CVE-2026-28788

Any authenticated user can overwrite any file's content by ID through the POST /api/v1/retrieval/process/files/batch endpoint. The endpoint performs no ownership check, so a regular user with read access to a shared knowledge base can obtain file UUIDs via GET /api/v1/knowledge/{id}/files and then overwrite those files, escalating from read to write. The overwritten content is served to the LLM via RAG, meaning the attacker controls what the model tells other users.

Details

The process_files_batch() function in backend/open_webui/routers/retrieval.py appears to be designed as an internal helper. The knowledge base router (add_files_to_knowledge_batch() in knowledge.py) imports and calls it directly after performing its own ownership and access control checks. The frontend never calls the retrieval route directly; all legitimate UI flows go through the knowledge base wrapper.

However, the function is also exposed as a standalone HTTP endpoint via @router.post(...). This direct route only requires get_verified_user (any authenticated user) and performs no ownership check of its own:

for file in form_data.files:
    text_content = file.data.get("content", "")  # attacker-controlled

    file_updates.append(FileUpdateForm(
        hash=calculate_sha256_string(text_content),
        data={"content": text_content},            # written to DB
    ))

for file_update, file_result in zip(file_updates, file_results):
    Files.update_file_by_id(id=file_result.file_id, form_data=file_update)
    #                       ^^^ no ownership check

There is no verification that file.user_id == user.id before the write. Any authenticated user who knows a file UUID can overwrite that file.

How an attacker obtains file UUIDs:

Same as with read access, any user who can see a knowledge base can retrieve file IDs for every document in it via GET /api/v1/knowledge/{id}/files. In deployments where knowledge bases are shared across teams, this gives any regular user a list of valid targets.

Suggested fix: Add an ownership check before writing:

for file in form_data.files:
    db_file = Files.get_file_by_id(file.id)
    if not db_file or (db_file.user_id != user.id and user.role != "admin"):
        file_errors.append(BatchProcessFilesResult(
            file_id=file.id, status="failed",
            error="Permission denied: not file owner",
        ))
        continue

Classification:

• CWE-639: Authorization Bypass Through User-Controlled Key
• OWASP API1:2023: Broken Object Level Authorization

Tested on Open WebUI 0.8.3 using a default Docker configuration.

PoC

Prerequisites:

• Default Open WebUI installation (Docker: ghcr.io/open-webui/open-webui:main)
• An admin or user creates a knowledge base with shared read access and uploads a file
• A regular user account exists (the attacker)

Obtaining the file UUID (attacker):

GET /api/v1/knowledge/{kb_id}/files

This returns metadata for all files in the KB, including their UUIDs.

Exploit (attacker):

python3 poc_exploit.py --url http://<host>:3000 --file-id <target-file-uuid> -t <attacker-jwt>

The PoC script: poc_exploit.py

1. Authenticates as the attacker
2. Overwrites the target file via POST /api/v1/retrieval/process/files/batch with a canary payload containing a unique marker string
3. Reads the file back and confirms the attacker's content replaced the original

Verifying RAG poisoning:

After the overwrite, log in as any other user, start a chat with the poisoned knowledge base attached, and ask about the document. The model's response will include the attacker's canary string (BOLA-<marker>), confirming that attacker-controlled content reached the LLM and influenced the response.

No special tooling is required. The script uses only Python 3 standard library (urllib).

Disclaimer on the use of AI powered tools

The research and reporting related to this vulnerability was aided by the help of AI tools.

Who is affected: Any multi-user Open WebUI deployment where knowledge bases are shared. The attacker needs a valid account (any role) and a target file UUID, which is available through any knowledge base they have read access to.

What can happen:

• RAG poisoning: The overwritten content is served to the LLM via RAG. The attacker controls what the model tells every user who queries that knowledge base. This includes the ability to inject instructions the model will follow, which could lead to further exploitation depending on what tools and capabilities are available in the deployment (e.g. code interpreter, function calling).
• Silent data corruption: The original file content is permanently replaced with no indication to the file owner or other users that it has changed.
• No audit trail: Nothing records that an unauthorized user modified the file.

The core issue is that a function designed as an internal helper is exposed as a public endpoint without its own authorization checks. A user with read-only access to a knowledge base can escalate to write access over any file in it.

CVE-2026-28788 has a CVSS score of 7.1 (High). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.8.6); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-28788? CVE-2026-28788 is a high-severity security vulnerability in open-webui (pip), affecting versions < 0.8.6. It is fixed in 0.8.6.
2. How severe is CVE-2026-28788? CVE-2026-28788 has a CVSS score of 7.1 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of open-webui are affected by CVE-2026-28788? open-webui (pip) versions < 0.8.6 is affected.
4. Is there a fix for CVE-2026-28788? Yes. CVE-2026-28788 is fixed in 0.8.6. Upgrade to this version or later.
5. Is CVE-2026-28788 exploitable, and should I be worried? Whether CVE-2026-28788 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-28788 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-28788? Upgrade open-webui to 0.8.6 or later.