Dorkbot is a botnet used to steal online payment, participate in distributed denial-of-service (DDoS) attacks, and deliver other types of malware to victims’ computers. According to Microsoft, the family of malware used in this botnet “has infected more than one million personal computers in over 190 countries over the course of the past year.” The United States Department of Homeland Security (DHS), in collaboration with the Federal Bureau of Investigation (FBI) and Microsoft, is releasing this Technical Alert to provide further information about Dorkbot.
Dorkbot-infected systems are used by cyber criminals to steal sensitive information (such as user account credentials), launch denial-of-service (DoS) attacks, disable security protection, and distribute several malware variants to victims’ computers. Dorkbot is commonly spread via malicious links sent through social networks instant message programs or through infected USB devices.
In addition, Dorkbot’s backdoor functionality allows a remote attacker to exploit infected system. According to Microsoft’s analysis, a remote attacker may be able to:
A system infected with Dorkbot may be used to send spam, participate in DDoS attacks, or harvest users' credentials for online services, including banking services.
Users are advised to take the following actions to remediate Dorkbot infections:
The above example does not constitute an exhaustive list. The U.S. Government does not endorse or support any particular product or vendor.
Compromised web servers with malicious web shells installed
This alert describes the frequent use of web shells as an exploitation vector. Web shells can be used to obtain unauthorized access and can lead to wider network compromise. This alert outlines the threat and provides prevention, detection, and mitigation strategies.
Consistent use of web shells by Advanced Persistent Threat (APT) and criminal groups has led to significant cyber incidents.
This product was developed in collaboration with US-CERT partners in the United Kingdom, Australia, Canada, and New Zealand based on activity seen targeting organizations across these countries. The detection and mitigation measures outlined in this document represent the shared judgement of all participating agencies.
A web shell is a script that can be uploaded to a web server to enable remote administration of the machine. Infected web servers can be either Internet-facing or internal to the network, where the web shell is used to pivot further to internal hosts.
A web shell can be written in any language that the target web server supports. The most commonly observed web shells are written in languages that are widely supported, such as PHP and ASP. Perl, Ruby, Python, and Unix shell scripts are also used.
Using network reconnaissance tools, an adversary can identify vulnerabilities that can be exploited and result in the installation of a web shell. For example, these vulnerabilities can exist in content management systems (CMS) or web server software.
Once successfully uploaded, an adversary can use the web shell to leverage other exploitation techniques to escalate privileges and to issue commands remotely. These commands are directly linked to the privilege and functionality available to the web server and may include the ability to add, delete, and execute files as well as the ability to run shell commands, further executables, or scripts.
Web shells are frequently used in compromises due to the combination of remote access and functionality. Even simple web shells can have a considerable impact and often maintain minimal presence.
Web shells are utilized for the following purposes:
While a web shell itself would not normally be used for denial of service (DoS) attacks, it can act as a platform for uploading further tools, including DoS capability.
Web shells such as China Chopper, WSO, C99 and B374K are frequently chosen by adversaries; however these are just a small number of known used web shells. (Further information linking to IOCs and SNORT rules can be found in the Additional Resources section).
Web shells can be delivered through a number of web application exploits or configuration weaknesses including:
The above tactics can be and are combined regularly. For example, an exposed admin interface also requires a file upload option, or another exploit method mentioned above, to deliver successfully.
A successfully uploaded shell script may allow a remote attacker to bypass security restrictions and gain unauthorized system access.
Installation of a web shell is commonly accomplished through web application vulnerabilities or configuration weaknesses. Therefore, identification and closure of these vulnerabilities is crucial to avoiding potential compromise. The following suggestions specify good security and web shell specific practices:
Due to the potential simplicity and ease of modification of web shells, they can be difficult to detect. For example, anti-virus products sometimes produce poor results in detecting web shells.
The following may be indicators that your system has been infected by a web shell. Note a number of these indicators are common to legitimate files. Any suspected malicious files should be considered in the context of other indicators and triaged to determine whether further inspection or validation is required.
