# CVE-2026-50128: Mastodon: Spoofing of attribution domains

> Attackers can bypass signature verification to spoof author attribution domains in Mastodon social network posts.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/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:** Not needed
- **Fix available:** Yes
- **Published:** 24 Jun 2026
- **Updated:** 25 Jun 2026
- **Also known as:** BIT-mastodon-2026-50128

## What is CVE-2026-50128?

CVE-2026-50128 is a medium-severity vulnerability in mastodon, published on 24 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Mastodon is a free, open-source social network server based on ActivityPub. From 4.3.0 until 4.5.11 and 4.4.18, Mastodon has a feature to let websites credit authors of their articles. To prevent false attribution claims, Mastodon uses the attributionDomains JSON-LD term, however, an error in how it is defined makes Linked Data Signatures on the toot:attributionDomains property ineffective. An attacker can arbitrarily modify the attributionDomains value of a legitimately signed Update activity and bypass Mastodon’s signature verification. This vulnerability is fixed in 4.5.11 and 4.4.18.

Weakness: CWE-354 (Improper Validation of Integrity Check Value)

## Which versions are affected by CVE-2026-50128?

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


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `mastodon` | Bitnami | From 4.3.0 | 4.4.18, 4.5.11 |

## How do I fix CVE-2026-50128?

Upgrade mastodon to a fixed version for your release line: 4.4.18 or 4.5.11.

## Is CVE-2026-50128 being exploited?

There is no known exploitation of CVE-2026-50128 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-50128 and fix it if it is affected.

What it is: CVE-2026-50128 is a medium-severity vulnerability in mastodon, published on 24 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-50128 in the wild and no public exploit code.

Affected packages and fixed versions:
- mastodon (Bitnami): affected from 4.3.0; fixed in 4.4.18, 4.5.11

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-2026-50128/ (Markdown: https://insights.sbomify.com/CVE-2026-50128/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-50128
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-50128
- OSV: https://osv.dev/vulnerability/CVE-2026-50128

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-50128/
Summaries are generated and may contain mistakes; the official description above is the record.