Summary
CVE-2026-55418 is a high-severity broken object-level authorization (IDOR) vulnerability in FastGPT, the open-source AI knowledge base and RAG platform developed by labring. Two file handler endpoints — the chat-file presign endpoint and the dataset preview endpoint — accept an S3 object key directly from the request without verifying that the key belongs to the requesting caller’s team, enabling any unauthenticated network attacker to read files belonging to other tenants. The vulnerability carries a CVSS v3.1 score of 8.6 (HIGH) and requires no authentication, no privileges, and no user interaction to exploit.
Technical details
- Root cause: Both the chat-file presign endpoint and the dataset preview endpoint authorize an unrelated resource and then sign or read an S3 object using a key taken directly from the HTTP request. No ownership check is performed to confirm that the supplied key belongs to the requesting team. Because S3 object keys are globally scoped within the shared bucket — with the tenant identifier encoded only as a path segment — an attacker can freely substitute another tenant’s key.
- Trigger conditions: An attacker supplies a known or guessed S3 object key belonging to a different team in a request to either vulnerable endpoint. No prior authentication or account access is required.
- Attack vector: Remotely exploitable over the network (AV:N) with low attack complexity (AC:L), no privileges required (PR:N), and no user interaction (UI:N).
- Impact: Full read access to private files — including knowledge base documents, uploaded datasets, and chat attachments — stored in another tenant’s S3 namespace. Scope is Changed (S:C) with High confidentiality impact (C:H) and no integrity or availability impact.
- CWE: CWE-639 – Authorization Bypass Through User-Controlled Key.
Affected software
- FastGPT (labring/FastGPT): all versions prior to v4.15.0-beta5
Severity
- CVSS v3.1 Base Score: 8.6 (HIGH)
- Vector String:
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N
Mitigation and recommended actions
- Immediate action: Upgrade FastGPT to v4.15.0-beta5 or later. The fix (pull request #7104) binds S3 object keys to authenticated resources, ensuring that the key supplied in a request is validated against the requesting team before any presign or read operation is performed.
- If immediate patching is not possible: Restrict network access to the FastGPT application to trusted networks or authenticated users only via a reverse proxy or firewall rule, reducing the unauthenticated attack surface until the patch can be applied.
IONIX Status
The IONIX research team is tracking ongoing exploitation attempts and recommends immediate patching. Potentially affected assets are outlined in this post.