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This is for internal use by the PaaS team. Public-facing documentation is located at

ADR0051: How we plan to migrate from cflinuxfs3 to cflinuxfs4


cflinuxfs3 is a configured and packaged deployment of Ubuntu Bionic which we use as the base layer (stack) for the containers we build from tenant source code. It is maintained by the CloudFoundry Foundation, who ensure that new releases of it contain fixes and mitigations for relevant CVEs.

In April 2023, Ubuntu Bionic (18.04) will reach the end of its standard support period. and will no longer receive security updates. The CloudFoundry Foundation also will cease support for cflinuxfs3 at the same time, with the same effect. Relying on it once it is out of support presents a greater risk to us, because any subsequently discovered exploitable vulnerability will not be mitigated or patched.

Ubuntu Jammy (22.04) is the successor to Ubuntu Bionic, and cflinuxfs4 succeeds cflinuxfs3.

Before April 2023, the CloudFoundry Foundation will introduce cflinuxfs4 to cf-deployment and make it the default stack, and after April it will remove cflinuxfs3 from cf-deployment by default but provide an ops file to reintroduce it optionally.

Complicating factors

Reaching the end of a support period for a piece of software is common, and upgrading to the next version isn’t typically a big deal. However, in the case of cflinuxfs3 to cflinuxfs4 there are some complicating factors:

  • Tenant applications implicitly depend on the operating system and the exact collection of libraries (and versions) supplied. Library major versions often change between OS releases (for example Bionic has openssl 1.1.1 while Jammy has openssl 3.0.2) and we cannot assume that changing the base layer of the container images won’t break applications.

    Tenants must test their applications against cflinuxfs4 and resolve any problems for themselves. We cannot provide much support in this, given the number of applications on the platform and the variety of technologies in use.

  • Tenants are often slow to respond to communications and calls to action. This is a problem we have faced through the lifetime of the platform, and we usually build this into our plans by providing a long window (for example 9 months for Postgres 10 deprecation) in which to act and issuing regular reminders. Unfortunately in this case we have not given ourselves a long lead time, and as of 14th Feb 2023 the CloudFoundry Foundation still has 3 buildpacks missing support for cflinuxfs4.

    Regardless, we must provide an adequate timeframe for tenants to act in.

  • GOV.UK PaaS is in the middle of being decommissioned, and every tenant is being asked to migrate away from the platform. For many of them this is consuming a significant proportion of their development capacity, and in most cases they will have to work out how to prioritize migration activities against the work to swap to cflinuxfs4.

    We must set our expectations of how quickly tenants will act accordingly. Some may choose not to transition to cflinuxfs4, in favour of trying to migrate away from the platform before any cflinuxfs3 deadline.


Given the complicating factors, and the risks associated with running an unsupported base operating system version, we will do the following

  1. Offer cflinuxfs4 on GOV.UK PaaS as soon as it is available, but keep cflinuxfs3 as the default
  2. Communicate the upcoming change to tenants
  3. Provide monthly reminders alongside decommissioning news
  4. Provide a 4-month window in which to make the change
  5. Make cflinuxfs4 the default at the end of the 4-month period
  6. Offer cflinuxfs3 until the platform is fully decommissioned to support those tenants who cannot, or choose not to, make the switch. Tenants who do this will be doing so at their own risk, and must clear it with their own Information Assurance teams, risk management processes etc.

Risk mitigations/controls

Basing tenant applications on an unsupported OS version presents us a medium level risk, due to the opportunity for vulnerabilities of varying severity to go un-patched, and exploitation to go unnoticed. To account for this we will monitor the Ubuntu Security Notices for Bionic so that we have continued visibility of any vulnerabilities. The Cloud Foundry Foundation Vulnerability Management Working Group will also continue to publish Ubuntu Security Notices for Bionic for as long as it is supported in any way in cf-deployment. On notification of a CVE the team will be responsible for understanding its scope and severity. We will inform tenants as necessary, and point them to mitigations they can put in place.

We mitigate the risk somewhat by reducing the number of instances in which a container is running with cflinuxfs3 as the base. We can do this by encouraging tenants to move to cflinuxfs4, and making it clear to them that if, at the end of the 4-month period, they are still using cflinuxfs3 then they are responsible for the risk to their own applications.




As a consequence of the above decision, we will continue to base tenant applications on Ubuntu Bionic (cflinuxfs3), by default, for 4 months (April - July 2023). For 3 of those months (May - July 2023), Ubuntu Bionic will be an unsupported operating system version. This is an elevated risk that we accept, and which tenants can mitigate for themselves by moving to cflinuxfs4.

We are also choosing to offer the unsupported operating system for the rest of the lifetime of the platform, but not as the default after a 4-month period. This will allow us to support tenants who either cannot or will not switch to cflinuxfs4, and allows the risk to be assumed by the tenant selectively.

We think our existing application isolation and security controls are sufficient to protect the platform and tenants in the event of a serious, exploitable vulnerability being discovered in Ubuntu Bionic after it leaves its standard support period.