivirus engines are releasing every day hundreds of new detection rules for these ever-changing samples but the latency of the process guarantees the delivery of many samples that are not yet identified by the anti-virus engines.

These emails are being quarantined by content checks and the attachments are blocked or disarmed by our QuickSand sandboxing service, which is a service that disarms active code when the typical operations that enable the dropper functionality are present. This is an approach that doesn’t have the drawbacks of signature and pattern-based approaches and proved to be quite effective in blocking unknown and ever-changing malware variants.

Despite these emails being blocked by these additional layers of protection, the phishing component is so strong that some users voluntarily override all of the safety checks and get infected anyway. This simple phishing trick can induce a recipient to go through a significant effort in order to help the malware authors: release the email from the quarantine, open it, launch the attachment, enable the macros and in some cases even report the email as a false positive.

This is how powerful deception can be.

In order to delay detection, phishing and malware websites often use some obfuscation technique.
Obfuscation techniques are double-edged swords. They hide the malicious content from dumb crawlers, bots and sandboxes, but smarter algorithms that know what to look for can detect the malware just by looking at it’s attempts to hide. This is one of the ways we can detect zero-day malware.

In this example we have a fake PayPal website. This page interleaves invisible spans between visible text in order to avoid detection by automated systems that perform heuristic analysis of the web page content.
You’ll get a clearer idea by looking at the following pictures.

This is the fake PayPal website as it is displayed in the browser:

PayPal phishing website

Notice the text just above the login box on the left of the page. The text says “Bitte geben Sie Ihre PayPal-Dated ein”. You will not find this phrase in the source code of the page because the phrase (and especially the word PayPal) has been interleaved with a lot of text enclosed in invisible spans. This text is present in the page but it is not displayed to the user.

Here is a part of the source code of the page (click on the image to enlarge it):

The parts in brown are the invisible spans, they contain a lot of random text that the browser is instructed not to display to the user.

The parts surrounded by yellow boxes are visible and displayed to the user. These parts compose the phrase you see on the webpage but a bot that scans the page and that doesn’t skip the invisible parts cannot find this phrase or even the word PayPal in the whole page.

Invisible content is perfectly normal in legit web pages, often some parts of the page are made visible only on specific events, often most of the page is initially invisible and made visible only when everything has been loaded. Having invisible content is not bad by itself and this is why crawlers and sandboxes don’t ignore it. Using it in this way is certainly suspicious.

Our UrlSand sandbox searches for this and other obfuscation/evasion techniques in order to detect malware.

 

Rodolfo Saccani
Libra Esva R&D Manager