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Security Audit Remediation: cabotage

by: Ee Durbin ยท 2023-11-14

This is part three in a three-part series. See part one here, and part two here.

This post is a deeper dive into the remediation of the security audit findings for cabotage - the codebase that deploys PyPI and its supporting services such as conveyor, camo, and inspector.

Relative to the warehouse codebase that is PyPI, cabotage is not as widely known. The goals of cabotage are to provide a seamless and secure way of deploying arbitrary services into a Kubernetes cluster in a "Twelve-Factor" style. There are also a number of firm opinions baked into cabotage that provide end-to-end TLS, protection against recovering secrets through the web UI, and isolation between tenants inside the cluster.

cabotage was initially developed in 2018 as part of the Mozilla Open Source Support Award that enabled the Python Software Foundation (PSF) to fund a team of contracted developers and a project manager to complete the development and deployment of warehouse and sunset the original PyPI codebase.

A primary goal of cabotage is to reduce the PSF Infrastructure's dependence on a specific provider for running PyPI, while providing self-service of configuration for project administrators and fully automated deployments. It is in-effect a "Platform as a Service" that deploys applications into bog-standard Kubernetes clusters, no YAML required.

To date, cabotage has deployed 3,901 releases to PyPI since 2018, and 7,377 releases in total across its current services "fleet".

The audit report can be found here. Reading that before you dive in will provide the fullest context.


Eleven findings resulted from the audit along with twelve code quality suggestions. This post will focus on the findings and their remediation. Some of the code quality suggestions were implemented, others deferred.

Here's a table of the items that are relevant to cabotage, and their status:

ID Title Severity Difficulty Status
TOB-PYPI-3 Vulnerable dependencies in cabotage Undetermined Low Remediated
TOB-PYPI-17 Use of shell=True in subprocesses Medium Medium Remediated
TOB-PYPI-18 Use of HMAC with SHA1 for GitHub webhook payload validation Low High Remediated
TOB-PYPI-19 Potential container image manipulation through malicious Procfile Medium High Remediated
TOB-PYPI-20 Repository confusion during image building Medium Medium Remediated
TOB-PYPI-21 Brittle X.509 certificate rewriting Informational Undetermined Accepted
TOB-PYPI-22 Unused dependencies in cabotage Informational Undetermined Remediated
TOB-PYPI-24 Missing resource integrity check of third-party resources Informational High Remediated
TOB-PYPI-25 Brittle secret filtering in logs Medium Low Remediated
TOB-PYPI-26 Routes missing access controls Low High Remediated
TOB-PYPI-28 Deployment hook susceptible to race condition due to temp files Informational High Remediated 1, 2

IDs are non-consecutive, as the audit report included findings for Warehouse as well.


TOB-PYPI-3: Vulnerable dependencies in cabotage

The maintenance of cabotage has been primarily driven by the need for new features or to mitigate issues raised. As a result dependency management and upgrades have often been done as a byproduct of other changes.

During review, there were a number of dependencies with known vulnerabilities found. Of the nine vulnerabilities noted, only GHSA-cg8c-gc2j-2wf7 was determined impact cabotage and was remediated by migrating to the latest release of the maintained fork of flask-security, flask-security-too (diff).

In order to avoid falling behind in this kind of maintenance, automated dependency management was added along with updates to all of known vulnerable dependencies (diff).

TOB-PYPI-17: Use of shell=True in subprocesses

An attack vector was identified in the way that cabotage calls out to buildctl when running container builds in development mode. A specifically crafted user-input has the ability to run arbitrary shell commands on the application host.

Ultimately this was not determined to be exploitable in the production instance of cabotage, since the shell commands were only used when building containers in local development mode. The use of shell=True was removed none-the-less as a matter of hygiene (diff).

TOB-PYPI-18: Use of HMAC with SHA1 for GitHub webhook payload validation

Similar to the SNS verification finding in TOB-PYPI-2 for warehouse, the endpoint that received webhook payloads from GitHub for automated deployments was using SHA1 HMAC signatures to validate authenticity when SHA256 HMAC signatures were available.

The remediation of this finding was much more direct than the SNS finding, as GitHub began sending the SHA256 signature in the header, does not require any changes to the configuration of the webhook, and uses standard HMAC signing supported by the Python standard library (diff).

TOB-PYPI-19: Potential container image manipulation through malicious Procfile

Along the same lines as TOB-PYPI-17, some user-supplied values had the ability to alter the cabotage controlled Dockerfile that specifies how release containers are built, which should not be modifiable. Through specifically crafted process names in the Procfile, a user could alter the resulting Dockerfile by injecting newlines.

Remediation was straightforward by adding additional validation of user supplied process names (diff).

