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TA15-314A: Compromised Web Servers and Web Shells - Threat Awareness and Guidance

US Cert latest breaches - Wed, 11/11/2015 - 01:12
Original release date: November 10, 2015 | Last revised: November 13, 2015
Systems Affected

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.

Description Web Shell Description

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.

How and why are they used by malicious adversaries?

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:

  1. To harvest and exfiltrate sensitive data and credentials;
  2. To upload additional malware for the potential of creating, for example, a watering hole for infection and scanning of further victims;
  3. To use as a relay point to issue commands to hosts inside the network without direct Internet access;
  4. To use as command-and-control infrastructure, potentially in the form of a bot in a botnet or in support of compromises to additional external networks. This could occur if the adversary intends to maintain long-term persistence.

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).

  • China Chopper A small web shell packed with features. Has several command and control features including a password brute force capability.
  • WSO Stands for “web shell by orb” and has the ability to masquerade as an error page containing a hidden login form.
  • C99 A version of the WSO shell with additional functionality. Can display the server’s security measures and contains a self-delete function.
  • B374K PHP based web shell with common functionality such as viewing processes and executing commands.
Delivery Tactics

Web shells can be delivered through a number of web application exploits or configuration weaknesses including:

  • Cross-Site Scripting;
  • SQL Injection;
  • Vulnerabilities in applications/services  (e.g., WordPress or other CMS applications);
  • File processing vulnerabilities (e.g., upload filtering or assigned permissions);
  • Remote File Include (RFI) and Local File Include (LFI) vulnerabilities;
  • Exposed Admin Interfaces (possible areas to find vulnerabilities mentioned above).

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.

Solution Prevention and Mitigation

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:

  • Employ regular updates to applications and the host operating system to ensure protection against known vulnerabilities.
  • Implement a least-privileges policy on the web server to:
    • Reduce adversaries’ ability to escalate privileges or pivot laterally to other hosts.
    • Control creation and execution of files in particular directories.
  • If not already present, consider deploying a demilitarized zone (DMZ) between your webfacing systems and the corporate network. Limiting the interaction and logging traffic between the two provides a method to identify possible malicious activity.
  • Ensure a secure configuration of web servers. All unnecessary services and ports should be disabled or blocked. All necessary services and ports should be restricted where feasible. This can include whitelisting or blocking external access to administration panels and not using default login credentials.
  • Utilize a reverse proxy or alternative service, such as mod_security, to restrict accessible URL paths to known legitimate ones.
  • Establish, and backup offline, a “known good” version of the relevant server and a regular change-management  policy to enable monitoring for changes to servable content with a file integrity system.
  • Employ user input validation to restrict local and remote file inclusion vulnerabilities.
  • Conduct regular system and application vulnerability scans to establish areas of risk. While this method does not protect against zero day attacks it will highlight possible areas of concern.
  • Deploy a web application firewall and conduct regular virus signature checks, application fuzzing, code reviews and server network analysis.

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.

  • Abnormal periods of high site usage (due to potential uploading and downloading activity);
  • Files with an unusual timestamp (e.g., more recent than the last update of the web applications installed);
  • Suspicious files in Internet-accessible locations (web root);
  • Files containing references to suspicious keywords such as cmd.exe or eval;
  • Unexpected connections in logs. For example:
    • A file type generating unexpected or anomalous network traffic (e.g., a JPG file making requests with POST parameters);
    • Suspicious logins originating from internal subnets to DMZ servers and vice versa.
  • Any evidence of suspicious shell commands, such as directory traversal, by the web server process. 

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.

References Revision History
  • November 10, 2015: Initial Release
  • November 13, 2015: Changes to Title and Systems Affected sections

This product is provided subject to this Notification and this Privacy & Use policy.

TA15-286A: Dridex P2P Malware

US Cert latest breaches - Tue, 10/13/2015 - 11:23
Original release date: October 13, 2015 | Last revised: October 15, 2015
Systems Affected

Microsoft Windows


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:

  • Use and maintain anti-virus software - Anti-virus software recognizes and protects your computer against most known viruses. Even though Dridex is designed to evade detection, security companies are continuously updating their software to counter these advanced threats. Therefore, it is important to keep your anti-virus software up-to-date (see Understanding Anti-Virus Software for more information).
  • Change your passwords - Your original passwords may have been compromised during the infection, so you should change them (see Choosing and Protecting Passwords for more information).
  • Keep your operating system and application software up-to-date - Install software patches so that attackers can't take advantage of known problems or vulnerabilities. Many operating systems offer automatic updates. You should enable automatic updates if this option is available (see Understanding Patches for more information).
  • Use anti-malware tools - Using a legitimate program that identifies and removes malware can help eliminate an infection. Users can consider employing a remediation tool (examples below) to help remove Dridex from your system.









       Trend Micro


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.

References Revision History
  • Initial Publication - October 13, 2015

This product is provided subject to this Notification and this Privacy & Use policy.

IBM PowerVC Version 1.2.3 Introduction and Configuration

IBM News Feed - Wed, 09/02/2015 - 13:30
Draft Redbook, last updated: Wed, 2 Sep 2015

- OpenStack compatibility for integration with cloud software stacks
- Integration of server and storage virtualization
- IBM PowerVM virtualization management

IBM® Power Virtualization Center (PowerVC) is an advanced enterprise virtualization management offering for IBM Power Systems™, based on the OpenStack framework.

