diff --git a/src/mcp/shared/auth_utils.py b/src/mcp/shared/auth_utils.py
index 3ba880f40d..78255c292e 100644
--- a/src/mcp/shared/auth_utils.py
+++ b/src/mcp/shared/auth_utils.py
@@ -1,16 +1,39 @@
 """Utilities for OAuth 2.0 Resource Indicators (RFC 8707) and PKCE (RFC 7636)."""
 
 import time
-from urllib.parse import urlparse, urlsplit, urlunsplit
+from urllib.parse import SplitResult, urlparse, urlsplit, urlunsplit
 
 from pydantic import AnyUrl, HttpUrl
 
 
+def _canonical_netloc(parsed: SplitResult) -> str:
+    """Normalize netloc by lowercasing and stripping explicit default ports (RFC 3986 §6.2.3)."""
+    scheme = parsed.scheme.lower()
+    netloc = parsed.netloc.lower()
+    try:
+        port = parsed.port
+    except ValueError:
+        # Malformed explicit port (e.g. non-numeric) - not canonicalizable, fall back as-is
+        return netloc
+
+    if (scheme == "http" and port == 80) or (scheme == "https" and port == 443):
+        # Strip default port while preserving userinfo and IPv6 brackets
+        userinfo = ""
+        if "@" in netloc:
+            userinfo = netloc.split("@", 1)[0] + "@"
+        hostname = parsed.hostname.lower() if parsed.hostname else ""
+        if ":" in hostname:  # IPv6 literal
+            hostname = f"[{hostname}]"
+        return f"{userinfo}{hostname}"
+    return netloc
+
+
 def resource_url_from_server_url(url: str | HttpUrl | AnyUrl) -> str:
     """Convert server URL to canonical resource URL per RFC 8707.
 
     RFC 8707 section 2 states that resource URIs "MUST NOT include a fragment component".
-    Returns absolute URI with lowercase scheme/host for canonical form.
+    RFC 3986 section 6.2.3 specifies normalization of default ports (80 for http, 443 for https).
+    Returns absolute URI with lowercase scheme/host and stripped default ports for canonical form.
 
     Args:
         url: Server URL to convert
@@ -23,7 +46,8 @@ def resource_url_from_server_url(url: str | HttpUrl | AnyUrl) -> str:
 
     # Parse the URL and remove fragment, create canonical form
     parsed = urlsplit(url_str)
-    canonical = urlunsplit(parsed._replace(scheme=parsed.scheme.lower(), netloc=parsed.netloc.lower(), fragment=""))
+    canonical_netloc = _canonical_netloc(parsed)
+    canonical = urlunsplit(parsed._replace(scheme=parsed.scheme.lower(), netloc=canonical_netloc, fragment=""))
 
     return canonical
 
@@ -43,9 +67,13 @@ def check_resource_allowed(requested_resource: str, configured_resource: str) ->
     Returns:
         True if the requested resource matches the configured resource
     """
-    # Parse both URLs
-    requested = urlparse(requested_resource)
-    configured = urlparse(configured_resource)
+    # Canonicalize both resource URLs (RFC 8707 & RFC 3986 default port normalization)
+    requested_canonical = resource_url_from_server_url(requested_resource)
+    configured_canonical = resource_url_from_server_url(configured_resource)
+
+    # Parse both canonical URLs
+    requested = urlparse(requested_canonical)
+    configured = urlparse(configured_canonical)
 
     # Compare scheme, host, and port (origin)
     if requested.scheme.lower() != configured.scheme.lower() or requested.netloc.lower() != configured.netloc.lower():
diff --git a/tests/shared/test_auth_utils.py b/tests/shared/test_auth_utils.py
index 5ae0e22b0c..d2b9ef6b69 100644
--- a/tests/shared/test_auth_utils.py
+++ b/tests/shared/test_auth_utils.py
@@ -29,9 +29,30 @@ def test_resource_url_from_server_url_preserves_query():
 
 
 def test_resource_url_from_server_url_preserves_port():
-    """Non-default ports should be preserved."""
+    """Non-default ports should be preserved while default ports are stripped per RFC 3986 §6.2.3."""
     assert resource_url_from_server_url("https://example.com:8443/path") == "https://example.com:8443/path"
     assert resource_url_from_server_url("http://example.com:8080/") == "http://example.com:8080/"
+    assert resource_url_from_server_url("https://example.com:443/path") == "https://example.com/path"
+    assert resource_url_from_server_url("http://example.com:80/path") == "http://example.com/path"
+    assert resource_url_from_server_url("http://example.com:80") == "http://example.com"
+    assert resource_url_from_server_url("https://example.com:443") == "https://example.com"
+
+
+def test_resource_url_from_server_url_strips_default_port_with_userinfo():
+    """Default port stripping must preserve userinfo in the netloc."""
+    assert resource_url_from_server_url("https://user:pass@example.com:443/mcp") == "https://user:pass@example.com/mcp"
+    assert resource_url_from_server_url("http://user@example.com:80/api") == "http://user@example.com/api"
+
+
+def test_resource_url_from_server_url_strips_default_port_with_ipv6():
+    """Default port stripping must preserve IPv6 literal brackets."""
+    assert resource_url_from_server_url("https://[::1]:443/path") == "https://[::1]/path"
+    assert resource_url_from_server_url("http://[2001:db8::1]:80/api") == "http://[2001:db8::1]/api"
+
+
+def test_resource_url_from_server_url_malformed_port_falls_back():
+    """A malformed explicit port must not raise; it should fall back to the original netloc."""
+    assert resource_url_from_server_url("https://example.com:99999999/path") == "https://example.com:99999999/path"
 
 
 def test_resource_url_from_server_url_lowercase_scheme_and_host():
@@ -69,9 +90,14 @@ def test_check_resource_allowed_different_domains():
 
 
 def test_check_resource_allowed_different_ports():
-    """Different ports should not match."""
+    """Different ports should not match, but explicit default ports are equivalent to omitted ports."""
     assert check_resource_allowed("https://example.com:8443/path", "https://example.com/path") is False
     assert check_resource_allowed("https://example.com:8080/", "https://example.com:8443/") is False
+    # Explicit default ports per RFC 3986 §6.2.3
+    assert check_resource_allowed("https://example.com:443/mcp", "https://example.com/mcp") is True
+    assert check_resource_allowed("https://example.com/mcp", "https://example.com:443/mcp") is True
+    assert check_resource_allowed("http://example.com:80/api", "http://example.com/api") is True
+    assert check_resource_allowed("http://example.com/api", "http://example.com:80/api") is True
 
 
 def test_check_resource_allowed_hierarchical_matching():
@@ -116,6 +142,11 @@ def test_check_resource_allowed_case_insensitive_origin():
     assert check_resource_allowed("https://Example.Com:8080/api", "https://example.com:8080/api") is True
 
 
+def test_check_resource_allowed_malformed_port_does_not_raise():
+    """A malformed explicit port must not raise; check_resource_allowed should return a bool."""
+    assert check_resource_allowed("https://example.com:99999999/path", "https://example.com/path") is False
+
+
 def test_check_resource_allowed_empty_paths():
     """Empty paths should be handled correctly."""
     assert check_resource_allowed("https://example.com", "https://example.com") is True