# CVE-2026-33868: Mastodon has a GET-Based Open Redirect via '/web/%2F<domain>'

> Unauthenticated attackers can redirect users to malicious sites via crafted URLs, enabling phishing and potential OAuth credential theft in Mastodon.

- **Severity:** Medium 4.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/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:** 27 Mar 2026
- **Updated:** 31 Mar 2026
- **Also known as:** BIT-mastodon-2026-33868, GHSA-xqw8-4j56-5hj6

## What is CVE-2026-33868?

CVE-2026-33868 is a medium-severity vulnerability in mastodon, published on 27 Mar 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

Mastodon is a free, open-source social network server based on ActivityPub. Prior to versions 4.5.8, 4.4.15, and 4.3.21, an unauthenticated Open Redirect vulnerability (CWE-601) exists in the `/web/*` route due to improper handling of URL-encoded path segments. An attacker can craft a specially encoded URL that causes the application to redirect users to an arbitrary external domain, enabling phishing attacks and potential OAuth credential theft. The issue occurs because URL-encoded slashes (`%2F`) bypass Rails path normalization and are interpreted as host-relative redirects. Versions 4.5.8, 4.4.15, and 4.3.21 patch the issue.

Weakness: CWE-601 (URL Redirection to Untrusted Site)

## Which versions are affected by CVE-2026-33868?

1 open source package ship an affected version, including mastodon. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `mastodon` | Bitnami | All versions | 4.3.21, 4.4.15, 4.5.8 |

Products: Joinmastodon Mastodon.

## How do I fix CVE-2026-33868?

Upgrade mastodon to a fixed version for your release line: 4.3.21, 4.4.15 or 4.5.8.

## Is CVE-2026-33868 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33868 exists.

Public exploit code:

- Mastodon - Open Redirect (nuclei): http/cves/2026/CVE-2026-33868.yaml

## Other vulnerabilities in mastodon

- [CVE-2023-36460](https://insights.sbomify.com/CVE-2023-36460/): Mastodon vulnerable to arbitrary file creation through media attachments (critical 10.0)
- [CVE-2022-2166](https://insights.sbomify.com/CVE-2022-2166/): Improper Restriction of Excessive Authentication Attempts in mastodon/mastodon (critical 9.8)
- [CVE-2022-24307](https://insights.sbomify.com/CVE-2022-24307/): Missing authorization in n/a (critical 9.8)
- [CVE-2018-21018](https://insights.sbomify.com/CVE-2018-21018/): Auth bypass in n/a (critical 9.8)
- [CVE-2024-23832](https://insights.sbomify.com/CVE-2024-23832/): Mastodon Remote user impersonation and takeover (critical 9.4)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-33868 and fix it if it is affected.

What it is: CVE-2026-33868 is a medium-severity vulnerability in mastodon, published on 27 Mar 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: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33868 exists.

Affected packages and fixed versions:
- mastodon (Bitnami): affected all versions; fixed in 4.3.21, 4.4.15, 4.5.8

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-2026-33868/ (Markdown: https://insights.sbomify.com/CVE-2026-33868/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-33868
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-33868
- OSV: https://osv.dev/vulnerability/CVE-2026-33868

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-33868/
Summaries are generated and may contain mistakes; the official description above is the record.