Windows, OS X, Linux systems, and web browsers with WPAD enabled
Web Proxy Auto-Discovery (WPAD) Domain Name System (DNS) queries that are intended for resolution on private or enterprise DNS servers have been observed reaching public DNS servers . In combination with the New generic Top Level Domain (gTLD) program’s incorporation of previously undelegated gTLDs for public registration, leaked WPAD queries could result in domain name collisions with internal network naming schemes  . Collisions could be abused by opportunistic domain registrants to configure an external proxy for network traffic, allowing the potential for man-in-the-middle (MitM) attacks across the Internet.
WPAD is a protocol used to ensure all systems in an organization utilize the same web proxy configuration. Instead of individually modifying configurations on each device connected to a network, WPAD locates a proxy configuration file and applies the configuration automatically.
The use of WPAD is enabled by default on all Microsoft Windows operating systems and Internet Explorer browsers. WPAD is supported but not enabled by default on Mac and Linux-based operating systems, as well as, Safari, Chrome, and Firefox browsers.
With the New gTLD program, previously undelegated gTLD strings are now being delegated for public domain name registration . These strings may be used by private or enterprise networks, and in certain circumstances, such as when a work computer is connected from a home or external network, WPAD DNS queries may be made in error to public DNS servers. Attackers may exploit such leaked WPAD queries by registering the leaked domain and setting up MitM proxy configuration files on the Internet.
Leaked WPAD queries could result in domain name collisions with internal network naming schemes. If an attacker registers a domain to answer leaked WPAD queries and configures a valid proxy, there is potential to conduct man-in-the-middle (MitM) attacks across the Internet.
The WPAD vulnerability is significant to corporate assets such as laptops. In some cases these assets are vulnerable even while at work but observations indicate that most assets become vulnerable when used outside an internal network (e.g. home networks, public Wi-Fi networks).
US-CERT encourages users and network administrators to implement the following recommendations to provide a more secure and efficient network infrastructure:
Outdated or misconfigured SAP systems
At least 36 organizations worldwide are affected by an SAP vulnerability . Security researchers from Onapsis discovered indicators of exploitation against these organizations’ SAP business applications.
The observed indicators relate to the abuse of the Invoker Servlet, a built-in functionality in SAP NetWeaver Application Server Java systems (SAP Java platforms). The Invoker Servlet contains a vulnerability that was patched by SAP in 2010. However, the vulnerability continues to affect outdated and misconfigured SAP systems.
SAP systems running outdated or misconfigured software are exposed to increased risks of malicious attacks.
The Invoker Servlet vulnerability affects business applications running on SAP Java platforms.
SAP Java platforms are the base technology stack for many SAP business applications and technical components, including:
The vulnerability resides on the SAP application layer, so it is independent of the operating system and database application that support the SAP system.
Exploitation of the Invoker Servlet vulnerability gives unauthenticated remote attackers full access to affected SAP platforms, providing complete control of the business information and processes on these systems, as well as potential access to other systems.
In order to mitigate this vulnerability, US-CERT recommends users and administrators implement SAP Security Note 1445998 and disable the Invoker Servlet. For more mitigation details, please review the Onapsis threat report .
In addition, US-CERT encourages that users and administrators:
These recommendations apply to SAP systems in public, private, and hybrid cloud environments.
Note: The U.S. Government does not endorse or support any particular product or vendor.
Microsoft Windows with Apple QuickTime installed
According to Trend Micro, Apple will no longer be providing security updates for QuickTime for Windows, leaving this software vulnerable to exploitation. 
All software products have a lifecycle. Apple will no longer be providing security updates for QuickTime for Windows. 
Computer systems running unsupported software are exposed to elevated cybersecurity dangers, such as increased risks of malicious attacks or electronic data loss. Exploitation of QuickTime for Windows vulnerabilities could allow remote attackers to take control of affected systems.
Computers running QuickTime for Windows will continue to work after support ends. However, using unsupported software may increase the risks from viruses and other security threats. Potential negative consequences include loss of confidentiality, integrity, or availability of data, as well as damage to system resources or business assets. The only mitigation available is to uninstall QuickTime for Windows. Users can find instructions for uninstalling QuickTime for Windows on the Apple Uninstall QuickTime page. 