TOB-PYPI-20: Repository confusion during image building

Due to a quirk in GitHub's API for fetching references, a given reference may return a concrete SHA/commit that belongs to a repository other than the one specified in the API call. In this case by providing a reference that resolves to a commit on a fork of the configured repository, a user of cabotage had the ability to intentionally (or mistakenly) configure cabotage to deploy code from a repository other than the one defined.

By adding additional validation inspired by Chainguard's clank tool, cabotage now verifies that the resulting SHA for a given reference belongs to the configured repository (diff).

TOB-PYPI-21: Brittle X.509 certificate rewriting

All containers built and deployed by cabotage are done so using short-lived authentication tokens for an internally deployed Docker registry instance. The cabotage application itself provides this authentication and must publish a public key that the registry can use to validate tokens.

In order to avoid handling private-key material in the application, cabotage relies heavily on Hashicorp Vault. The transit backend for vault did not support publishing the required x509 certificate that Docker registry required when cabotage was originally developed in 2018, so some clever use of the cryptography library was employed to create the necessary file ref.

In the audit it was determined that this work around was brittle in the event that an attacker had the ability to alter the length of the signature, resulting in an invalid x509 certificate and broken authentication for registry clients.

In practice, this has not been observed in the five and a half years that it has been in production and the result of a successful attack would only lead to deployments being halted. As such, we have accepted this finding for the time being and will investigate the newly released x509 support in vault 1.15 and adopt it if able (issue).

TOB-PYPI-22: Unused dependencies in cabotage

Similar to TOB-PYPI-3, dependency management for cabotage was lacking. This led to a handful of dependencies being installed that could be additional exposure to vulnerabilities or attacks.

By adopting pip-tools to compile and pin dependencies, only the projects necessary are installed (diff).

TOB-PYPI-24: Missing resource integrity check of third-party resources

When adding support for a new feature, third party JavaScript was added without subresource integrity information being added. This addition guards against malicious replacement of JavaScript an is good practice when loading any third party code.

Remediation was simple, by ensuring that all CDN loaded JavaScript had the correct value set (diff).

TOB-PYPI-25: Brittle secret filtering in logs

There was a brief period where cabotage supported building from private GitHub repositories, which necessitated filtering build logs and removing the plaintext authentication tokens.

This filtering was naive, but also no longer required. Remediation was removal of the filtering code, and a comment directing a future developer to the correct way of providing such authentication for builds in the future, should building from private GitHub repositories be supported (diff).

TOB-PYPI-26: Routes missing access controls

Another vestigial piece of code that allowed for the build context necessary for container builds was identified as allowing for potentially non-public information to be leaked if a release id (UUIDv4) was guessed or surmised.

This route was unauthenticated as a shortcut rather than adding a new authentication method to cabotage itself.

This code was made defunct when cabotage began building from contexts pulled directly from GitHub and supplied via Kubernetes secrets. Remediation was again, a simple removal of the code (diff).

TOB-PYPI-28: Deployment hook susceptible to race condition due to temp files

A final vestigial piece of code was also flagged as part of the audit which was created to fetch and re-package source code from GitHub for deployments. This had the very outside potential of being exploitable if an attacker gained access to the filesystem that the cabotage app uses for temporary files.

This was similarly made defunct when cabotage began building from contexts pulled directly from GitHub. Remediation was a final simple removal of code (diff-0), and a refactor of how temporary files are created and opened (diff-1).


In addition to the specific findings, the Trail of Bits team also made a number of "Code Quality Recommendations" and analyzed the overall maturity of the codebase. Those sections of the report highlight one of the two themes I see in the report regarding cabotage:

  1. Overall the development experience and continuous integration environment for cabotage is lacking.

  2. There are countless minutiae that one must consider when writing code with security in mind.

In the end, no show stopping or easily exploitable security issues were found, which is a relief! Many of the most interesting security findings were only exploitable by a malicious internal actor who already had configuration permissions in cabotage, was deploying their app there in the first place, or had access to the underlying systems.

The takeaway I have as the sole author and maintainer of cabotage is pretty resounding, and addresses both themes from the report:

Projects with solo maintainers do not benefit from the accountability that comes with collaborative development, are prone to deprioritizing critical improvements to developer experience and testing, and don't have the extra sets of eyes that often assist in spotting small bugs or improper handling of security sensitive software.

So if you're interested in infrastructure and projects that make deploying software securely and reliably more straightforward, I'd love to talk more. Swing by the cabotage repo and consider helping build the software that deploys PyPI, and will soon be deploying more and more of the Python Software Foundation's infrastructure as we migrate from previously gratis PaaS hosting providers.