Omnigent: Shared Agent Bundle Overwrite Leads to Authenticated Runner RCE

Summary

An authenticated user with edit access to their own session can overwrite a shared/template agent by uploading a full agent bundle through PUT /sessions/{session_id}/agent.

Shared/template agents are shown as not MCP-editable, but this upload path still accepts a replacement bundle. By adding a stdio MCP server to the shared agent, the attacker can cause future runner sessions using that shared agent to start an attacker-controlled command.

Details

The vulnerable endpoint is the full agent bundle upload route: PUT /sessions/{session_id}/agent
This route checks whether the caller can edit the session: LEVEL_EDIT session permission check
But it does not check whether the bound agent is a shared/template agent.
Shared/template agents have:

 1agent.session_id is None

The API already exposes that these agents are not meant to be MCP-editable: mcp_servers_editable is false for shared/template agents
The direct MCP edit endpoint correctly blocks shared/template agents: session_mcp_servers.py shared-agent guard

 1if agent.session_id is None:
 2    raise OmnigentError("Built-in agents are read-only through this endpoint.")

The full bundle upload endpoint is missing that same guard. It stores the uploaded bundle and updates the agent row: agent_store.update(agent.id, new_loc)
For shared/template agents, this changes the shared agent itself, not only the attacker's session.
This becomes command execution because stdio MCP servers start the configured command as a runner subprocess: tools/mcp.py stdio subprocess launch

Suggested fix: add the same agent.session_id is None guard to PUT /sessions/{session_id}/agent.

Impact

This is authenticated RCE against Omnigent runner hosts. A normal authenticated user with edit access to their own session can poison a shared/template agent bundle, even though that agent is shown as not MCP-editable. The poisoned bundle can then be used by future sessions created from the same shared agent.

Because stdio MCP servers run as local subprocesses, the attacker-controlled command executes with the permissions of the Omnigent runner process. In a shared or company-hosted deployment, this can let an attacker read runner-accessible files and credentials, modify workspace data, disrupt runner availability, pivot to internal services reachable from the runner, or establish an interactive remote shell if outbound networking is allowed.

This is high impact because it turns normal shared-agent session access into command execution on runner infrastructure used by other sessions.