For investigating many types of shells, a search engine can be very helpful. Often, web shells will be used to spread malware onto a server and the search engines are able to see it. But many web shells check the User-Agent and will display differently for a search engine spider (a program that crawls through links on the Internet, grabbing content from sites and adding it to search engine indexes) than for a regular user. To find a shell, you may need to change your User-Agent to one of the search engine bots. Some browsers have plugins that allow you to easily switch a User-Agent. Once the shell is detected, simply delete the file from the server.
Client characteristics can also indicate possible web shell activity. For example, the malicious actor will often visit only the URI where the web shell script was created, but a standard user usually loads the webpage from a linked page/referrer or loads additional content/resources. Thus, performing frequency analysis on the web access logs could indicate the location of a web shell. Most legitimate URI visits will contain varying user-agents, whereas a web shell is generally only visited by the creator, resulting in limited user-agent variants.
Dridex, a peer-to-peer (P2P) bank credential-stealing malware, uses a decentralized network infrastructure of compromised personal computers and web servers to execute command-and-control (C2). The United States Department of Homeland Security (DHS), in collaboration with the Federal Bureau of Investigation (FBI) and the Department of Justice (DOJ), is releasing this Technical Alert to provide further information about the Dridex botnet.
Dridex is a multifunctional malware package that leverages obfuscated macros in Microsoft Office and extensible markup language (XML) files to infect systems. The primary goal of Dridex is to infect computers, steal credentials, and obtain money from victims’ bank accounts. Operating primarily as a banking Trojan, Dridex is generally distributed through phishing email messages. The emails appear legitimate and are carefully crafted to entice the victim to click on a hyperlink or to open a malicious attached file. Once a computer has been infected, Dridex is capable of stealing user credentials through the use of surreptitious keystroke logging and web injects.
A system infected with Dridex may be employed to send spam, participate in distributed denial-of-service (DDoS) attacks, and harvest users' credentials for online services, including banking services.
Users are recommended to take the following actions to remediate Dridex infections:
The above are examples only and do not constitute an exhaustive list. The U.S. Government does not endorse or support any particular product or vendor.
US-CERT has observed an increase in Domain Name System (DNS) traffic from client systems within internal networks to publically hosted DNS servers. Direct client access to Internet DNS servers, rather than controlled access through enterprise DNS servers, can expose an organization to unnecessary security risks and system inefficiencies. This Alert provides recommendations for improving security related to outbound DNS queries and responses.
Client systems and applications may be configured to send DNS requests to servers other than authorized enterprise DNS caching name servers (also called resolving, forwarding or recursive name servers). This type of configuration poses a security risk and may introduce inefficiencies to an organization.
Unless managed by perimeter technical solutions, client systems and applications may connect to systems outside the enterprise’s administrative control for DNS resolution. Internal enterprise systems should only be permitted to initiate requests to and receive responses from approved enterprise DNS caching name servers. Permitting client systems and applications to connect directly to Internet DNS infrastructure introduces risks and inefficiencies to the organization, which include:
Implement the recommendations below to provide a more secure and efficient DNS infrastructure. Please note that these recommendations focus on improving the security of outbound DNS query or responses and do not encompass all DNS security best practices.
Microsoft Windows Systems, Adobe Flash Player, and Linux
Between June and July 2015, the United States Computer Emergency Readiness Team (US-CERT) received reports of multiple, ongoing and likely evolving, email-based phishing campaigns targeting U.S. Government agencies and private sector organizations. This alert provides general and phishing-specific mitigation strategies and countermeasures.
US-CERT is aware of three phishing campaigns targeting U.S. Government agencies and private organizations across multiple sectors. All three campaigns leveraged website links contained in emails; two sites exploited a recent Adobe Flash vulnerability (CVE-2015-5119) while the third involved the download of a compressed (i.e., ZIP) file containing a malicious executable file. Most of the websites involved are legitimate corporate or organizational sites that were compromised and are hosting malicious content.
Systems infected through targeted phishing campaigns act as an entry point for attackers to spread throughout an organization’s entire enterprise, steal sensitive business or personal information, or disrupt business operations.
Phishing Mitigation and Response Recommendations
Educate Your Users
Organizations should remind users that they play a critical role in protecting their organizations form cyber threats. Users should:
Basic Cyber Hygiene
Practicing basic cyber hygiene would address or mitigate the vast majority of security breaches handled by today’s security practitioners:
For more information on cybersecurity best practices, users and administrators are encouraged to review US-CERT Security Tip: Handling Destructive Malware to evaluate their capabilities encompassing planning, preparation, detection, and response. Another resource is ICS-CERT Recommended Practice: Improving Industrial Control Systems Cybersecurity with Defense-In-Depth Strategies.
