fix(wasm): close DNS-rebinding TOCTOU in the plugin HTTP client

crates/barbacane-wasm/src/http_client.rs

@@ -4,7 +4,7 @@
 
 use std::collections::HashMap;
 use std::hash::{Hash, Hasher};
-use std::net::IpAddr;
+use std::net::{IpAddr, SocketAddr};
 use std::path::PathBuf;
 use std::sync::Arc;
 use std::time::Duration;
@@ -116,6 +116,11 @@ impl HttpClient {
             // Disable redirect following: a permitted host could otherwise 3xx
             // to an internal/metadata target, bypassing the SSRF guard below.
             .redirect(reqwest::redirect::Policy::none())
+            // Enforce the SSRF guard at connect-time resolution to close the
+            // DNS-rebinding TOCTOU window.
+            .dns_resolver(Arc::new(GuardedResolver {
+                allow_internal: config.allow_internal_egress,
+            }))
             .build()
             .map_err(HttpClientError::BuildError)?;
 
@@ -165,7 +170,11 @@ impl HttpClient {
             .pool_idle_timeout(self.base_config.pool_idle_timeout)
             .connect_timeout(self.base_config.connect_timeout)
             .timeout(self.base_config.default_timeout)
-            .redirect(reqwest::redirect::Policy::none());
+            .redirect(reqwest::redirect::Policy::none())
+            // Enforce the SSRF guard at connect-time resolution (DNS-rebinding).
+            .dns_resolver(Arc::new(GuardedResolver {
+                allow_internal: self.base_config.allow_internal_egress,
+            }));
 
         // Add client certificate (mTLS)
         if let (Some(cert_path), Some(key_path)) = (&tls_config.client_cert, &tls_config.client_key)
@@ -544,6 +553,54 @@ fn is_forbidden_outbound_header(name: &str) -> bool {
     FORBIDDEN.iter().any(|h| name.eq_ignore_ascii_case(h))
 }
 
+/// A reqwest DNS resolver that enforces the SSRF guard at the moment of
+/// resolution. reqwest connects to exactly the addresses this returns and does
+/// not resolve again, which closes the DNS-rebinding TOCTOU window: a hostile
+/// resolver cannot answer with a public IP for the pre-flight [`ssrf_guard`]
+/// check and then rebind to an internal IP when the connection is made, because
+/// the connect-time resolution is this one and it filters internal addresses.
+#[derive(Debug, Clone)]
+struct GuardedResolver {
+    allow_internal: bool,
+}
+
+/// Keep only the addresses a plugin is permitted to connect to. With egress
+/// disallowed, internal/loopback/link-local/metadata addresses are dropped; an
+/// empty result means every resolved address was internal (the caller treats
+/// that as blocked).
+fn permitted_addrs(
+    resolved: impl Iterator<Item = SocketAddr>,
+    allow_internal: bool,
+) -> Vec<SocketAddr> {
+    if allow_internal {
+        resolved.collect()
+    } else {
+        resolved.filter(|a| !ip_is_internal(&a.ip())).collect()
+    }
+}
+
+impl reqwest::dns::Resolve for GuardedResolver {
+    fn resolve(&self, name: reqwest::dns::Name) -> reqwest::dns::Resolving {
+        let allow_internal = self.allow_internal;
+        Box::pin(async move {
+            let host = name.as_str().to_string();
+            // The port is irrelevant to resolution; reqwest applies the real
+            // target port to the returned IPs.
+            let resolved = tokio::net::lookup_host((host.as_str(), 0u16)).await?;
+            let addrs = permitted_addrs(resolved, allow_internal);
+            if addrs.is_empty() {
+                let err: Box<dyn std::error::Error + Send + Sync> = format!(
+                    "no permitted address for host '{host}': all resolved \
+                     addresses are internal, or none were returned"
+                )
+                .into();
+                return Err(err);
+            }
+            Ok(Box::new(addrs.into_iter()) as reqwest::dns::Addrs)
+        })
+    }
+}
+
 /// SSRF guard for the HTTP client: reject requests whose target resolves to an
 /// internal address.
 async fn ssrf_guard(url: &reqwest::Url, allow_internal: bool) -> Result<(), HttpClientError> {
@@ -1141,6 +1198,28 @@ mod tests {
         assert_ne!(tls1.cache_key(), tls3.cache_key());
     }
 
+    #[test]
+    fn permitted_addrs_filters_internal_unless_egress_allowed() {
+        let internal: SocketAddr = "127.0.0.1:0".parse().unwrap();
+        let link_local: SocketAddr = "169.254.169.254:0".parse().unwrap();
+        let external: SocketAddr = "8.8.8.8:0".parse().unwrap();
+
+        // Egress disallowed: internal + link-local (cloud metadata) dropped.
+        assert_eq!(
+            permitted_addrs(vec![internal, link_local, external].into_iter(), false),
+            vec![external]
+        );
+
+        // Disallowed and every address internal -> empty (caller blocks).
+        assert!(permitted_addrs(vec![internal, link_local].into_iter(), false).is_empty());
+
+        // Egress allowed: everything passes through.
+        assert_eq!(
+            permitted_addrs(vec![internal, external].into_iter(), true),
+            vec![internal, external]
+        );
+    }
+
     #[test]
     fn forbidden_outbound_headers_are_denied_case_insensitively() {
         for h in [