FireEye Labs has been tracking a recent spike in malicious email
detections that we attribute to a campaign that began in 2013. While
malicious email campaigns are nothing new, this one is significant in
that we are observing mass-targeting attackers adopting the malware
evasion methods pioneered by the stealthier APT attackers. And this is
certainly a high-volume business, with anywhere from a few hundred to
ten thousand malicious emails sent daily – usually distributing
between 50 and 500,000 emails per outbreak.
Through the FireEye
Dynamic Threat Intelligence (DTI) cloud, FireEye Labs discovered
that each and every major spike in email blasts brought a change in
the attributes of their attack. These changes have made it difficult
for anti-virus, IPS, firewalls and file-based sandboxes to keep up
with the malware and effectively protect endpoints from infection.
Worse, if past is prologue, we can expect other malicious,
mass-targeting email operators to adopt this approach to bypass
This blog will cover the trends of the campaign, as well as provide
a short technical analysis of the payload.
Figure 1: Attack Architecture
The campaign first appeared in late December of 2013 and has since
been seen in fairly cyclical patterns each month. It appears that the
threat actors behind this campaign are fairly responsive to published
blogs and reports surrounding their malware techniques, tweaking their
malware accordingly to continuously try and evade detection with success.
In late 2013, malware labeled as Kuluoz, the specific spam component
of the Asprox botnet, was discovered to be the main payload of what
would become the first malicious email campaign. Since then, the
threat actors have continuously tweaked the malware by changing its
hardcoded strings, remote access commands, and encryption keys.
Previously, Asprox malicious email campaigns targeted various
industries in multiple countries and included a URL link in the body.
The current version of Asprox includes a simple zipped email
attachment that contains the malicious payload “exe.” Figure 2 below
represents a sample message while Figure 3 is an example of the
various court-related email headers used in the campaign.
Figure 2 Email Sample
Figure 3 Email Headers
Some of the recurring campaign that Asporox used includes themes
focused around airline tickets, postal services and license keys. In
recent months however, the court notice and court request-themed
emails appear to be the most successful phishing scheme theme for the campaign.
The following list contains examples of email subject variations,
specifically for the court notice theme:
- Urgent court notice
- Notice to Appear in Court
- Notice of appearance in court
- Warrant to appear
- Pretrial notice
- Court hearing notice
of your case
- Mandatory court appearance
The campaign appeared to increase in volume during the month of May.
Figure 4 shows the increase in activity of Asprox compared to other
crimewares towards the end of May specifically. Figure 5 highlights
the regular monthly pattern of overall malicious emails. In
comparison, Figure 6 is a compilation of all the hits from our analytics.
Figure 4 Worldwide Crimeware Activity
Figure 5 Overall Asprox Botnet tracking
Figure 6 Asprox Botnet Activity Unique Samples
These malicious email campaign spikes revealed that FireEye
appliances, with the support of DTI cloud, were able to provide a full
picture of the campaign (blue), while only a fraction of the emailed
malware samples could be detected by various Anti-Virus vendors (yellow).
Figure 7 FireEye Detection vs.
By the end of May, we observed a big spike on the unique binaries
associated with this malicious activity. Compared to the previous days
where malware authors used just 10-40 unique MD5s or less per day, we
saw about 6400 unique MD5s sent out on May 29th. That is a
16,000% increase in unique MD5s over the usual malicious email
campaign we’d observed. Compared to other recent email campaigns,
Asprox uses a volume of unique samples for its campaign.
Figure 8 Asprox Campaign Unique Sample Tracking
Figure 9 Geographical Distribution of
Figure 10 Distribution of Industries Affected
Brief Technical Analysis
Figure 11 Attack Architecture
The infiltration phase consists of the victim receiving a phishing
email with a zipped attachment containing the malware payload
disguised as an Office document. Figure 11 is an example of one of the
more recent phishing attempts.
Figure 12 Malware Payload Icon
Once the victim executes the malicious payload, it begins to start
an svchost.exe process and then injects its code into the newly
created process. Once loaded into memory, the injected code is then
unpacked as a DLL. Notice that Asprox uses a hardcoded mutex that can
be found in its strings.
- Typical Mutex Generation
- Create svchost.exe process
- Code injection
Once the dll is running in memory it then creates a copy of itself
in the following location:
It’s important to note that the process will first check itself in
the startup registry key, so a compromised endpoint will have the
following registry populated with the executable:
The malware uses various encryption techniques to communicate with
the command and control (C2) nodes. The communication uses an RSA
(i.e. PROV_RSA_FULL) encrypted SSL session using the Microsoft Base
Cryptographic Provider while the payloads themselves are RC4
encrypted. Each sample uses a default hardcoded public key shown below.
Default Public Key
-----BEGIN PUBLIC KEY-----
-----END PUBLIC KEY-----
First Communication Packet
Bot ID RC4 Encrypted URL
User-Agent: <host useragent>
Host: <host ip>:443
In comparison to the campaign at the end of 2013, the current
campaign uses one of the newer versions of the Asprox family where
threat actors added the command “ear.”
if ( wcsicmp(Str1, L"idl") )
if ( wcsicmp(Str1, L"run") )
if ( wcsicmp(Str1, L"rem") )
if ( wcsicmp(Str1, L"ear")
if ( wcsicmp(Str1, L"rdl") )
if ( wcsicmp(Str1, L"red") )
if ( !wcsicmp(Str1, L"upd") )
|C2 commands ||Description |
commands idles the process to wait for
commands This commands idles the
process to wait for commands
from a partner site and execute from a specified
path Download from a partner
site and execute from a specified path
itself Remove itself
another executable and create autorun entry
executable and create autorun entry
inject into svchost, and run Download, inject into
svchost, and run
and update Download and
registry Modify the
C2 Campaign Characteristics
For the two major malicious email campaign
spikes in April and May of 2014, separate sets of C2 nodes were used
for each major spike.
|April ||May-June |
The data reveals that each of the Asprox botnet’s malicious email
campaigns changes its method of luring victims and C2 domains, as well
as the technical details on monthly intervals. And, with each new
improvement, it becomes more difficult for traditional security
methods to detect certain types of malware.
Nart Villeneuve, Jessa dela Torre, and David Sancho. Asprox Reborn.
Trend Micro. 2013. http://www.trendmicro.com/cloud-content/us/pdfs/security-intelligence/white-papers/wp-asprox-reborn.pdf