Microsoft Windows systems with Adobe Flash Player installed.
Used in conjunction, recently disclosed vulnerabilities in Adobe Flash and Microsoft Windows may allow a remote attacker to execute arbitrary code with system privileges. Since attackers continue to target and find new vulnerabilities in popular, Internet-facing software, updating is not sufficient, and it is important to use exploit mitigation and other defensive techniques.
The following vulnerabilities illustrate the need for ongoing mitigation techniques and prioritization of updates for highly targeted software:
By convincing a user to visit a website or open a file containing specially crafted Flash content, an attacker could combine any one of the three Adobe Flash vulnerabilities with the Microsoft Windows vulnerability to take full control of an affected system.
A common attack vector for exploiting a Flash vulnerability is to entice a user to load Flash content in a web browser, and most web browsers have Flash installed and enabled. A second attack vector for Flash vulnerabilities is through a file (such as an email attachment) that embeds Flash content. Another technique leverages Object Linking and Embedding (OLE) capabilities in Microsoft Office documents to automatically download Flash content from a remote server.
An attacker who is able to execute arbitrary code through the Flash vulnerability could exploit the Adobe Type Manager vulnerability to gain elevated system privileges. The Adobe Type Manager vulnerability allows the attacker to bypass sandbox defenses (such as those found in Adobe Reader and Google Chrome) and low integrity protections (such as Protected Mode Internet Explorer and Protected View for Microsoft Office).
The Adobe Flash vulnerabilities can allow a remote attacker to execute arbitrary code. Exploitation of the Adobe Type Manager vulnerability could then allow the attacker to execute code with system privileges.
Since attackers regularly target widely deployed, Internet-accessible software such as Adobe Flash and Microsoft Windows, it is important to prioritize updates for these products to defend against known vulnerabilities.
Since attackers regularly discover new vulnerabilities for which updates do not exist, it is important to enable exploit mitigation and other defensive techniques.
The Adobe Flash vulnerabilities (CVE-2015-5119, CVE-2015-5122, CVE-2015-5123) are addressed in Adobe Security Bulletins APSB15-16 and APSB15-18. Users are encouraged to review the Bulletins and apply the necessary updates.
The Microsoft Windows Adobe Type Manager vulnerability (CVE-2015-2387) is addressed in Microsoft security Bulletin MS15-077. Users are encouraged to review the Bulletin and apply the necessary updates.
Do not run untrusted Flash content. Most web browsers have Flash enabled by default, however, it may be possible to enable click-to-play features. For information see http://www.howtogeek.com/188059/how-to-enable-click-to-play-plugins-in-every-web-browser/
EMET can be used to help prevent exploitation of the Flash vulnerabilities. In particular, Attack Surface Reduction (ASR) can be configured to help restrict Microsoft Office and Internet Explorer from loading the Flash ActiveX control. See the following link for additional information: http://www.microsoft.com/en-us/download/details.aspx?id=46366
Securing end-to-end communications plays an important role in protecting privacy and preventing some forms of man-in-the-middle (MITM) attacks. Recently, researchers described a MITM attack used to inject code, causing unsecured web browsers around the world to become unwitting participants in a distributed denial-of-service attack. That same code can be employed to deliver an exploit for a particular vulnerability or to take other arbitrary actions.
A MITM attack occurs when a third party inserts itself between the communications of a client and a server. MITM attacks as a general class are not new. Classic MITM attacks (e.g., ARP Spoofing) focus on redirecting network communications. By definition, network infrastructure under attacker control is vulnerable to MITM. However, as technology evolves, new methods for performing MITM attacks evolve as well.
Currently, there is no single technology or configuration to prevent all MITM attacks. However, increasing the complexity with multiple layers of defense may raise the cost for the attacker. Increasing the attacker’s cost in time, effort, or money can be an effective deterrent to avoiding future network compromise.
