EXIM Vulnerability (CVE-2019-13917) – Next Steps

Technical Advice

The vulnerability described on this and previous pages affects various versions of the EXIM mail transfer agent (4.85 – 4.92) that are in non-default configuration. If you have one of these versions, you may be at risk of being exploited by this vulnerability. The potential threat and harm associated to it, has been independently scored as posing a critical risk to most organisations.

IMPORTANT: While SEROCU are notifying organisations of this vulnerability, the team will NOT be asking for any information or other details. They will only be providing information. Any unsolicited contact claiming to be from the Police should always be treated with caution. For more information read about Verifying Authenticity.

What to do now

1) Verify this Vulnerability Exists

It is essential that you engage with the senior leadership of the organisation throughout this process. We have also provided non-technical information for them, which can be read on the manager’s information pages for this vulnerability.

The key steps should be to establish :

  • Do you use a version of the EXIM mail transfer agent between 4.8 – 4.92?
  • Has the default configuration been changed in any way?
  • If yes, have you installed all available patches and updates already?

2) Establish if this Vulnerability has already been exploited on your systems

If this vulnerability is present within your systems, it is then important to establish whether criminals have already taken advantage of this in preparation to attack or to exploit you. History shows us that cyber-criminals often loiter in the systems of their victims for a period which may be days to months before a visible attack happens.

The Operation Configured team do not have any information or intelligence to indicate whether or not your organisation has or has not been compromised as a result of this vulnerability.

How you detect this depends on the nature of your systems. It is likely to involve some degree of looking at computer logs for clues or ‘Indicators of Compromise’. Some suggestions are below:

  • Administrators are encouraged to review network security devices protecting Exim servers, both for identifying prior exploitation and for ensuring network-based protection for any unpatched Exim servers.
  • Routinely verifying no unauthorised system modifications, such as additional accounts and SSH keys, have occurred can help detect a compromise. To detect these modifications, administrators can use file integrity monitoring software that alerts an administrator or blocks unauthorized changes on the system.

Locating other forms of persistence

NCSC guidance also provide general advice on how to prevent and detect lateral movement in organisation networks.

Attackers are known to establish persistence on networks by creating scheduled tasks to periodically execute a binary.  By reviewing the list of current scheduled tasks on a device, suspicious activity can be established, identified, investigated and eradicated.

If you believe your systems have been compromised, please read our advice on reporting cybercrime:

3) Establish the Threat, Risk and Harm this Poses

Not every vulnerability will pose the same risk to all organisations. The level of risk this presents this organisation should be used to influence how you move forwards resolving this vulnerability – in particular, the speed at which you do this. Technical advice is essential to making a proper assessment of risk, but the final decision of what risk is acceptable is a managerial or strategic decision.

The Threat – is what may happen. In this case, that means the exploitation of this vulnerability.

The Harm – is the likely adverse consequence of the threat actually happening. The question you need to ask yourself is: “Based on what I have read about this, what are criminals likely to be able to do to the organisation if they do exploit this vulnerability – what harm will this cause?” Harm may manifest itself in many ways – financial, operational, personal, etc… The harm may be direct or indirect – for example, there may be obvious financial harms (cost of responding to and resolving an incident, lost revenue, etc…) as well as further organisational harms such as regulatory fines.

Consequences of ignoring or choosing not to resolve this vulnerability may include, but are not limited to:

  • Access to sensitive / confidential data
  • Add unauthorised privileged accounts / users on the server
  • Disable network security settings
  • Update SSH configurations to enable additional remote access (backdoor access)
  • Execute additional scripts to enable further exploitation
  • Denial of Service attacks
  • Theft of data and electricity (crypto-mining)
  • Increased degradation of affected hardware
  • Corruption of your backups
  • Using you to attack others that you work with, damaging your reputation
  • Ransomware attack against your systems
  • Fines from regulators such as the Information Commissioner’s Office (ICO)
  • Your organisation ceasing operations because of system failure, reputational damage or financial losses

The Risk – this is the likelihood the threat will occur. It may well be that you are confident this vulnerable system is well protected through other measures and the risk is low as a result. It is important you understand the risk in the context of this organisation and your systems. Determining the acceptable degree of risk is a skill in itself and underestimating the risk can have catastrophic consequences for the organisation.

4) Isolate and Mitigate the Immediate Risk

You may not be able to immediately resolve this vulnerability – this is something highly dependent on the nature of the systems and organisation and the availability of a fix for this vulnerability. It is essential that you develop an understanding of how best to resolve this vulnerability including any potential adverse consequences.

Because of this, it is important to explore how best to mitigate the immediate risk – can it be removed or reduced, at least temporarily, until the bigger issue is resolved. This often means isolating the affected system so that it cannot talk to other parts of the network – and probably cutting it off from the public internet. Doing this quickly may prevent a serious attack in the very near future. It may sometimes be quicker, easier and safer overall to fully fix the vulnerability from the outset.

5) Plan to Resolve this Vulnerability

Vendor Advice – EXIM

Administrators update the affected Exim servers through their Linux distribution’s package manager or by downloading the latest fully patched version from https://exim.org/mirrors.html, to provide the strongest level of defence against any potential attacks. 

Using a previous version of Exim leaves a system vulnerable to exploitation. System administrators should continually check software versions and update as new versions become available.

If it is not possible to patch the out-of-date Exim server straight away, there are short-term steps to take to reduce the current risk of exploitation, see NCSC guidance.

6) Plan for the Future

The final phase of responding to this notification is learning. This is an opportunity for the organisation to improve things and become independently proactive in identifying and resolving this kind of problem. A cyber attack is one of the most likely adverse incidents to face any organisation in the modern world and requires a dedicated strategy, ownership at a strategic level and a plan for regular review with iterative improvement.

For more guidance:

Disclaimer: The advice provided on this website is for general information only and is not intended to replace specific professional advice relevant to your organisation. Information on the website is not comprehensive and may not reflect the most recent legislation, practice, advice or application to your specific circumstances.
The South-East Regional Organised Crime Unit (SEROCU) does not accept any responsibility for any loss which may arise from reliance on information or materials published on this website.  It is not responsible for the content of external internet sites that link to this site or which are linked from it.