diff --git a/crawl4ai/utils.py b/crawl4ai/utils.py
index 279c27708..306f7f86c 100644
--- a/crawl4ai/utils.py
+++ b/crawl4ai/utils.py
@@ -2552,12 +2552,18 @@ def is_external_url(url: str, base_domain: str) -> bool:
         if not parsed.netloc:  # Relative URL
             return False
 
-        # Strip port and 'www.' from both domains for comparison
-        url_domain = parsed.netloc.lower().split(":")[0].replace("www.", "")
-        base = base_domain.lower().split(":")[0].replace("www.", "")
-
-        # Check if URL domain ends with base domain
-        return not url_domain.endswith(base)
+        # Strip port and a leading 'www.' from both domains for comparison.
+        # removeprefix (not replace) so a host containing 'www.' somewhere
+        # other than the front is left untouched.
+        url_domain = parsed.netloc.lower().split(":")[0].removeprefix("www.")
+        base = base_domain.lower().split(":")[0].removeprefix("www.")
+
+        # Internal means the base domain itself or one of its subdomains. A
+        # bare endswith(base) also matches look-alike domains that merely end
+        # with the base string (notexample.com for example.com), which are
+        # genuinely different sites an attacker can register.
+        is_internal = url_domain == base or url_domain.endswith(f".{base}")
+        return not is_internal
     except Exception:
         return False
 
diff --git a/tests/regression/test_reg_utils.py b/tests/regression/test_reg_utils.py
index dfc63c42d..f637689b4 100644
--- a/tests/regression/test_reg_utils.py
+++ b/tests/regression/test_reg_utils.py
@@ -15,6 +15,7 @@
     efficient_normalize_url_for_deep_crawl,
     sanitize_input_encode,
     generate_content_hash,
+    is_external_url,
 )
 from crawl4ai.cache_context import CacheContext, CacheMode
 
@@ -498,3 +499,48 @@ def test_image_description_threshold_exists(self):
         """IMAGE_DESCRIPTION_MIN_WORD_THRESHOLD should exist."""
         from crawl4ai.config import IMAGE_DESCRIPTION_MIN_WORD_THRESHOLD
         assert isinstance(IMAGE_DESCRIPTION_MIN_WORD_THRESHOLD, (int, float))
+
+
+# ===================================================================
+# is_external_url
+# ===================================================================
+
+class TestIsExternalUrl:
+    """Verify is_external_url only treats the base domain and its subdomains
+    as internal, and does not mistake look-alike domains for internal ones."""
+
+    def test_base_domain_is_internal(self):
+        """The base domain itself is internal."""
+        assert is_external_url("http://example.com", "example.com") is False
+
+    def test_subdomain_is_internal(self):
+        """A real subdomain of the base domain is internal."""
+        assert is_external_url("http://blog.example.com", "example.com") is False
+
+    def test_www_is_internal(self):
+        """A leading www. is stripped, so www.example.com is internal."""
+        assert is_external_url("http://www.example.com", "example.com") is False
+
+    def test_prefixed_lookalike_is_external(self):
+        """A host that merely ends with the base string is a different site."""
+        assert is_external_url("http://malicious-example.com", "example.com") is True
+
+    def test_bare_lookalike_is_external(self):
+        """notexample.com is not a subdomain of example.com."""
+        assert is_external_url("http://notexample.com/path", "example.com") is True
+
+    def test_base_as_prefix_is_external(self):
+        """The base domain appearing as a prefix is a different site."""
+        assert is_external_url("http://example.com.evil.com", "example.com") is True
+
+    def test_unrelated_domain_is_external(self):
+        """An unrelated domain is external."""
+        assert is_external_url("http://other.org", "example.com") is True
+
+    def test_relative_url_is_internal(self):
+        """A relative URL has no netloc and is treated as internal."""
+        assert is_external_url("/path/page", "example.com") is False
+
+    def test_mailto_is_external(self):
+        """Non-http schemes such as mailto: are external."""
+        assert is_external_url("mailto:hi@example.com", "example.com") is True