Generally, encryption and digital certificates provide an effective safeguard against MITM attacks, assuring both the confidentiality and integrity of communications. As a result, modern MITM attacks have focused on taking advantage of weaknesses in the cryptographic infrastructure (e.g., certificate authorities (CAs), web browser certificate stores) or the encryption algorithms and protocols themselves.
MITM attacks are critical because of the wide range of potential impacts—these include the exposure of sensitive information, modification of trusted data, and injection of data.
Employing multiple network and browser protection methods forces an attacker to develop different tactics, techniques, and procedures to circumvent the new security configuration.
US-CERT recommends reviewing the following mitigations to reduce vulnerability to MITM attacks:
US-CERT recommends upgrading TLS to 1.1 or higher and ensuring TLS 1.0 and SSL 1, 2, 3.x are disabled, unless required. TLS 1.0 clients can fall back to version 3.0 of the SSL protocol, which is vulnerable to a padding oracle attack when Cypher-Block Chaining mode is used. This method is commonly referred to as the "POODLE" (Padding Oracle on Downgraded Legacy Encryption) attack. Vulnerable TLS implementations can be updated by applying the patch provided by the vendor. Vendor information is available in the National Vulnerability Database (NVD) entry for CVE-2014-3566  or in CERT Vulnerability Note VU#577193 . See US-CERT TA14-290A  for additional information on this vulnerability.
Certificate pinning  is a method of associating X.509 certificate and its public key to a specific CA or root. Typically, certificates are validated by checking a verifiable chain of trust back to a trusted root certificate. Certificate pinning bypasses this validation process and allows the user to trust “this certificate only” or “trust only certificates signed by this certificate.” Please use the following resources to configure your browser for certificate pinning:
Microsoft Certificate Trust
The Microsoft Enhanced Mitigation Experience Toolkit (EMET) 5.2 employs a feature named "Certificate Trust" for SSL/TLS certificate pinning. This feature is intended to detect and stop MITM attacks that leverage Public Key Infrastructure. 
To use the Certificate Trust, you must provide a list of websites you want to protect and certificate pinning rules applicable to those websites. In order to do this, work with the Certificate Trust Configuration feature of the graphical application or use the Configuration Wizard to automatically configure EMET with the recommended settings.  Also, ensure period defaults are updated through patching.
Browser Certificate Pinning
Google Chrome and Mozilla Firefox, among others, perform certificate pinning. They conduct a variation of certificate pinning using the HTTP Strict Transport Security (HSTS), which pre-loads a specific set of public key hashes into the HSTS configuration, limiting valid certificates to only those with the specified indicated public key. Chrome uses HTTPS pins for most Google properties. It uses whitelisted public keys which include keys from Verisign, Google Internet Authority, Equifax, and GeoTrust. Thus, Chrome will not accept certificates for Google properties from other CAs.
Firefox 32 on desktop and later (Firefox 34 and later on Android) has the ability to use certificate pinning. It also has the ability to enforce built-in pinsets (mapping of public keys) information to domains. Firefox will pin all sites that Chrome already does, pin their own sites after audit and cleansing, and pin other popular sites that are already in good standing. Please visit this site on How to Use Pinning  and for more information.
DANE is a protocol that allows certificates (X.509) commonly used for TLS. DANE is bound to DNS which uses Domain Name System Security Extensions (DNSSEC). A working group in the Internet Engineering Task Force of DANE developed a new type of DNS record that allows a domain itself to sign statements about which entities are authorized to represent it. 
Network notary servers aim to improve the security of communications between computers and websites by enabling browsers to verify website authenticity without relying on CAs. CAs are often considered a security risk because they can be compromised.  As a result, browsers can deem fraudulent sites trustworthy and are left vulnerable to MITM attacks.
Each network notary server, or group of servers, is public and can be operated by public/private organizations or individuals. These servers regularly monitor websites and build a history of each site’s certificate data over time. When a browser equipped with a network notary add-on communicates with a website and obtains its certificate information, a user-designated network notary server supplies the browser with historical certificate data for that site. If certificate information provided by the website is inconsistent with the notary’s historical data, a MITM attack could be at play. 
Systems running unpatched software from Adobe, Microsoft, Oracle, or OpenSSL.
