Cisco Releases Adaptive Security Appliance (ASA) Security Updates

In late October Cisco released a series of 4 security advisories to resolve 4 high severity CVEs (defined) that could result in a denial of service (DoS)(defined) condition for the affected Cisco networking Adaptive Security Appliance (ASA) software.

Why Should These Issues Be Considered Important?
If you make use of Cisco ASA software an unauthenticated remote attacker (namely an attacker that does not have any prior access to your Cisco software) could potentially prevent that software from performing it’s job by causing that software to reload (stop functioning and then restart).

Reloading could be called a denial of service (DoS) condition since while your software is reloading it’s not doing what it was intended to do within your organization. The attacker would only need to send the software a specifically crafted DHCPv6 (see Aside below for a definition) or UDP (defined) packets (when exploiting the VPN ISAKMP issue which involves IKE (Internet Key Exchange (IKE)) v1; see Aside 3 below for a definition) to exploit these issues.

In the case of the first 2 advisories concerning how the ASA software processes DNS requests (see this post for a non-technical explanation and see Aside 2 below for a more formal definition of DNS) the attacker would only need to send the ASA software specifically crafted packets that will cause the software to generate a DNS request packet.

The above means of attack makes it reasonable easy for an attacker to take advantage of these issues to interrupt the normal operation of your ASA software. Finally, there are no workarounds available for these issues (apart from disabling the affected components, which is not really an option if you make use of them).

How Can I Protect Myself From These Issues?
At this time the Cisco Product Security Incident Response Team (PSIRT) is not aware of any of these issues being exploited by attackers since these issues were discovered during internal security testing.

If your organization uses any of the above mentioned Cisco ASA software please follow the directions within the four Cisco security advisories mentioned below to install the necessary security updates:

Cisco ASA Software DNS Denial of Service Vulnerability Advisory 1
Cisco ASA Software DNS Denial of Service Vulnerability Advisory 2
Cisco ASA Software DHCPv6 Relay Denial of Service Vulnerability
Cisco ASA Software VPN ISAKMP Denial of Service Vulnerability

Thank you.

What is DHCP?

Dynamic Host Configuration Protocol (DHCP) is a protocol that automatically assigns an IP address (defined) to a computing device to enable it to communicate with other devices on that network.

The IP addresses provided can be static (fixed) or dynamic (temporary; these addresses exist for a time known as the leasing time, when the lease expires the device can choose to renew the lease for another lease period e.g. 12 hours). The IP address assigned by DHCP comes from a pool (collection) of free address available for use on that network. The process of being automatically assigned an IP address is similar to being given a phone number so that you can call other phone numbers to speak to other people.

DHCP can also provide other information such as the IP address of the DNS server to a device enabling it to access websites on the internet when a person types a website address into their web browser address bar (DNS is explained in more detail below).

Finally DHCP provides the newly established device on that network with the IP address of the default gateway of that network enabling the device to communicate with other networks (e.g. the wider internet). The default gateway acts as a bridging point from one network to another (usually networks using different protocols e.g. ATM (defined) or Frame Relay (defined)). For example, in your home your wireless router acts as both your default gateway and your DNS server (unless you decide to use custom DNS settings). This router connects your devices (which are part of your Local Network (LAN) to the internet (a Wide Area Network, WAN)).

Please note that DHCPv6 is the IPv6 (defined) equivalent of DHCP (which is used with current generation IPv4 networks).

Aside 2:
What is DNS?

DNS (Domain Name Service) works very much like looking a phone number up in a phone book. By doing so it translates website names e.g. into an IP address (defined) allowing for example your web browser to connect to Google’s server to display Google’s homepage. However this communication between computers could also be used for any other desired purpose.

DNS can also be used with email services to locate a mail server for you to send a message from your computer to that domain e.g. to An MX (mail exchange record) maps that domain name ( to a list of mail transfer agents (MTA) for that domain. MTAs transfer a message using SMTP (defined) from MTA to MTA until it reaches the MTA for the messages destination.

DNS usually uses UDP (defined) port 53 to communicate with other DNS servers to find the IP address for the website name that you entered. DNS servers also communicate/synchronize with one another to stay up to date with the appropriate domain name to IP address translations using a process known as DNS zone (defined) transfers.

Aside 2:
What is Internet Key Exchange (IKE)?

Internet Key Exchange is part of a wider security feature known as IPSec.

IPSec (Internet Protocol Security) is a set of protocols that provide a means of setting up a secure channel of communication between 2 computing devices. Many VPNs (Virtual Private Networks)(defined) used by employees to access data and computers (usually servers) when outside of the office use IPSec to secure the connection between the employee’s device and their corporate office.

IPSec is a framework (recommended means of accomplishing something) and thus it does not stipulate specific hashing algorithms (e.g. SHA-1) or encryption algorithms e.g. RSA or ECC to use when creating a secure channel between 2 devices. Moreover, how the 2 devices exchange public keys are not specified.

A commonly used key exchange mechanism used when IPSec is securing a channel is Internet Key Exchange (IKE)(defined within RFC 2828). This standard is made up of ISAKMP (Internet Security Association and Key Management Protocol (ISAKMP)) and OAKLEY protocols. ISAKMP provides the necessary means of exchanging the encryption keys while OAKLEY actually carries out the exchange.

The establishment of the secure channel happens in two phases described in detail within this Cisco article. The Diffie-Hellman algorithm is used to agree on the public encryption for use within this secure channel within phase 1.

IKE is used with IPSec to provide the following benefits:

  • Removes the need to manually set the IPSec security parameters while establishing the connection between two devices.
  • Protects against replay attacks (summarized details of such are provided in this thread (this is a long thread, I would advise searching for the keyword “session” within that page)).
  • Provides the ability to set a limited lifetime for the IPSec communication channel which takes advantage of the capability for encryption keys to change during an individual IPSec session (essentially providing the capabilities and extra security of a temporary session key.


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