Read-Only Open WebUI Users Can Modify Collaborative Documents via Socket.IO

Read-Only Users Can Modify Collaborative Documents via Socket.IO

Affected Component

Socket.IO collaborative document editing handler:

• backend/open_webui/socket/main.py (lines 667-721, ydoc:document:update handler)

Affected Versions

Current main branch and likely all versions with collaborative note editing.

Description

The ydoc:document:update Socket.IO event handler checks whether the sender is a member of the document's Socket.IO room (line 678) but does not verify that the sender has write permission. Users with read-only access join the document room via ydoc:document:join, which only requires read permission (line 520). Once in the room, the user can emit ydoc:document:update events that modify the in-memory Yjs document state and are broadcast to all other collaborators in real time.

The document_save_handler (line 600) correctly checks write permission before persisting to the database, so the attacker cannot directly save changes. However, the tampered content is visible to all collaborators, and if any user with write access saves the document, the injected content is persisted.

# ydoc:document:update handler (line 667) — only checks room membership, not write permission
async def on_document_update(sid, data):
    document_id = normalize_document_id(data.get('document_id', ''))
    # ...
    room = f'doc_{document_id}'
    if room not in sio.rooms(sid):  # Room membership check only
        return
    # Applies update to Yjs state and broadcasts to all users
    YDOC_MANAGER.apply_update(document_id, update)
    await sio.emit('ydoc:document:update', {...}, room=room, skip_sid=sid)

Compare with ydoc:document:join (line 520) which checks permission:

# Only checks READ permission — so read-only users join the room
if not has_access(user_id, type, id, 'read', db=db):
    return

CVSS 3.1 Breakdown

Attack Scenario

1. User A creates a note and shares it with User B with read permission.
2. User B opens the note, which triggers ydoc:document:join — the server checks read permission and adds User B to the document room.
3. User B emits ydoc:document:update with a crafted Yjs update payload via the Socket.IO connection (bypassing any frontend read-only enforcement).
4. The server applies the update to the Yjs document state and broadcasts it to all collaborators.
5. User A sees the injected content appear in their editor in real time.
6. If User A saves the document (intentionally or via autosave), the tampered content is persisted to the database — User A's save passes the write permission check since User A is the owner.

Impact

• Read-only users can inject, modify, or delete content in collaborative documents
• Modifications are broadcast in real time to all collaborators, causing confusion or disruption
• If a write-access user saves (including autosave), the tampered content is permanently persisted
• Undermines the read/write permission model for collaborative editing

Preconditions

• Attacker must have a valid user account with read access to a shared note
• The note must be open for collaborative editing (at least one other user viewing it, or the attacker can wait for a write-access user to open and save)

Consolidation

This advisory covers read-only users modifying collaborative notes over Socket.IO. Two handlers were affected, both fixed in v0.9.0:

• ydoc:document:update — checked only room membership, not write permission, so a read-only collaborator could inject in-memory document updates broadcast to other collaborators (persistence indirect). @Classic298.
• document_save_handler — checked permission='read' while persisting via Notes.update_note_by_id, so a read-only collaborator could persist note changes directly. Reported by @hacnho

One CVE for the consolidated advisory.