Cyber threat actors continue to exploit unpatched software to conduct attacks against critical infrastructure organizations. As many as 85 percent of targeted attacks are preventable .
This Alert provides information on the 30 most commonly exploited vulnerabilities used in these attacks, along with prevention and mitigation recommendations.
It is based on analysis completed by the Canadian Cyber Incident Response Centre (CCIRC) and was developed in collaboration with our partners from Canada, New Zealand, the United Kingdom, and the Australian Cyber Security Centre.
Unpatched vulnerabilities allow malicious actors entry points into a network. A set of vulnerabilities are consistently targeted in observed attacks.
A successful network intrusion can have severe impacts, particularly if the compromise becomes public and sensitive information is exposed. Possible impacts include:
The attack vectors frequently used by malicious actors such as email attachments, compromised “watering hole” websites, and other tools often rely on taking advantage of unpatched vulnerabilities found in widely used software applications. Patching is the process of repairing vulnerabilities found in these software components.
It is necessary for all organizations to establish a strong ongoing patch management process to ensure the proper preventive measures are taken against potential threats. The longer a system remains unpatched, the longer it is vulnerable to being compromised. Once a patch has been publicly released, the underlying vulnerability can be reverse engineered by malicious actors in order to create an exploit. This process has been documented to take anywhere from 24-hours to four days. Timely patching is one of the lowest cost yet most effective steps an organization can take to minimize its exposure to the threats facing its network.
Executives should ensure their organization’s information security professionals have patched the following software vulnerabilities. Please see patching information for version specifics.
|CVE||Affected Products||Patching Information|
|CVE-2006-3227||Internet Explorer||Microsoft Malware Protection Encyclopedia Entry|
|CVE-2008-2244||Office Word||Microsoft Security Bulletin MS08-042|
Office for Mac
Open XML File Format Converter for Mac
Office Excel Viewer
Office Compatibility Pack for Word, Excel, and PowerPoint
|Microsoft Security Bulletin MS09-067|
|CVE-2009-3674||Internet Explorer||Microsoft Security Bulletin MS09-072|
|CVE-2010-0806||Internet Explorer||Microsoft Security Bulletin MS10-018|
Office for Mac
Open XML File Format Converter for Mac
|Microsoft Security Bulletin MS10-087|
|CVE-2011-0101||Excel||Microsoft Security Bulletin MS11-021|
|Microsoft Security Bulletin MS12-027|
Host Integration Server
Visual FoxPro Visual Basic
|Microsoft Security Bulletin MS12-060|
|CVE-2012-4792||Internet Explorer||Microsoft Security Bulletin MS13-008|
|CVE-2013-0074||Silverlight and Developer Runtime||Microsoft Security Bulletin MS13-022|
|CVE-2013-1347||Internet Explorer||Microsoft Security Bulletin MS13-038|
|CVE-2014-0322||Internet Explorer||Microsoft Security Bulletin MS14-012|
Office Word Viewer
Office Compatibility Pack
Office for Mac
Word Automation Services on SharePoint Server
Office Web Apps
Office Web Apps Server
|Microsoft Security Bulletin MS14-017|
|CVE-2014-1776||Internet Explorer||Microsoft Security Bulletin MS14-021|
|CVE-2014-4114||Windows||Microsoft Security Bulletin MS14-060|
|CVE||Affected Products||Patching Information|
|CVE-2012-1723||Java Development Kit, SDK, and JRE||Oracle Java SE Critical Patch Update Advisory - June 2012|
|CVE-2013-2465||Java Development Kit and JRE||Oracle Java SE Critical Patch Update Advisory - June 2013|
|CVE||Affected Products||Patching Information|
|CVE-2009-3953||Reader Acrobat ||Adobe Security Bulletin APSB10-02|
|CVE-2010-0188||Reader Acrobat||Adobe Security Bulletin APSB10-07|
|CVE-2010-2883||Reader Acrobat ||Adobe Security Bulletin APSB10-21|
|Adobe Security Bulletin APSB11-07|
Adobe Security Bulletin APSB11-08
|CVE-2011-2462||Reader Acrobat ||Adobe Security Bulletin APSB11-30|
|CVE-2013-0625||ColdFusion||Adobe Security Bulletin APSB13-03|
|CVE-2013-0632||ColdFusion||Adobe Security Bulletin APSB13-03|
|CVE-2013-2729||Reader Acrobat||Adobe Security Bulletin APSB13-15|
|CVE-2013-3336||ColdFusion||Adobe Security Bulletin APSB13-13|
|CVE-2013-5326||ColdFusion||Adobe Security Bulletin APSB13-27|
AIR SDK & Compiler
|Adobe Security Bulletin APSB14-22|
|CVE||Affected Products||Patching Information|
|CVE-2014-0160||OpenSSL||CERT Vulnerability Note VU#720951|
As part of a comprehensive security strategy, network administrators should implement the following four mitigation strategies, which can help prevent targeted cyber attacks.
