CVE
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.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
- Exploited
- No known exploitation
- Fix
- Fixed in 4.59.1
- Who is exposed
- Specific setups only
Do first: Check whether your setup matches what the bug needs. Affected Hydro versions 4.16.1 to 5.1.0 (UI up to 4.59.0) The user bio is rendered as markdown through the XSS filter
- Last modified
- Whether it needs action now or is a reference record, by fixed rules, with why. Last checked is when cve.now last re-read it.What do these mean? →
- Reference
- Last checked
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 17/100 · provisional
- How many real-world setups can hit the bug, from the vendor's advisory and CISA data. It does not know your setup.What do these mean? →
- Specific setups only
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
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.
Read the original 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.
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
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure8/25Specific setups only, reachable over the network
- Reach?/20Unknown. No tracked technology or package matched
- Severity9.3/10CVSS 9.3
- No patch0/5No signal
What would change the score
- Would score about 67 if added to CISA KEV
- Would score about 62 if CISA's SSVC rates exploitation active
- Would score about 24 if CISA's SSVC rates it automatable
What to do
Check whether your setup matches what the bug needs.
Affected Hydro versions 4.16.1 to 5.1.0 (UI up to 4.59.0) The user bio is rendered as markdown through the XSS filterUpgrade to a fixed version: 4.59.1.
Check your systems with an agent
Paste this into a coding or ops agent. It checks your systems read-only and reports a table per asset.
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"}.Timeline
- CVE published
- First seen by cve.now
- Posted on X by cve.now
Affected versions
- hydro-dev @hydrooj/ui-default
fixed in 4.59.1
Evidence 6
CVE record · 2
Establishes: The identifier and description.
- Record
- Record
Other links · 4
- Patch
- Repository
- Link
- Link
Corrections
None so far.
Something wrong on this page? Report an error. Every fix is listed here.
Questions about CVE-2026-126800
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: cve.now feed data
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: vul.yulun.ac.cn, vulncheck.com
Which versions are affected by CVE-2026-126800?
@hydrooj/ui-default: before 4.59.1.Source: cve.now feed data
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: vul.yulun.ac.cn, vulncheck.com
How do I fix CVE-2026-126800?
Upgrade @hydrooj/ui-default to 4.59.1 or later, which is the listed fixed version.Source: vul.yulun.ac.cn, vulncheck.com