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CVE-2026-46413 is an authorization bypass vulnerability in Discourse, an open‑source discussion platform. The flaw resides in the `ExternalUploadManager` component, which handles direct‑to‑S3 multipart uploads. In affected versions, the manager does not properly enforce role‑based permissions when processing upload requests destined for the admin backup store. As a result, any authenticated regular user—who should only be able to upload files to user‑scoped locations—can manipulate the routing logic to point their multipart upload session toward the internal backup bucket.
The vulnerability stems from missing authorization checks (CWE‑862) in the upload workflow. When a client initiates a multipart upload, Discourse creates an upload record and generates pre‑signed URLs for each part. Normally, the destination is validated against the user’s allowed storage scope. However, in versions prior to the fix, the `ExternalUploadManager` accepts a `target` parameter that can be overridden to `”backup”` without verifying that the requester holds administrative privileges. This allows a low‑privileged user to obtain pre‑signed URLs that write directly to the S3 bucket used for admin backups.
Once the attacker has these URLs, they can upload arbitrary file parts, and finally complete the multipart upload to assemble a full file inside the backup store. The uploaded file can be a malicious payload, a large junk file to exhaust storage, or a replacement for an existing backup. Because backups often contain sensitive data (database dumps, user exports, configuration files), unauthorized modification or insertion can lead to data corruption, denial of service, or even remote code execution if the backup is later restored or processed by administrative tasks.
The attack is conducted remotely over the network, requires only low‑privileged credentials (any registered user), and does not need user interaction. The CVSS v3.1 score assigned by GitHub, Inc. is 6.5 (Medium) with the vector AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:N—indicating high integrity impact but no confidentiality or availability loss. The vulnerability affects all Discourse instances using S3 for backups (the default in many cloud deployments) and running any version before the fixed releases. The patches were backported to versions 2026.6.0, 2026.5.1, 2026.4.2, and 2026.1.5. Administrators are strongly urged to upgrade immediately, as no public exploit has been disclosed yet, but the simplicity of the flaw makes it a prime target for automated scanners.
DailyCVE Form:
Platform: Discourse
Version: prior 2026.6.0
Vulnerability: Improper access control
Severity: Medium (6.5)
date: July 9 2026
Prediction: Patch already released
What Undercode Say:
Check current Discourse version cd /var/discourse rake about | grep "Discourse" Or if using the official Docker image docker exec -it discourse_app rake about | grep "Discourse" Verify if your instance is vulnerable (versions < 2026.6.0, < 2026.5.1, < 2026.4.2, < 2026.1.5) Example output: Discourse 2026.4.1 -> VULNERABLE Simulate a malicious multipart upload attempt (for testing in a sandbox) This curl attempts to start an upload to the backup store as a regular user curl -X POST "https://your-discourse.com/uploads.json" \ -H "Content-Type: multipart/form-data" \ -H "Api-Key: YOUR_API_KEY" \ -H "Api-Username: regular_user" \ -F "files[]=@/tmp/test.txt" \ -F "target=backup" \ -F "multipart=true" If the response returns S3 pre‑signed URLs without an authorization error, the instance is vulnerable. Upgrade to a fixed version using the official update script cd /var/discourse git pull ./launcher rebuild app
Exploit:
An attacker with a valid user account on the Discourse instance can craft a multipart upload request that sets the `target` parameter to "backup". By intercepting the legitimate upload flow (or by directly calling the `ExternalUploadManager` endpoints), they obtain pre‑signed S3 URLs that point to the admin backup bucket. They then upload a file—for example, a malicious script or a large payload—and complete the multipart session. The file is now stored inside the backup store, potentially overwriting critical backup archives or injecting hostile content that may be executed during restore operations.
Protection:
- Upgrade immediately to Discourse versions 2026.6.0, 2026.5.1, 2026.4.2, or 2026.1.5 (or any later release). These releases add proper authorization checks in `ExternalUploadManager` to reject non‑admin requests for the backup store.
- If an immediate upgrade is not possible, disable S3‑based backups temporarily and switch to local storage (
backup_location = local) until the patch can be applied. - Monitor S3 bucket logs for unexpected uploads to the backup prefix, and set up alerts for any write activity originating from non‑admin user sessions.
- Rotate S3 credentials and review IAM policies to ensure that only the Discourse application has write access to the backup bucket, mitigating the impact of compromised pre‑signed URLs.
Impact:
Successful exploitation allows a regular user to arbitrarily create, overwrite, or corrupt files in the admin backup store. This can lead to:
– Data integrity loss – Backup files may be replaced with malicious or outdated data, compromising disaster recovery capabilities.
– Denial of service – Uploading massive files can exhaust storage quotas and prevent legitimate backups from being created.
– Potential remote code execution – If backups are automatically processed or restored by administrative scripts, an attacker could inject executable code that runs with elevated privileges.
– Privacy breach – Although the vulnerability does not directly expose data, an attacker could replace a backup with a crafted file that, when restored, leaks sensitive information.
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Sources:
Reported By: nvd.nist.gov
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