|1||Use application whitelisting to help prevent malicious software and unapproved programs from running.||Application whitelisting is one of the best security strategies as it allows only specified programs to run, while blocking all others, including malicious software.|
|2||Patch applications such as Java, PDF viewers, Flash, web browsers and Microsoft Office.||Vulnerable applications and operating systems are the target of most attacks. Ensuring these are patched with the latest updates greatly reduces the number of exploitable entry points available to an attacker.|
|3||Patch operating system vulnerabilities.|
|4||Restrict administrative privileges to operating systems and applications based on user duties.||Restricting these privileges may prevent malware from running or limit its capability to spread through the network.|
It is recommended that users review US-CERT Security Tip (ST13-003) and CCIRC’s Mitigation Guidelines for Advanced Persistent Threats for additional background information and to assist in the detection of, response to, and recovery from malicious activity linked to advance persistent threats [2, 3].
The Simda botnet – a network of computers infected with self-propagating malware – has compromised more than 770,000 computers worldwide .
The United States Department of Homeland Security (DHS), in collaboration with Interpol and the Federal Bureau of Investigation (FBI), has released this Technical Alert to provide further information about the Simda botnet, along with prevention and mitigation recommendations.
Since 2009, cyber criminals have been targeting computers with unpatched software and compromising them with Simda malware . This malware may re-route a user’s Internet traffic to websites under criminal control or can be used to install additional malware.
The malicious actors control the network of compromised systems (botnet) through backdoors, giving them remote access to carry out additional attacks or to “sell” control of the botnet to other criminals . The backdoors also morph their presence every few hours, allowing low anti-virus detection rates and the means for stealthy operation .
A system infected with Simda may allow cyber criminals to harvest user credentials, including banking information; install additional malware; or cause other malicious attacks. The breadth of infected systems allows Simda operators flexibility to load custom features tailored to individual targets.
Users are recommended to take the following actions to remediate Simda infections:
Kaspersky Lab : http://www.kaspersky.com/security-scan
Trend Micro: http://housecall.trendmicro.com/
Cyber Defense Institute: http://www.cyberdefense.jp/simda/
The above are examples only and do not constitute an exhaustive list. The U.S. government does not endorse or support any particular product or vendor.
Misconfigured Domain Name System (DNS) servers that respond to global Asynchronous Transfer Full Range (AXFR) requests.
A remote unauthenticated user may request a DNS zone transfer from a public-facing DNS server. If improperly configured, the DNS server may respond with information about the requested zone, revealing internal network structure and potentially sensitive information.
AXFR is a protocol for “zone transfers” for replication of DNS data across multiple DNS servers. Unlike normal DNS queries that require the user to know some DNS information ahead of time, AXFR queries reveal resource records including subdomain names . Because a zone transfer is a single query, it could be used by an adversary to efficiently obtain DNS data.
A well-known problem with DNS is that zone transfer requests can disclose domain information; for example, see CVE-1999-0532 and a 2002 CERT/CC white paper . However, the issue has regained attention due to recent Internet scans still showing a large number of misconfigured DNS servers. Open-source, tested scripts are now available to scan for the possible exposure, increasing the likelihood of exploitation .
A remote unauthenticated user may observe internal network structure, learning information useful for other directed attacks.
Configure your DNS server to respond only to zone transfer (AXFR) requests from known IP addresses. Many open-source resources give instructions on reconfiguring your DNS server. For example, see this AXFR article for information on testing and fixing the configuration of a BIND DNS server. US-CERT does not endorse or support any particular product or vendor.