Restrict go2rtc exec sources by default (#21543)

* Restrict go2rtc exec sources by default

* add docs

docker/main/rootfs/usr/local/go2rtc/create_config.py

@@ -22,6 +22,11 @@ sys.path.remove("/opt/frigate")
 
 yaml = YAML()
 
+# Check if arbitrary exec sources are allowed (defaults to False for security)
+ALLOW_ARBITRARY_EXEC = os.environ.get(
+    "GO2RTC_ALLOW_ARBITRARY_EXEC", "false"
+).lower() in ("true", "1", "yes")
+
 FRIGATE_ENV_VARS = {k: v for k, v in os.environ.items() if k.startswith("FRIGATE_")}
 # read docker secret files as env vars too
 if os.path.isdir("/run/secrets"):
@@ -109,14 +114,26 @@ if LIBAVFORMAT_VERSION_MAJOR < 59:
     elif go2rtc_config["ffmpeg"].get("rtsp") is None:
         go2rtc_config["ffmpeg"]["rtsp"] = rtsp_args
 
-for name in go2rtc_config.get("streams", {}):
+
+def is_restricted_source(stream_source: str) -> bool:
+    """Check if a stream source is restricted (echo, expr, or exec)."""
+    return stream_source.strip().startswith(("echo:", "expr:", "exec:"))
+
+
+for name in list(go2rtc_config.get("streams", {})):
     stream = go2rtc_config["streams"][name]
 
     if isinstance(stream, str):
         try:
-            go2rtc_config["streams"][name] = go2rtc_config["streams"][name].format(
-                **FRIGATE_ENV_VARS
-            )
+            formatted_stream = stream.format(**FRIGATE_ENV_VARS)
+            if not ALLOW_ARBITRARY_EXEC and is_restricted_source(formatted_stream):
+                print(
+                    f"[ERROR] Stream '{name}' uses a restricted source (echo/expr/exec) which is disabled by default for security. "
+                    f"Set GO2RTC_ALLOW_ARBITRARY_EXEC=true to enable arbitrary exec sources."
+                )
+                del go2rtc_config["streams"][name]
+                continue
+            go2rtc_config["streams"][name] = formatted_stream
         except KeyError as e:
             print(
                 "[ERROR] Invalid substitution found, see https://docs.frigate.video/configuration/restream#advanced-restream-configurations for more info."
@@ -124,15 +141,33 @@ for name in go2rtc_config.get("streams", {}):
             sys.exit(e)
 
     elif isinstance(stream, list):
-        for i, stream in enumerate(stream):
+        filtered_streams = []
+        for i, stream_item in enumerate(stream):
             try:
-                go2rtc_config["streams"][name][i] = stream.format(**FRIGATE_ENV_VARS)
+                formatted_stream = stream_item.format(**FRIGATE_ENV_VARS)
+                if not ALLOW_ARBITRARY_EXEC and is_restricted_source(formatted_stream):
+                    print(
+                        f"[ERROR] Stream '{name}' item {i + 1} uses a restricted source (echo/expr/exec) which is disabled by default for security. "
+                        f"Set GO2RTC_ALLOW_ARBITRARY_EXEC=true to enable arbitrary exec sources."
+                    )
+                    continue
+
+                filtered_streams.append(formatted_stream)
             except KeyError as e:
                 print(
                     "[ERROR] Invalid substitution found, see https://docs.frigate.video/configuration/restream#advanced-restream-configurations for more info."
                 )
                 sys.exit(e)
 
+        if filtered_streams:
+            go2rtc_config["streams"][name] = filtered_streams
+        else:
+            print(
+                f"[ERROR] Stream '{name}' was removed because all sources were restricted (echo/expr/exec). "
+                f"Set GO2RTC_ALLOW_ARBITRARY_EXEC=true to enable arbitrary exec sources."
+            )
+            del go2rtc_config["streams"][name]
+
 # add birdseye restream stream if enabled
 if config.get("birdseye", {}).get("restream", False):
     birdseye: dict[str, Any] = config.get("birdseye")

docs/docs/configuration/restream.md

@@ -185,10 +185,35 @@ In this configuration:
 - `front_door` stream is used by Frigate for viewing, recording, and detection. The `#backchannel=0` parameter prevents go2rtc from establishing the audio output backchannel, so it won't block two-way talk access.
 - `front_door_twoway` stream is used for two-way talk functionality. This stream can be used by Frigate's WebRTC viewer when two-way talk is enabled, or by other applications (like Home Assistant Advanced Camera Card) that need access to the camera's audio output channel.
 
+## Security: Restricted Stream Sources
+
+For security reasons, the `echo:`, `expr:`, and `exec:` stream sources are disabled by default in go2rtc. These sources allow arbitrary command execution and can pose security risks if misconfigured.
+
+If you attempt to use these sources in your configuration, the streams will be removed and an error message will be printed in the logs.
+
+To enable these sources, you must set the environment variable `GO2RTC_ALLOW_ARBITRARY_EXEC=true`. This can be done in your Docker Compose file or container environment:
+
+```yaml
+environment:
+  - GO2RTC_ALLOW_ARBITRARY_EXEC=true
+```
+
+:::warning
+
+Enabling arbitrary exec sources allows execution of arbitrary commands through go2rtc stream configurations. Only enable this if you understand the security implications and trust all sources of your configuration.
+
+:::
+
 ## Advanced Restream Configurations
 
 The [exec](https://github.com/AlexxIT/go2rtc/tree/v1.9.10#source-exec) source in go2rtc can be used for custom ffmpeg commands. An example is below:
 
+:::warning
+
+The `exec:`, `echo:`, and `expr:` sources are disabled by default for security. You must set `GO2RTC_ALLOW_ARBITRARY_EXEC=true` to use them. See [Security: Restricted Stream Sources](#security-restricted-stream-sources) for more information.
+
+:::
+
 NOTE: The output will need to be passed with two curly braces `{{output}}`
 
 ```yaml