In early 2016, destructive ransomware variants such as Locky and Samas were observed infecting computers belonging to individuals and businesses, which included healthcare facilities and hospitals worldwide. Ransomware is a type of malicious software that infects a computer and restricts users’ access to it until a ransom is paid to unlock it.
The United States Department of Homeland Security (DHS), in collaboration with Canadian Cyber Incident Response Centre (CCIRC), is releasing this Alert to provide further information on ransomware, specifically its main characteristics, its prevalence, variants that may be proliferating, and how users can prevent and mitigate against ransomware.
Ransomware is a type of malware that infects computer systems, restricting users’ access to the infected systems. Ransomware variants have been observed for several years and often attempt to extort money from victims by displaying an on-screen alert. Typically, these alerts state that the user’s systems have been locked or that the user’s files have been encrypted. Users are told that unless a ransom is paid, access will not be restored. The ransom demanded from individuals varies greatly but is frequently $200–$400 dollars and must be paid in virtual currency, such as Bitcoin.
Ransomware is often spread through phishing emails that contain malicious attachments or through drive-by downloading. Drive-by downloading occurs when a user unknowingly visits an infected website and then malware is downloaded and installed without the user’s knowledge.
Crypto ransomware, a malware variant that encrypts files, is spread through similar methods and has also been spread through social media, such as Web-based instant messaging applications. Additionally, newer methods of ransomware infection have been observed. For example, vulnerable Web servers have been exploited as an entry point to gain access into an organization’s network.
The authors of ransomware instill fear and panic into their victims, causing them to click on a link or pay a ransom, and users systems can become infected with additional malware. Ransomware displays intimidating messages similar to those below:
In 2012, Symantec, using data from a command and control (C2) server of 5,700 computers compromised in one day, estimated that approximately 2.9 percent of those compromised users paid the ransom. With an average ransom of $200, this meant malicious actors profited $33,600 per day, or $394,400 per month, from a single C2 server. These rough estimates demonstrate how profitable ransomware can be for malicious actors.
This financial success has likely led to a proliferation of ransomware variants. In 2013, more destructive and lucrative ransomware variants were introduced, including Xorist, CryptorBit, and CryptoLocker. Some variants encrypt not just the files on the infected device, but also the contents of shared or networked drives. These variants are considered destructive because they encrypt users’ and organizations’ files, and render them useless until criminals receive a ransom.
Samas, another variant of destructive ransomware, was used to compromise the networks of healthcare facilities in 2016. Unlike Locky, Samas propagates through vulnerable Web servers. After the Web server was compromised, uploaded Ransomware-Samas files were used to infect the organization’s networks.
Systems infected with ransomware are also often infected with other malware. In the case of CryptoLocker, a user typically becomes infected by opening a malicious attachment from an email. This malicious attachment contains Upatre, a downloader, which infects the user with GameOver Zeus. GameOver Zeus is a variant of the Zeus Trojan that steals banking information and is also used to steal other types of data. Once a system is infected with GameOver Zeus, Upatre will also download CryptoLocker. Finally, CryptoLocker encrypts files on the infected system, and requests that a ransom be paid.
The close ties between ransomware and other types of malware were demonstrated through the recent botnet disruption operation against GameOver Zeus, which also proved effective against CryptoLocker. In June 2014, an international law enforcement operation successfully weakened the infrastructure of both GameOver Zeus and CryptoLocker.
Ransomware not only targets home users; businesses can also become infected with ransomware, leading to negative consequences, including
Paying the ransom does not guarantee the encrypted files will be released; it only guarantees that the malicious actors receive the victim’s money, and in some cases, their banking information. In addition, decrypting files does not mean the malware infection itself has been removed.
Infections can be devastating to an individual or organization, and recovery can be a difficult process that may require the services of a reputable data recovery specialist.
US-CERT recommends that users and administrators take the following preventive measures to protect their computer networks from ransomware infection:
Individuals or organizations are discouraged from paying the ransom, as this does not guarantee files will be released. Report instances of fraud to the FBI at the Internet Crime Complaint Center.
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