# CVE-2020-26293: Possible XSS bypass if style tag is allowed

> Attackers can execute scripts via crafted HTML if the style tag is explicitly allowed in HtmlSanitizer, requiring an upgrade to fix.

- **Severity:** Medium 6.1 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N`)
- **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:** 4 Jan 2021
- **Updated:** 4 Aug 2024
- **Also known as:** GHSA-8j9v-h2vp-2hhv

## What is CVE-2020-26293?

CVE-2020-26293 is a medium-severity cross-site scripting vulnerability in mganss HtmlSanitizer, published on 4 Jan 2021. 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

HtmlSanitizer is a .NET library for cleaning HTML fragments and documents from constructs that can lead to XSS attacks. In HtmlSanitizer before version 5.0.372, there is a possible XSS bypass if style tag is allowed. If you have explicitly allowed the `<style>` tag, an attacker could craft HTML that includes script after passing through the sanitizer. The default settings disallow the `<style>` tag so there is no risk if you have not explicitly allowed the `<style>` tag. The problem has been fixed in version 5.0.372.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation), CWE-74 (Improper Neutralization of Special Elements in Output Used by a Downstream Component)

## Which versions are affected by CVE-2020-26293?

2 open source packages ship an affected version, including HtmlSanitizer. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `HtmlSanitizer` | NuGet | All versions | 5.0.372 |
| `HtmlSanitizer` | NuGet | All versions | 5.0.372 |

Products: Htmlsanitizer Project Htmlsanitizer.

## How do I fix CVE-2020-26293?

Upgrade HtmlSanitizer to 5.0.372 or later.

## Is CVE-2020-26293 being exploited?

There is no known exploitation of CVE-2020-26293 in the wild and no public exploit code.

## Other vulnerabilities in mganss HtmlSanitizer

- [CVE-2026-25543](https://insights.sbomify.com/CVE-2026-25543/): HtmlSanitizer has a bypass via template tag (medium 6.3)
- [CVE-2023-44390](https://insights.sbomify.com/CVE-2023-44390/): HtmlSanitizer vulnerable to Cross-site Scripting in Foreign Content (medium 6.1)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2020-26293 and fix it if it is affected.

What it is: CVE-2020-26293 is a medium-severity cross-site scripting vulnerability in mganss HtmlSanitizer, published on 4 Jan 2021. 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-2020-26293 in the wild and no public exploit code.

Affected packages and fixed versions:
- HtmlSanitizer (NuGet): affected all versions; fixed in 5.0.372
- HtmlSanitizer (NuGet): affected all versions; fixed in 5.0.372

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-2020-26293/ (Markdown: https://insights.sbomify.com/CVE-2020-26293/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2020-26293
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2020-26293
- OSV: https://osv.dev/vulnerability/CVE-2020-26293

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From sbomify Insights: https://insights.sbomify.com/CVE-2020-26293/
Summaries are generated and may contain mistakes; the official description above is the record.