CVE-2026-55103 — Discourse ActivityPub adds ActivityPub support to Discourse.¶

Key facts

CVE-2026-55103 is a High (8.6) vulnerability: Discourse ActivityPub adds ActivityPub support to Discourse.. This entry ships a detection-only PoC and a Docker test lab.


CVE	CVE-2026-55103
Severity	High (8.6)
Detection / prevention	—
PoC	poc.py
Source	2026/CVE-2026-55103/

Details¶

English | 한국어

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.

Vulnerability Details¶

Property	Value
CVE ID	CVE-2026-55103
CVSS Score	8.6

Proof of Concept¶

Download¶

# Download PoC script
curl -O https://raw.githubusercontent.com/fankh/vulnerability-poc/main/2026/CVE-2026-55103/poc.py

Usage¶

# Run PoC against target
python3 poc.py --target <TARGET_URL>

# With authentication
python3 poc.py --target <TARGET_URL> --username <USER> --password <PASS>

⚠️ Warning: This PoC is for authorized security testing only. It uses detection-only techniques and does not exploit or damage target systems.

Test Lab¶

This repository includes a Docker-based test environment with both vulnerable and patched applications.

Quick Start¶

# Clone and navigate to CVE directory
cd CVE-2026-55103

# Start both applications
docker-compose up -d

# Run tests
./run-tests.sh    # Linux/Mac
./run-tests.ps1   # Windows PowerShell

Application Endpoints¶

Application	URL	Status
Vulnerable App	http://localhost:8080	Shows vulnerability
Patched App	http://localhost:8081	Demonstrates fix

Directory Structure¶

├── README.md           # English documentation
├── README_KO.md        # Korean documentation
├── poc.py              # PoC detection script
├── docker-compose.yml  # Container orchestration
├── run-tests.sh        # Linux/Mac test runner
├── run-tests.ps1       # Windows test runner
├── vulnerable-app/     # Vulnerable version
│   ├── app.py
│   ├── Dockerfile
│   └── requirements.txt
└── patched-app/        # Fixed version
    ├── app.py
    ├── Dockerfile
    └── requirements.txt

References¶

https://github.com/discourse/discourse-activity-pub/commit/1c8518b0f2abdf31cbea3e0b242f74d8994a9ba5
https://github.com/discourse/discourse-activity-pub/security/advisories/GHSA-jw39-rch6-mhwf

Generated by PoC Generator on 2026-10-16T04:35:23.823550735

Frequently asked questions¶

What is CVE-2026-55103?¶

CVE-2026-55103 is a High (8.6) vulnerability: Discourse ActivityPub adds ActivityPub support to Discourse.. This entry ships a detection-only PoC and a Docker test lab.

How do I detect CVE-2026-55103?¶

Run the detection-only PoC included with this entry against your Docker test lab to confirm whether a target is affected; pair it with your own IDS logging.

How do I block or mitigate CVE-2026-55103?¶

Upgrade to a fixed version per the vendor advisory; the NVD record lists the affected versions. Use the included PoC to verify your remediation.

For authorized security testing and research only. See the full source and NVD record.