---
id: "CVE-2026-126800"
page: "https://infrastack.ai/live/CVE-2026-126800"
kind: "vulnerability"
aliases: ["CVE-2026-126800"]
title: "Hydro UI: stored script injection through markdown code blocks steals sessions"
severity: "critical"
cvss: 9.3
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["hydro", "hydrooj", "package", "self-hosted", "ui-default"]
not_affected_if: ["version-4.59.1-or-later"]
published_at: "2026-11-11T21:16:43+00:00"
first_seen_at: "2026-11-11T21:29:08+00:00"
updated_at: "2026-11-11T21:30:32+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-126800", "first_seen_at": "2026-11-11T21:29:08+00:00"}]
exploit_sources: []
---

# Hydro UI: stored script injection through markdown code blocks steals sessions

Link for people: https://infrastack.ai/live/CVE-2026-126800 (this .md is for agents)

The Hydro online judge's default UI package before 4.59.1 fails to escape double quotes in language class names during markdown rendering, so a registered user can store script in their profile bio. When an administrator or another user views that bio, the script runs and can read the non-httpOnly sid session cookie, letting an attacker take over that session.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check the installed version of `@hydrooj/ui-default` in the Hydro installation; versions below `4.59.1` are affected.
- Look for `markdown-it-xss.ts` in the `@hydrooj/ui-default` package and confirm the language class handling.

## Fix

- `@hydrooj/ui-default` moves to `4.59.1` or later.

## Timeline

- 2026-11-11T21:29:08+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-126800

## Documents

- [advisory] Third-party advisory: www.vulncheck.com https://www.vulncheck.com/advisories/hydro-hydrooj-ui-default-before-4.59.1-stored-xss-via-markdown-language-class
- [fix] Patch: github.com https://github.com/hydro-dev/Hydro/commit/3bae56fe8edddff12346bcb1c8281269c94688d8

## From the sources

Hydro @hydrooj/ui-default before 4.59.1 contains a stored cross-site scripting vulnerability that allows registered users to inject script by placing double quotes in language- class tokens within markdown-it-xss.ts. Attackers can store a payload in their profile bio that executes when administrators or other users view it, stealing the non-httpOnly sid session cookie for session hijacking.

---

_Free to read, with fair use of about 60 pages a minute. For higher limits or API access: sales@infrastack.ai_