# CVE-2026-55103: Discourse ActivityPub: Fix inbound actor validation and apply outbound request SSRF hardening

> Remote attackers can edit, delete, or reply to federated posts without proper permissions due to missing identity and ownership checks in inbound ActivityPub processing.

- **Severity:** High 8.6 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 15 Oct 2026
- **Updated:** 28 Oct 2026
- **Also known as:** GHSA-jw39-rch6-mhwf

## What is CVE-2026-55103?

CVE-2026-55103 is a high-severity missing authorization vulnerability in discourse-activity-pub, published on 15 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Discourse ActivityPub adds ActivityPub support to Discourse. Prior to commit 1c8518b0f2abdf31cbea3e0b242f74d8994a9ba5, inbound ActivityPub processing does not sufficiently verify that the actor authenticated by the signed request matches the Activity actor and owns the referenced federated post. A remote attacker can exploit these missing identity and ownership checks to edit, delete, or reply to federated posts without the required permissions. This vulnerability is fixed with commit 1c8518b0f2abdf31cbea3e0b242f74d8994a9ba5.

Weakness: CWE-863 (Incorrect Authorization), CWE-345 (Insufficient Verification of Data Authenticity)

## Which versions are affected by CVE-2026-55103?


## How do I fix CVE-2026-55103?

No fix has been published for CVE-2026-55103 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-55103 being exploited?

There is no known exploitation of CVE-2026-55103 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-55103 and fix it if it is affected.

What it is: CVE-2026-55103 is a high-severity missing authorization vulnerability in discourse-activity-pub, published on 15 Oct 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-2026-55103 in the wild and no public exploit code.

Affected product: discourse-activity-pub. No package-level version data is available, so check the vendor advisory linked from the reference page.

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-2026-55103/ (Markdown: https://insights.sbomify.com/CVE-2026-55103/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-55103
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-55103
- OSV: https://osv.dev/vulnerability/CVE-2026-55103

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-55103/
Summaries are generated and may contain mistakes; the official description above is the record.