A critical XSS vulnerability affected hackage-server and hackage.haskell.org. HTML and JavaScript files provided in source packages or via the documentation upload facility were served as-is on the main hackage.haskell.org domain. As a consequence, when a user with latent HTTP credentials browses to the package pages or documentation uploaded by a malicious package maintainer, their session can be hijacked to upload packages or documentation, amend maintainers or other package metadata, or perform any other action the user is authorised to do.

A critical XSS vulnerability affected hackage-server and hackage.haskell.org. HTML and JavaScript files provided in source packages or via the documentation upload facility were served as-is on the main hackage.haskell.org domain. As a consequence, when a user with latent HTTP credentials browses to the package pages or documentation uploaded by a malicious package maintainer, their session can be hijacked to upload packages or documentation, amend maintainers or other package metadata, or perform any other action the user is authorised to do.

Affected: Hackage: hackage-server

# Hackage package and doc upload stored XSS vulnerability

*Author: Fraser Tweedale (Haskell SRT)*

## Executive summary

A **critical XSS vulnerability** affected *hackage-server* and `hackage.haskell.org`. HTML and JavaScript files provided in source packages or via the documentation upload facility were served *as-is* on the main `hackage.haskell.org` domain. As a consequence, when a user with latent HTTP credentials browses to the package pages or documentation uploaded by a malicious package maintainer, their **session can be hijacked** to upload packages or documentation, amend maintainers or other package metadata, or perform any other action the user is authorised to do.

This issue was discovered and reported to the Haskell Security Response Team (SRT) in June 2024. Remediations were progressively developed and deployed throughout 2025. **The known issues have been fully mitigated on `hackage.haskell.org` as of January 2026.**

The mitigations involve serving user content from a sister domain. In the case of `hackage.haskell.org`, this domain is `hackage-content.haskell.org`.

Operators of other *hackage-server* instances should update to the `master` branch from the upstream repository (https://github.com/haskell/hackage-server; commit `9a1887607d9b8d1a3b8b02c990ee144c0d402b79` or later), and configure the user content domain (new flag `--user-content-uri`).

## Scope of the issue

### Package documentation uploads

Any Hackage user can upload documentation bun…