Security considerations — Pygments

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# Security considerations

Pygments provides no guarantees on execution time, which needs to be taken into consideration when using Pygments to process arbitrary user inputs. For example, if you have a web service which uses Pygments for highlighting, there may be inputs which will cause the Pygments process to run “forever” and/or use significant amounts of memory. This can subsequently be used to perform a remote denial-of-service attack on the server if the processes are not terminated quickly.

Unfortunately, it’s practically impossible to harden Pygments itself against those issues: Some regular expressions can result in “catastrophic backtracking”, but other bugs like incorrect matchers can also cause similar problems, and there is no way to find them in an automated fashion (short of solving the halting problem.) Pygments has extensive unit tests, automated randomized testing, and is also tested by OSS-Fuzz, but we will never be able to eliminate all bugs in this area.

Our recommendations are:

- Ensure that the Pygments process is terminated after a reasonably short timeout. In general Pygments should take seconds at most for reasonably-sized input.

- Limit the number of concurrent Pygments processes to avoid oversubscription of resources.

The Pygments authors will treat any bug resulting in long processing times with high priority – it’s one of those things that will be fixed in a patch release. When reporting a bug where you suspect super-linear execution times, please make sure to attach an input to reproduce it.

## pip-audit, CVEs, and GitHub security advisories

We’ve seen various CVEs and security advisories filed against Pygments ignoring the guideline above. Typically, they’ll report a ReDoS (remote denial-of-service) vulnerability in Pygments. Generally those advisories will provide some input that causes a regex to go into catastrophic backtracking or similar, resulting in long processing times. First of all, please note that Pygments on its own does not allow remote input – it’s a command line application or library, but it has no RPC mechanism, nor does it start a server by default. The “remote” part only appears when Pygments is wrapped in a service that allows arbitrary user input. In this case, we strongly recommend to take the security considerations above into account, specifically sandboxing and resource limitations.

If your CI pipeline fails because of automated checks for CVEs, we kindly ask you to review the particular issue and consider adding an exception before asking for an urgent fix. Especially if you’re using Pygments for your documentation and not wrapping it into a service, you are not vulnerable (the worst thing that can happen is that someone writes a PR which will cause a CI job that builds the docs to time out, but that’s what timeouts are for, and if someone can send a PR there are many ways to attack your CI infrastructure.) pip-audit for example allows you to selectively ignore individual security issues, making it easy for you to fix your CI while we prepare a new release.

We try to fix security issues, but we do ask everyone to treat user-provided input as dangerous and protect in depth against it. If you find a way to make Pygments execute arbitrary code, we’ll do our best to fix it quickly, but if it is a hang, the guidance above applies. We also recommend reading: https://lwn.net/Articles/944399/