Disable the local companion in managed Chrome

The Chrome Web Store extension ID is cejgnmmhbbpdmjnfppjdfkocebngehfg. The registered native messaging host is com.steipete.summarize, and its manifest allows only this exact origin:

{
  "allowed_origins": ["chrome-extension://cejgnmmhbbpdmjnfppjdfkocebngehfg/"]
}

The extension declares nativeMessaging in optional_permissions, not permissions. A personal install therefore starts without local-companion capability. Chrome asks for the named permission only after the user selects a Daemon runtime or clicks Enable local companion. Direct AI and Browser media modes do not request it.

Chrome's current permissions documentation says most named permissions may be optional and lists the exceptions; nativeMessaging is not an exception. The generated production manifest is also tested to keep it optional.

#Exact managed policy

Use Chrome's browser-enforced permission block as the primary per-extension boundary. Add the host block and user-level-host restriction as independent defense in depth, then set the extension's managed UX policy so users see why Daemon controls are unavailable.

Example Linux managed policy at /etc/opt/chrome/policies/managed/summarize.json:

{
  "ExtensionSettings": {
    "cejgnmmhbbpdmjnfppjdfkocebngehfg": {
      "installation_mode": "allowed",
      "blocked_permissions": ["nativeMessaging"],
      "runtime_blocked_hosts": ["http://localhost", "http://127.0.0.1", "http://[::1]"]
    }
  },
  "NativeMessagingBlocklist": ["com.steipete.summarize"],
  "NativeMessagingUserLevelHosts": false,
  "3rdparty": {
    "extensions": {
      "cejgnmmhbbpdmjnfppjdfkocebngehfg": {
        "daemonAllowed": false
      }
    }
  }
}

blocked_permissions is the decisive per-extension browser control: Chrome prevents this extension from acquiring or retaining nativeMessaging. NativeMessagingBlocklist prevents Chrome from launching the named host even for another extension. NativeMessagingUserLevelHosts=false allows only administrator-installed system hosts; it is not a complete block on its own.

runtime_blocked_hosts is an independent browser-enforced guard against local HTTP. The production manifest retains http://localhost/* and http://127.0.0.1/* for configured Direct local providers; daemon requests never use those grants. The exact-ID policy above blocks this extension from interacting with localhost, IPv4 loopback, or IPv6 loopback on any port. Cloud Direct providers and Browser mode on ordinary sites continue to work. Chrome policy host patterns omit the path; adding /* here is not valid for this policy.

To disable every native messaging host, use "NativeMessagingBlocklist": ["*"]. If the company needs selected hosts, combine the wildcard block with NativeMessagingAllowlist entries. To force-install Summarize, change installation_mode to force_installed and add:

{ "update_url": "https://clients2.google.com/service/update2/crx" }

The per-ID ExtensionSettings entry is intentional. When a specific extension entry exists, only installation_mode and update_url inherit from the "*" defaults; repeat blocked_permissions on this exact ID.

On Windows, configure the equivalent values under:

HKLM\Software\Policies\Google\Chrome\ExtensionSettings with the JSON dictionary above.
HKLM\Software\Policies\Google\Chrome\NativeMessagingBlocklist\1 as the string

com.steipete.summarize (or *).

HKLM\Software\Policies\Google\Chrome\NativeMessagingUserLevelHosts as DWORD 0.
HKLM\Software\Policies\Google\Chrome\3rdparty\extensions\cejgnmmhbbpdmjnfppjdfkocebngehfg\policy

with daemonAllowed as DWORD 0.

On macOS, deploy the three Chrome policies in the com.google.Chrome managed-preferences domain and set daemonAllowed=false in com.google.Chrome.extensions.cejgnmmhbbpdmjnfppjdfkocebngehfg.

In Google Admin, configure the same per-ID ExtensionSettings and native-messaging policies, then set this extension's policy JSON to:

{ "daemonAllowed": false }

Verify at chrome://policy, restart the extension if policy was applied during an active request, and confirm:

Runtime settings show Disabled by administrator.
Direct remains selected for AI and Browser remains selected for media.
Daemon controls are disabled and no permission prompt appears.
Chrome rejects nativeMessaging for extension ID cejgnmmhbbpdmjnfppjdfkocebngehfg.
Chrome refuses to launch com.steipete.summarize when its host block applies.
Chrome blocks the extension from local HTTP origins when runtime_blocked_hosts applies.

daemonAllowed=false is UX and fail-closed application behavior, not a substitute for the Chrome policies. The service worker checks managed policy and current permission again immediately before every native-host connection.

#Transport and remaining limitation

The service worker sends daemon requests as bounded framed messages to the exact native host. The bridge accepts only GET, POST, and DELETE requests for /health and /v1/*, requires the configured daemon port to match, filters request headers, and chunks responses below Chrome's 1 MiB native-message limit. The separate retained loopback host grants preserve configured Direct local providers; daemon routing never selects them by URL.

The companion daemon itself still exposes its bearer-token-authenticated HTTP API on loopback, and the native process proxies to that API. Native Messaging removes the extension's direct network capability; it does not give the daemon operating-system peer identity. Another local process—or a different extension independently granted loopback access—that obtains the token can still call the daemon. Moving the daemon API entirely onto an OS-authenticated local socket would be a separate hardening project.

Chrome match patterns cannot express “all HTTP hosts except loopback.” The Store manifest removes <all_urls> while retaining only HTTPS plus localhost and IPv4 loopback for existing Direct local provider compatibility. Privileged background fetches of a different plain-HTTP host or media asset are no longer guaranteed. Enterprise deployments that prohibit local providers should keep the browser-enforced runtime_blocked_hosts policy above.

The npm-installed Windows CLI currently lacks the standalone .exe launcher that Chrome requires for a native host path. macOS and Linux install the host end to end; Windows daemon mode is blocked until release packaging includes that executable shim. Do not work around this with a .cmd host or by routing daemon requests through the Direct-provider loopback grants.

Chrome references: