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TA15-240A: Controlling Outbound DNS Access

Original release date: August 28, 2015 | Last revised: August 30, 2015

Systems Affected

Networked systems

Overview

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.

Description

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.   

Impact

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.

Solution

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.

Source: CERT

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

Original release date: August 01, 2015 | Last revised: April 06, 2016

Systems Affected

Microsoft Windows Systems, Adobe Flash Player, and Linux

Overview

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.

Description

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.

Impact

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.

Solution

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 search 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.

Source: CERT