Exim MTA

vSOC SPOT Report: Exim Remote Code Execution Vulnerability


On March 6th, 2018, a security researcher by the name of Meh Chang of Devcore, a Taiwanese security consulting firm, published a remote code execution vulnerability that is present in the mail transfer agent, Exim. Exim is a mail transfer agent (MTA for short) for Unix servers that was developed at the University of Cambridge. Its use is very widespread, estimated to be used on hundreds of thousands of different servers, and it is the default mail transfer agent on some popular web control panels, such as cPanel. It is also the default mail transfer agent in the Debian and Ubuntu Linux distributions. Due to the widespread use of Exim, we believe this vulnerability is particularly dangerous. The vulnerability was first disclosed to Exim on February 2nd, 2018, and a patch was published on February 10th to resolve this issue. This vulnerability is currently being tracked under CVE-2018-6789.

Attack Details

The attack exploits the Base64 decode function of the Exim MTA. The AUTH function of Exim, in most cases, uses Base64 encoding to communicate with the client. Exim uses a buffer to store the decoded Base64 data. Chang found that it was possible to use a certain invalid Base64 string to cause Exim to allocate less space for the buffer than it consumed, creating a buffer overflow. Normally this buffer overflow is harmless, but it is possible to craft the Base64 string to a certain length to overwrite critical data.

Remote execution is possible depending on the use of the Access Control List (ACL) strings in Exim. Chang found that it was possible to overwrite the ACL strings, and then initiate an ACL Check using the ‘MAIL FROM’ SMTP command. When an ACL Check is performed, any code in these strings will be executed if it encounters ${run{cmd}}.

Potential Impact

There have been no known active exploits or proofs of concept of this vulnerability, but this is expected to change in the days following the disclosure due to the ease of exploiting it. Also, the estimated number of machines affected by this vulnerability is very high. A successful exploit of this vulnerability could allow the attacker to gain full access to the mail server. This could then be used to compromise privileged information through the use of reading emails, or the copying, modifying, sending, or deleting of email. This server can then be used as a launching point for further attacks within your network. Even if you are not using Exim within your environment for mail, you could still be vulnerable if Exim is installed and there are open SMTP ports that allow incoming mail.

What You Should Do

Exim has already published Exim 40.9.1 to fix this vulnerability. ALL versions of Exim prior to 40.9.1 are vulnerable to this. Patches are available for Debian, Fedora, SuSE, and Ubuntu Linux distributions as standard packages. Some vulnerability scanners have already added checks for this vulnerability, such as Qualys, Rapid7 and Tenable. We would recommend you review your environment for any indication of vulnerable mail servers and ensure these are updated

GuidePoint’s vSOC will provide additional information as it is made public to help protect our clients.

Supporting Information

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vSOC SPOT Report: Cisco Adaptive Security Appliance RCE & Denial of Service Vulnerability

Update (2018-01-31): SNORT Signatures

After further research, vSOC has located Snort signatures published by the fox-srt team, which can detect exploitation of this vulnerability.

# IDS signatures for https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20180129-asa1:

alert udp any any -> any 500 (msg:"FOX-SRT - Suspicious - Possible Fragmented Cisco IKE/isakmp Packet HeapSpray (CVE-2018-0101)"; flow:to_server; content:"|84|"; offset:16; depth:1; content:"|02 08 |"; distance:1; within:2; fast_pattern; byte_test:4,>,5000,4,relative; byte_test:2,>,5000,10,relative; byte_extract:4,36,fragment_match; byte_test:4,=,fragment_match,52,relative; byte_test:4,=,fragment_match,136,relative; byte_test:4,=,fragment_match,236,relative; threshold:type limit, track by_dst, count 1, seconds 600; classtype:attempted-admin; sid:21002339; rev:4;)

alert udp any any -> any 500 (msg:"FOX-SRT - Exploit - Possible Shellcode in Cisco IKE/isakmp - tcp/CONNECT/"; content:"tcp/CONNECT/"; fast_pattern:only; threshold:type limit, track by_src, count 1, seconds 600; priority:1; classtype:attempted-admin; sid:21002340; rev:2;)

These alerts have been provided by fox-srt and can be found at their GitHub site: https://gist.github.com/fox-srt/09401dfdfc15652b22956b9cc59f71cb


On Monday January 29th, Cisco released a statement to customers that they had identified a vulnerability (CVE-2018-0101) affecting Cisco ASA (Adaptive Security Appliance) and Cisco Firepower Threat Defense Appliances via the Secure Sockets Layer (SSL) VPN functionality of the devices which could allow an unauthenticated remote attacker to create a denial of service condition by reloading the device to remotely execute specially crafted malicious code. The vulnerability is due to an attempt to double free a region of memory when the webvpn feature is turned on for the Cisco ASA device.

Attack Details

The vulnerability makes this very easy to exploit and as a result, it was rated a 10 out of 10 on the CVSS (Common Vulnerability Scoring System). The attack involves an attacker sending multiple crafted malformed XML packets to the Cisco ASA devices and Cisco Firepower software. If the exploit is successful, the attacker will then have the ability to execute unauthorized code on the devices. Depending on the nature of the code, the attacker can gain full control over the device. This attack does not require physical access and can be carried out remotely. The ASA device(s) are only vulnerable if they have the webvpn feature enabled within the OS settings.

Attack Detection and Prevention

Attack patterns will vary once exploits are developed and used in the wild. Some possible detection methods include monitoring XML packets sent to Cisco ASA hosts via packet capture, or to monitor for sudden regular spikes in traffic sent to Cisco ASA hosts, as these spikes would likely be an attempt to force constant restarts on the device. To determine whether the webvpn service is enabled, administrators can use the command show running-config webvpn at the command line. Additionally, the show version command can be run to verify which version of Cisco ASA Software is running on the device. The Cisco Adaptive Security Device Manager (ASDM) can also show the software release in the table that appears by the login window, or in the upper-left corner of the ASDM interface.

The show version command will also show the release version for Cisco Firepower Threat Defense (FTD) devices. Version 6.2.2 of FTD devices are vulnerable because it incorporates code from both Firepower and ASA devices, as it was the first release that supported the Remote Access VPN feature.

Affected Systems

  • 3000 Series Industrial Security Appliance (ISA)
  • ASA 5500 Series Adaptive Security Appliances
  • ASA 5500-X Series Next-Generation Firewalls
  • ASA Services Module for Cisco Catalyst 6500 Series Switches and Cisco 7600 Series Routers
  • ASA 1000V Cloud Firewall
  • Adaptive Security Virtual Appliance (ASAv)
  • Firepower 2100 Series Security Appliance
  • Firepower 4110 Security Appliance
  • Firepower 9300 ASA Security Module
  • Firepower Threat Defense Software


Cisco has released several tables showing the versions to update to and the original ASA major release. It is recommended to ensure both the ASA devices and FTD software is updated to the version released to counteract the vulnerability.

Cisco ASA Major Releases

[1] Cisco ASA Major release table (Cisco, 2018)

Cisco FTD Major Relases[1] Cisco FTD Major release table (Cisco, 2018)


There are no workarounds for this vulnerability. However, Cisco has already released updates that address this vulnerability. Versions that include this fix are listed in the ASA Major release table above.

Additional Resouces