diff --git a/src/browser/navigation-guard.test.ts b/src/browser/navigation-guard.test.ts
index 58ea7a4cd..8a8350cdb 100644
--- a/src/browser/navigation-guard.test.ts
+++ b/src/browser/navigation-guard.test.ts
@@ -1,4 +1,4 @@
-import { describe, expect, it, vi } from "vitest";
+import { afterEach, describe, expect, it, vi } from "vitest";
 import { SsrFBlockedError, type LookupFn } from "../infra/net/ssrf.js";
 import {
   assertBrowserNavigationAllowed,
@@ -12,6 +12,10 @@ function createLookupFn(address: string): LookupFn {
 }
 
 describe("browser navigation guard", () => {
+  afterEach(() => {
+    vi.unstubAllEnvs();
+  });
+
   it("blocks private loopback URLs by default", async () => {
     await expect(
       assertBrowserNavigationAllowed({
@@ -95,6 +99,29 @@ describe("browser navigation guard", () => {
     expect(lookupFn).toHaveBeenCalledWith("example.com", { all: true });
   });
 
+  it("blocks strict policy navigation when env proxy is configured", async () => {
+    vi.stubEnv("HTTP_PROXY", "http://127.0.0.1:7890");
+    const lookupFn = createLookupFn("93.184.216.34");
+    await expect(
+      assertBrowserNavigationAllowed({
+        url: "https://example.com",
+        lookupFn,
+      }),
+    ).rejects.toBeInstanceOf(InvalidBrowserNavigationUrlError);
+  });
+
+  it("allows env proxy navigation when private-network mode is explicitly enabled", async () => {
+    vi.stubEnv("HTTP_PROXY", "http://127.0.0.1:7890");
+    const lookupFn = createLookupFn("93.184.216.34");
+    await expect(
+      assertBrowserNavigationAllowed({
+        url: "https://example.com",
+        lookupFn,
+        ssrfPolicy: { dangerouslyAllowPrivateNetwork: true },
+      }),
+    ).resolves.toBeUndefined();
+  });
+
   it("rejects invalid URLs", async () => {
     await expect(
       assertBrowserNavigationAllowed({
diff --git a/src/browser/navigation-guard.ts b/src/browser/navigation-guard.ts
index c089cacee..73d1b41ba 100644
--- a/src/browser/navigation-guard.ts
+++ b/src/browser/navigation-guard.ts
@@ -6,6 +6,28 @@ import {
 
 const NETWORK_NAVIGATION_PROTOCOLS = new Set(["http:", "https:"]);
 const SAFE_NON_NETWORK_URLS = new Set(["about:blank"]);
+const ENV_PROXY_KEYS = [
+  "HTTP_PROXY",
+  "HTTPS_PROXY",
+  "ALL_PROXY",
+  "http_proxy",
+  "https_proxy",
+  "all_proxy",
+] as const;
+
+function hasEnvProxyConfigured(): boolean {
+  for (const key of ENV_PROXY_KEYS) {
+    const value = process.env[key];
+    if (typeof value === "string" && value.trim().length > 0) {
+      return true;
+    }
+  }
+  return false;
+}
+
+function allowsPrivateNetworkNavigation(policy?: SsrFPolicy): boolean {
+  return policy?.dangerouslyAllowPrivateNetwork === true || policy?.allowPrivateNetwork === true;
+}
 
 function isAllowedNonNetworkNavigationUrl(parsed: URL): boolean {
   // Keep non-network navigation explicit; about:blank is the only allowed bootstrap URL.
@@ -56,6 +78,16 @@ export async function assertBrowserNavigationAllowed(
     );
   }
 
+  // Browser network stacks may apply env proxy routing at connect-time, which
+  // can bypass strict destination-binding intent from pre-navigation DNS checks.
+  // In strict mode, fail closed unless private-network navigation is explicitly
+  // enabled by policy.
+  if (hasEnvProxyConfigured() && !allowsPrivateNetworkNavigation(opts.ssrfPolicy)) {
+    throw new InvalidBrowserNavigationUrlError(
+      "Navigation blocked: strict browser SSRF policy cannot be enforced while env proxy variables are set",
+    );
+  }
+
   await resolvePinnedHostnameWithPolicy(parsed.hostname, {
     lookupFn: opts.lookupFn,
     policy: opts.ssrfPolicy,