# CVE-2026-1721: Reflected Cross-Site Scripting (XSS) vulnerability in AI Playground site

> Attackers can execute arbitrary JavaScript via the OAuth callback error parameter, stealing chat history and accessing connected MCP servers.

- **Severity:** Medium 6.2 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:N/SC:H/SI:L/SA: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:** 13 Feb 2026
- **Updated:** 27 Feb 2026
- **Also known as:** GHSA-cvhv-6xm6-c3v4

## What is CVE-2026-1721?

CVE-2026-1721 is a medium-severity cross-site scripting vulnerability in agents, published on 13 Feb 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

Summary

A Reflected Cross-Site Scripting (XSS) vulnerability was discovered in the AI Playground's OAuth callback handler. The `error_description` query parameter was directly interpolated into an HTML script tag without proper escaping, allowing attackers to execute arbitrary JavaScript in the context of the victim's session.

Root cause

The OAuth callback handler in `site/ai-playground/src/server.ts` directly interpolated the `authError` value, sourced from the `error_description` query parameter,  into an inline `<script>` tag.

Impact

An attacker could craft a malicious link that, when clicked by a victim, would:

  *  Steal user chat message history - Access all LLM interactions stored in the user's session.

  *  Access connected MCP Servers - Interact with any MCP servers connected to the victim's session (public or authenticated/private), potentially allowing the attacker to perform actions on the victim's behalf

Mitigation:

  *  PR:  https://github.com/cloudflare/agents/pull/841 https://github.com/cloudflare/agents/pull/841 

  *  Agents-sdk users should upgrade to agents@0.3.10

  *  Developers using configureOAuthCallback with custom error handling in their own applications should ensure all user-controlled input is escaped before interpolation.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation)

## Which versions are affected by CVE-2026-1721?

2 open source packages ship an affected version, including agents.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `agents` | npm | All versions | 0.3.10 |
| `agents` | npm | All versions | 0.3.10 |

## How do I fix CVE-2026-1721?

Upgrade agents to 0.3.10 or later.

## Is CVE-2026-1721 being exploited?

There is no known exploitation of CVE-2026-1721 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-1721 and fix it if it is affected.

What it is: CVE-2026-1721 is a medium-severity cross-site scripting vulnerability in agents, published on 13 Feb 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-2026-1721 in the wild and no public exploit code.

Affected packages and fixed versions:
- agents (npm): affected all versions; fixed in 0.3.10
- agents (npm): affected all versions; fixed in 0.3.10

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-2026-1721/ (Markdown: https://insights.sbomify.com/CVE-2026-1721/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-1721
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-1721
- OSV: https://osv.dev/vulnerability/CVE-2026-1721

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-1721/
Summaries are generated and may contain mistakes; the official description above is the record.