TA15-240A: Controlling Outbound DNS Access

US Cert latest breaches - Fri, 08/28/2015 - 17:31
Original release date: August 28, 2015 | Last revised: August 30, 2015
Systems Affected

Networked systems


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:

  • Bypassed enterprise monitoring and logging of DNS traffic; this type of monitoring is an important tool for detecting potential malicious network activity.
  • Bypassed enterprise DNS security filtering (sinkhole/redirect or blackhole/block) capabilities; this may allow clients to access malicious domains that would otherwise be blocked.
  • Client interaction with compromised or malicious DNS servers; this may cause inaccurate DNS responses for the domain requested (e.g., the client is sent to a phishing site or served malicious code).
  • Lost protections against DNS cache poisoning and denial-of-service attacks. The mitigating effects of a tiered or hierarchical (e.g., separate internal and external DNS servers, split DNS, etc.) DNS architecture used to prevent such attacks are lost.  
  • Reduced Internet browsing speed since enterprise DNS caching would not be utilized.

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.  

  • Configure operating systems and applications (including lower-tier DNS servers intended to forward queries to controlled enterprise DNS servers) to use only authorized DNS servers within the enterprise for outbound DNS resolution.
  • Configure enterprise perimeter network devices to block all outbound User Datagram Protocol (UDP) and Transmission Control Protocol (TCP) traffic to destination port 53, except from specific, authorized DNS servers (including both authoritative and caching/forwarding name servers).  
    • Additionally, filtering inbound destination port 53 TCP and UDP traffic to only allow connections to authorized DNS servers (including both authoritative and caching/forwarding name servers) will provide additional protections. 
  • Refer to Section 12 of the NIST Special Publication 800-81-2 for guidance when configuring enterprise recursive DNS resolvers. [1]
References Revision History
  • August 28, 2015: Initial Release

This product is provided subject to this Notification and this Privacy & Use policy.

IBM PowerVP: Introduction and Technical Overview

IBM News Feed - Fri, 08/14/2015 - 13:30
Redpaper, published: Fri, 14 Aug 2015

This IBM® Redpaper™ publication is a comprehensive guide that covers IBM Power Virtualization Performance (PowerVP™) for IBM Power Systems™ Version 1.1.3.

TA15-213A: Recent Email Phishing Campaigns – Mitigation and Response Recommendations

US Cert latest breaches - Sat, 08/01/2015 - 22:01
Original release date: August 01, 2015
Systems Affected

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

  • Implement perimeter blocks for known threat indicators:
    • Email server or email security gateway filters for email indicators
    • Web proxy and firewall filters for websites or Internet Protocol (IP) addresses linked in the emails or used by related malware
    • DNS server blocks (blackhole) or redirects (sinkhole) for known related domains and hostnames
  • Remove malicious emails from targeted user mailboxes based on email indicators (e.g., using Microsoft ExMerge).
  • Identify recipients and possible infected systems:
    • Search email server logs for applicable sender, subject, attachments, etc. (to identify users that may have deleted the email and were not identified in purge of mailboxes)
    • Search applicable web proxy, DNS, firewall or IDS logs for activity the malicious link clicked.
    • Search applicable web proxy, DNS, firewall or IDS logs for activity to any associated command and control (C2) domains or IP addresses associated with the malware.
    • Review anti-virus (AV) logs for alerts associated with the malware.  AV products should be configured to be in quarantine mode. It is important to note that the absence of AV alerts or a clean AV scan should not be taken as conclusive evidence a system is not infected.
    • Scan systems for host-level indicators of the related malware (e.g., YARA signatures)
  • For systems that may be infected:
    • Capture live memory of potentially infected systems for analysis
    • Take forensic images of potentially infected systems for analysis
    • Isolate systems to a virtual local area network (VLAN) segmented form the production agency network (e.g., an Internet-only segment)
  • Report incidents, with as much detail as possible, to the NCCIC.

Educate Your Users

Organizations should remind users that they play a critical role in protecting their organizations form cyber threats. Users should:

  • Exercise caution when opening email attachments, even if the attachment is expected and the sender appears to be known.  Be particularly wary of compressed or ZIP file attachments.
  • Avoid clicking directly on website links in emails; attempts to verify web addresses independently (e.g., contact your organization’s helpdesk or sear the Internet for the main website of the organization or topic mentioned in the email).
  • Report any suspicious emails to the information technology (IT) helpdesk or security office immediately.

Basic Cyber Hygiene

Practicing basic cyber hygiene would address or mitigate the vast majority of security breaches handled by today’s security practitioners:

  • Privilege control (i.e., minimize administrative or superuser privileges)
  • Application whitelisting / software execution control (by file or location)
  • System application patching (e.g., operating system vulnerabilities, third-party vendor applications)
  • Security software updating (e.g., AV definitions, IDS/IPS signatures and filters)
  • Network segmentation (e.g., separate administrative networks from business-critical networks with physical controls and virtual local area networks)
  • Multi-factor authentication (e.g., one-time password tokens, personal identity verification (PIV cards)

Further Information

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.

References Revision History
  • August 1, 2015: Initial Release

This product is provided subject to this Notification and this Privacy & Use policy.

IBM PowerVP: Introduction and Technical Overview

IBM News Feed - Fri, 07/24/2015 - 13:30
Draft Redpaper, last updated: Fri, 24 Jul 2015

This IBM® Redpaper™ publication is a comprehensive guide that covers IBM Power Virtualization Performance (PowerVP™) for IBM Power Systems™ Version 1.1.3.

IBM Power System E850 Technical Overview and Introduction

IBM News Feed - Wed, 06/17/2015 - 13:30
Redpaper, published: Wed, 17 Jun 2015

This IBM® Redpaper™ publication is a comprehensive guide covering the IBM Power System E850 (8408-E8E) server that supports IBM AIX®, and Linux operating systems.

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