# CVE-2026-25540: Information disclosure in mastodon

> Attackers can poison the cache to serve incorrect ActivityPub content, leaking data to blocked users or hiding it from legitimate ones.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 4 Feb 2026
- **Updated:** 5 Feb 2026
- **Also known as:** BIT-mastodon-2026-25540, GHSA-ccpr-m53r-mfwr

## What is CVE-2026-25540?

CVE-2026-25540 is a medium-severity vulnerability in mastodon, published on 4 Feb 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. Prior to versions 4.3.19, 4.4.13, 4.5.6, Mastodon is vulnerable to web cache poisoning via `Rails.cache. When AUTHORIZED_FETCH is enabled, the ActivityPub endpoints for pinned posts and featured hashtags have contents that depend on the account that signed the HTTP request. However, these contents are stored in an internal cache and reused with no regards to the signing actor. As a result, an empty response generated for a blocked user account may be served to requests from legitimate non-blocked actors, or conversely, content intended for non-blocked actors may be returned to blocked actors. This issue has been patched in versions 4.3.19, 4.4.13, 4.5.6.

Weakness: CWE-524 (Use of Cache Containing Sensitive Information)

## Which versions are affected by CVE-2026-25540?

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


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `mastodon` | Bitnami | All versions | 4.5.6 |

Products: Joinmastodon Mastodon.

## How do I fix CVE-2026-25540?

Upgrade mastodon to 4.5.6 or later.

## Is CVE-2026-25540 being exploited?

There is no known exploitation of CVE-2026-25540 in the wild and no public exploit code.

## 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-2026-25540 and fix it if it is affected.

What it is: CVE-2026-25540 is a medium-severity vulnerability in mastodon, published on 4 Feb 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-2026-25540 in the wild and no public exploit code.

Affected packages and fixed versions:
- mastodon (Bitnami): affected all versions; fixed in 4.5.6

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-2026-25540/ (Markdown: https://insights.sbomify.com/CVE-2026-25540/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-25540
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-25540
- OSV: https://osv.dev/vulnerability/CVE-2026-25540

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-25540/
Summaries are generated and may contain mistakes; the official description above is the record.