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Cyren Security Blog

The Cyren Security Blog is where Cyren engineers and thought leaders provide insights, research and analysis on a range of current cybersecurity topics.

Fake Invoice Carries “Rescoms” Malware

Emails containing malicious attachments equipped with keyloggers and screen capture capabilities are targeting businesses worldwide, with noted attacks in Asia, Russia, and the Middle East. The campaign is designed to look like it comes from real affiliates and employees working for a well-known pharmaceutical distributor in order to make the emails more convincing and lure the recipients into opening the attached document.

The malware exploits two known Microsoft vulnerabilities—CVE-2017-0199 and CVE-2017-8759 and includes obfuscation tools, such as sandbox detection.

Cyren detects and blocks this threat as XML/CVE170199, CVE-2017-8759!Camelot,W32/TinyDL.A and W32/Rescoms.G.

How It Works

An email arrives from what appears to be a reputable person and company in the pharmaceutical industry containing an attachment that looks like an invoice or statement.

Figure 1 : Email Sample

To initiate the installation of the main malware, this attack first exploits the Microsoft vulnerability CVE-2017-0199 to automatically update the document with malicious content—in this case, a file named “free.doc” accessed directly from the threat actor’s server.

Figure 2: CVE-2017-0199 exploit automatically updates using “free.doc” directly from the threat actor’s server.

Figure 3 : MS Word prompts user to update document from linked files.

The downloaded document contains a linked document object (with hidden text) that when executed exploits a second vulnerability known as CVE-2017-8759, which takes advantage of a vulnerability in MS Office’s SOAP WSDL Parser.

Figure 4 : Hidden linked document object

Figure 5: CVE-2017-8759 Exploit

The CVE-2017-8759 exploit runs .Net code, which drops and installs an executable binary in the Windows temporary directory. This file (which Cyren detects as W32/TinyDL.A) downloads the main malware component and saves it in %LOCALAPPDATA%avast.exe. Cyren detects the main malware component as W32/Rescoms.G.

Analysis—Payload W32/Rescoms.G

The Backdoor payload dump strings suggest that it is a variant of Remcos RAT. Checking the latest free version of the Remote Access Trojan reveals the different capabilities it can do on an infected system.

Figure 6: Builder Options

Figure 7: Installation Options

Figure 8: Process Injection and Sandbox Detection Options

Figure 9: Keylogging Options

Figure 10: Screen Capture Options

Figure 11: Remote Options

Digging deeper on the backdoor payload, we can find the settings in the resource section of the file.

Figure 11: 1st byte is the size of the RC4 key and the actual key next to it

This version still uses the RC4 encryption and with the settings decrypted, it reveals that it will try to connect to the following remote host and use “pass” as the password.

C&C :
port : 2404
password: pass


With malware exploding around the globe, it is critical that companies put essential steps in place to protect from new and existing threats.

Cloud-based Email and Web Security

Email and web security gateways instantly filters and block malicious or unwanted email and malware threats for all users on your network, regardless of user location or device type.


It is common for threat actors to use recently disclosed/patched vulnerabilities since they know that companies are sometimes notoriously bad at updating and applying patches to their networks.

The attack takes advantage of two known exploits that Microsoft has identified and provided fixes for. Updating software and applying patches is a critical step to safeguarding your networks.

Disabling Links

In addition to cloud-based security and system patches, another option in this kind of threat scenario is to disable the “automatic links at open” function, also used successfully with the recent DDE vulnerability. (Please note that we only tested it on Microsoft Word 2016.)

File->Options->Advanced->General->Uncheck Update automatic links at open.

Indicators of Compromise

SHA256/URL Description/Detection
Case 1:
074ede6276def79b95c342289bc940deb05fdeaf4bc4896e7fc3c8f6578e6c57 E-mail, Subject: Payment confirmation attached.
7b2c39c838c6629543f1185750b4e41612810722947b4818d15dbbaad99033e1 E-mail, Subject: Payment confirmation attached
8505309fd4e78db3f6efdc49bae5b8e045d89d6dd665d245cfa0101b4d39a6bb Filename: Nov Payment.docx, XML/CVE170199
hxxp:// Link to CVE-2017-8759 exploit
7affc3737c52bf39a03e725184836efc647dd001ec90229d6c4ec887fb741fd6 CVE-2017-8759!Camelot
hxxp:// Link to CVE-2017-8759 payload
60ce039c026d191d996448963aa186e96d50c445b7bb07360318393edbf7ccdb Filename: xin.png, XML/DropExe.A
445a15c88ed27e01e668e17bd47c6793ba5361b98a903c40d1a1f22eeb453684 Filename: TMP<random>.exe, W32/TinyDL.A
e5da506ab6ccb1b3d24cd1fc3030a3e7e65b29b75e92a02a6c2ff57e73ddf85e Filename: avast.exe, W32/Rescoms.G
Case 2:
hxxp:// Download link
856cf2046fb797d83df6beb620956a9845a99f479ef134cec7db0743f462f958 Filename: whmpqn.doc, CVE-2017-8759!Camelot
2a8da20bd8bac77805b4facd4fabb04a49b5f6c6af8085023ee64568463189dc Filename: epraeb, CVE178759
3f065107b25dfd77e14ebef919419d00cde19d5372168161cb6d8ae7782eb32d Filename: usa.exe, W32/Injector.GAV
Additional IOC’s:

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