Coder vulnerable to privilege escalation could lead to a cross workspace compromise

Summary

Insecure session handling opened room for a privilege escalation scenario in which prebuilt workspaces could be compromised by abusing a shared system identity.

Details

Coder automatically generates a session token for a user when a workspace is started. It is automatically exposed via coder_workspace_owner.session_token. Prebuilt workspaces are initially owned by a built-in prebuilds system user.

When a prebuilt workspace is claimed, a new session token is generated for the user that claimed the workspace, but the previous session token for the prebuilds user was not expired. Any Coder workspace templates that persist this automatically generated session token are potentially impacted.

For example, the coder-login module allows template authors to automatically configure the coder CLI to be authenticated with the Coder deployment.

This causes a script to be run on workspace startup that runs the command coder login using the automatically generated user session token c.f. https://github.com/coder/registry/blob/8677e7d52b374b025c4820d09049dc6b008beee8/registry/coder/modules/coder-login/main.tf#L23

This module was originally written before the inception of the prebuilds feature in Coder, which essentially creates a "pre-warmed" pool of workspaces owned by a particular prebuilds system user.

When this module is invoked on a prebuilt workspace, it causes the Coder CLI inside the workspace to persist the session token for the prebuilds user to disk. Because the coder-login module checks if the CLI is authenticated, the script exits early before updating the session token, leaving the Coder CLI authenticated as the prebuildsuser upon claim:

Impact

Important: Deployments that have never utilized the prebuilds feature are not affected by this vulnerability.

This vulnerability requires a previously authenticated user to claim a prebuilt workspace from a template configured to store the prebuilds user session token as described above.

The prebuilds user has no specific roles, so its credential has the same level of access as a regular member user, meaning a user with the prebuilds user's credential can move laterally to any other prebuilt workspace, or create new workspaces as the prebuilds user.

This means that a malicious authenticated actor can potentially execute code on other workspaces owned by the prebuilds user and potentially access information of other users once they claim a previously poisoned workspace.

Remediation

Fixed in https://github.com/coder/coder/pull/19667

Note that prebuilt workspaces claimed previous to the fix may have been affected by this vulnerability. It is recommended that users re-create their workspace if it was created from a prebuild.

To identify potentially affected workspaces, run the following SQL query against the Coder database:

SELECT w.id, w.name, w.owner_id, u.email
FROM workspaces w
LEFT JOIN users u ON u.id = w.owner_id
WHERE w.owner_id <> 'c42fdf75-3097-471c-8c33-fb52454d81c0'::uuid
  AND NOT w.deleted
  AND EXISTS (
    SELECT 1
    FROM workspace_builds AS wb
    WHERE wb.workspace_id = w.id
      AND wb.initiator_id = 'c42fdf75-3097-471c-8c33-fb52454d81c0'::uuid
  );

Previous manually created tokens for the prebuilds user are left as-is. You may wish to review these via coder tokens list --all

Premium license customers can also take advantage of the audit logs feature to query actions performed by the prebuilds user.

Following filter can be used to verify if no API keys were created by the prebuilds user

would indicate a potentially malicious action aimed at achieving persistence

username:prebuilds resource_type:api_key

Following filter can be used to list all write operations performed by the prebuilds user (including failed attempts to update various settings)

username:prebuilds action:write

Patched versions

• 2.24.4
• 2.25.2
• 2.26.0