fix(cdp): gate file:// navigation behind --allow-file-access

GHSA-q55h-vfv9-qcr5. Any process able to reach the CDP port could
send Page.navigate {"url":"file:///etc/shadow"} and read arbitrary
local files (the process's UID's worth of read access). When obscura
runs as root inside Docker or as a systemd service that translates
straight into /etc/shadow, /root/.ssh/id_rsa, /proc/1/environ, etc.

Validate at the CDP boundary, not in obscura-net, so direct CLI
callers ("obscura fetch file://...") keep working. A new
BrowserContext.allow_file_access flag defaults to false; Page.navigate
returns an error when the requested URL is file:// and the flag is
off. CdpContext::new_with_security threads the flag from the server
entry point, and "obscura serve --allow-file-access" opts in for
local-HTML testing workflows.

Tested:
- Default obscura serve: CDP Page.navigate {file:///etc/passwd} ->
  refused with descriptive error.
- obscura serve --allow-file-access: same call succeeds.
- obscura fetch file://... CLI path: unchanged, still reads the file.
- All earlier PoCs (GHSA-6pj9 JS->file SOP, GHSA-4789 sub-resource
  SSRF, GHSA-4789 CSS template injection) still pass.
- cargo test --workspace clean.

crates/obscura-cdp/src/dispatch.rs

@@ -57,12 +57,23 @@ impl CdpContext {
         stealth: bool,
         user_agent: Option<String>,
     ) -> Self {
-        let default_context = Arc::new(BrowserContext::with_full_options(
+        Self::new_with_security(proxy, stealth, user_agent, false)
+    }
+
+    pub fn new_with_security(
+        proxy: Option<String>,
+        stealth: bool,
+        user_agent: Option<String>,
+        allow_file_access: bool,
+    ) -> Self {
+        let mut ctx = BrowserContext::with_full_options(
             "default".to_string(),
             proxy,
             stealth,
             user_agent,
-        ));
+        );
+        ctx.allow_file_access = allow_file_access;
+        let default_context = Arc::new(ctx);
         // Pre-seed with the default-frame execution context ids that
         // `Runtime.enable` (1) and post-navigation re-emission (2) advertise
         // via Runtime.executionContextCreated. Anything else has to be

crates/obscura-cdp/src/domains/page.rs

@@ -4,6 +4,14 @@ use serde_json::{json, Value};
 use crate::dispatch::CdpContext;
 use crate::types::CdpEvent;
 
+fn url_is_file_scheme(raw: &str) -> bool {
+    url::Url::parse(raw)
+        .map(|u| u.scheme().eq_ignore_ascii_case("file"))
+        .unwrap_or_else(|_| {
+            raw.trim_start().to_ascii_lowercase().starts_with("file:")
+        })
+}
+
 pub async fn handle(
     method: &str,
     params: &Value,
@@ -16,6 +24,18 @@ pub async fn handle(
             let url = params.get("url").and_then(|v| v.as_str())
                 .ok_or("url required")?;
 
+            // Block CDP-initiated file:// navigation by default.
+            // Anyone who can reach the CDP port (default localhost,
+            // but Docker images bind 0.0.0.0) could otherwise read
+            // any file the obscura process can read. Opt in via
+            // `obscura serve --allow-file-access` when local-HTML
+            // testing is the intended workflow.
+            if url_is_file_scheme(url) && !ctx.default_context.allow_file_access {
+                return Err(
+                    "Page.navigate to file:// is disabled. Restart with `obscura serve --allow-file-access` to enable.".to_string()
+                );
+            }
+
             let wait_until = params.get("waitUntil")
                 .and_then(|v| {
                     if let Some(s) = v.as_str() {

crates/obscura-cdp/src/lib.rs

@@ -3,4 +3,7 @@ pub mod dispatch;
 pub mod types;
 pub mod domains;
 
-pub use server::{start, start_with_full_options, start_with_host, start_with_options};
+pub use server::{
+    start, start_with_full_options, start_with_host, start_with_host_and_security,
+    start_with_options,
+};

crates/obscura-cdp/src/server.rs

@@ -55,6 +55,17 @@ pub async fn start_with_host(
     proxy: Option<String>,
     stealth: bool,
     user_agent: Option<String>,
+) -> anyhow::Result<()> {
+    start_with_host_and_security(port, host, proxy, stealth, user_agent, false).await
+}
+
+pub async fn start_with_host_and_security(
+    port: u16,
+    host: &str,
+    proxy: Option<String>,
+    stealth: bool,
+    user_agent: Option<String>,
+    allow_file_access: bool,
 ) -> anyhow::Result<()> {
     let ip: std::net::IpAddr = host
         .parse()
@@ -67,13 +78,16 @@ pub async fn start_with_host(
         "DevTools endpoint: ws://{}:{}/devtools/browser",
         host, port
     );
+    if allow_file_access {
+        info!("file:// navigation enabled (--allow-file-access). Do not expose this port to untrusted networks.");
+    }
 
     let local = tokio::task::LocalSet::new();
     local
         .run_until(async {
             let (msg_tx, msg_rx) = mpsc::unbounded_channel::<ServerMessage>();
 
-            let _processor_handle = tokio::task::spawn_local(cdp_processor(msg_rx, proxy, stealth, user_agent));
+            let _processor_handle = tokio::task::spawn_local(cdp_processor(msg_rx, proxy, stealth, user_agent, allow_file_access));
 
             loop {
                 match listener.accept().await {
@@ -100,8 +114,9 @@ async fn cdp_processor(
     proxy: Option<String>,
     stealth: bool,
     user_agent: Option<String>,
+    allow_file_access: bool,
 ) {
-    let mut ctx = CdpContext::new_with_full_options(proxy, stealth, user_agent);
+    let mut ctx = CdpContext::new_with_security(proxy, stealth, user_agent, allow_file_access);
     let (itx, irx) = mpsc::unbounded_channel::<obscura_js::ops::InterceptedRequest>();
     ctx.intercept_tx = Some(itx);
     let mut intercept_rx: Option<mpsc::UnboundedReceiver<obscura_js::ops::InterceptedRequest>> = Some(irx);