# CVE-2026-50159: Mermaid allows CSS injection applying to sibling elements of the diagram

> Attackers can inject CSS via diagram text to alter page appearance or behavior, requiring an upgrade to Mermaid.

- **Severity:** Medium 5.3 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:L/VA:N/SC:L/SI:L/SA:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 6 Aug 2026
- **Updated:** 7 Aug 2026
- **Also known as:** CLEANSTART-2026-TH91746, DEBIAN-CVE-2026-50159, GHSA-6x64-9x62-f2gx, MINI-3vwg-m673-3jwr, MINI-79x4-8jrf-8w7x, MINI-7xh7-6c8q-cxr9, MINI-9wwq-4pr7-w24w, UBUNTU-CVE-2026-50159

## What is CVE-2026-50159?

CVE-2026-50159 is a medium-severity cross-site scripting vulnerability in mermaid-js mermaid, published on 6 Aug 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

Mermaid is a JavaScript tool that uses Markdown-inspired text to create and modify diagrams and charts. Prior to 10.9.8 and 11.16.1, Mermaid is vulnerable to CSS injection via sibling combinator selectors generated from diagram-supplied class or id names. An attacker who can supply diagram text can inject arbitrary CSS into the rendered page, potentially altering the appearance or behavior of unrelated page elements. This issue is fixed in versions 10.9.8 and 11.16.1.

Weakness: CWE-94 (Improper Control of Generation of Code)

## Which versions are affected by CVE-2026-50159?

4 open source packages ship an affected version, including mermaid.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `mermaid` | npm | All versions | 10.9.8 |
| `mermaid` | npm | All versions | 10.9.8 |
| `mermaid` | npm | From 11.0.0-alpha.1 | 11.16.1 |
| `mermaid` | npm | From 11.0.0-alpha.1 | 11.16.1 |

## How do I fix CVE-2026-50159?

Upgrade mermaid to a fixed version for your release line: 10.9.8 or 11.16.1.

## Is CVE-2026-50159 being exploited?

There is no known exploitation of CVE-2026-50159 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-50159 and fix it if it is affected.

What it is: CVE-2026-50159 is a medium-severity cross-site scripting vulnerability in mermaid-js mermaid, published on 6 Aug 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2026-50159 in the wild and no public exploit code.

Affected packages and fixed versions:
- mermaid (npm): affected all versions; fixed in 10.9.8
- mermaid (npm): affected all versions; fixed in 10.9.8
- mermaid (npm): affected from 11.0.0-alpha.1; fixed in 11.16.1
- mermaid (npm): affected from 11.0.0-alpha.1; fixed in 11.16.1

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-50159/ (Markdown: https://insights.sbomify.com/CVE-2026-50159/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-50159
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-50159
- OSV: https://osv.dev/vulnerability/CVE-2026-50159

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-50159/
Summaries are generated and may contain mistakes; the official description above is the record.