Allow ephemeral ports for loopback redirect URIs

Native OAuth clients such as Claude Code publish loopback redirect URIs
without a port (http://localhost/callback) and pick an ephemeral port at
request time, as described in RFC 8252 section 7.3. The authorize flow
matched the requested redirect URI against the registered set with an
exact string comparison, so http://localhost:3118/callback was rejected
with invalid_redirect_uri even for a trusted, allow-listed client.

Make OAuth2Client.IsRedirectURIAllowed the single source of truth for
redirect matching: it keeps exact matching and adds loopback-aware
matching that ignores the port when scheme, host, path, and query agree.
The redundant document-level check and its duplicate loopback helper in
the CIMD resolver are removed, so both the registered-client and CIMD
paths now rely on one matcher.

Also add a pkg/netx package for the loopback helper.

Signed-off-by: Bryan Frimin <bryan@probo.com>

pkg/coredata/oauth2_client.go

@@ -19,6 +19,7 @@ import (
 	"errors"
 	"fmt"
 	"maps"
+	"net/url"
 	"slices"
 	"time"
 
@@ -26,6 +27,7 @@ import (
 	"go.gearno.de/kit/pg"
 	"go.probo.inc/probo/pkg/gid"
 	"go.probo.inc/probo/pkg/iam/policy"
+	"go.probo.inc/probo/pkg/netx"
 	"go.probo.inc/probo/pkg/page"
 	"go.probo.inc/probo/pkg/uri"
 )
@@ -54,7 +56,42 @@ type (
 )
 
 func (c *OAuth2Client) IsRedirectURIAllowed(rawURI string) bool {
-	return slices.Contains(c.RedirectURIs, uri.URI(rawURI))
+	if slices.Contains(c.RedirectURIs, uri.URI(rawURI)) {
+		return true
+	}
+
+	// RFC 8252 §7.3: native apps using a loopback redirect URI choose an
+	// ephemeral port at request time, so the port must be ignored when
+	// matching against the registered loopback redirect URIs.
+	requested, err := url.Parse(rawURI)
+	if err != nil || !netx.IsLoopback(requested.Hostname()) {
+		return false
+	}
+
+	for _, registered := range c.RedirectURIs {
+		candidate, err := url.Parse(registered.String())
+		if err != nil {
+			continue
+		}
+
+		if candidate.Scheme != requested.Scheme {
+			continue
+		}
+
+		if !netx.IsLoopback(candidate.Hostname()) {
+			continue
+		}
+
+		if candidate.Hostname() != requested.Hostname() {
+			continue
+		}
+
+		if candidate.Path == requested.Path && candidate.RawQuery == requested.RawQuery {
+			return true
+		}
+	}
+
+	return false
 }
 
 func (c *OAuth2Client) HasGrantType(grantType OAuth2GrantType) bool {

pkg/coredata/oauth2_client_test.go

@@ -0,0 +1,96 @@
+// Copyright (c) 2026 Probo Inc <hello@probo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package coredata_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"go.probo.inc/probo/pkg/coredata"
+	"go.probo.inc/probo/pkg/uri"
+)
+
+func TestOAuth2Client_IsRedirectURIAllowed(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name       string
+		registered []uri.URI
+		requested  string
+		want       bool
+	}{
+		{
+			name:       "exact https match",
+			registered: []uri.URI{"https://chatgpt.com/connector/oauth/callback"},
+			requested:  "https://chatgpt.com/connector/oauth/callback",
+			want:       true,
+		},
+		{
+			name:       "https mismatch",
+			registered: []uri.URI{"https://chatgpt.com/connector/oauth/callback"},
+			requested:  "https://evil.example/callback",
+			want:       false,
+		},
+		{
+			name:       "loopback ignores port when registered without port",
+			registered: []uri.URI{"http://localhost/callback"},
+			requested:  "http://localhost:3118/callback",
+			want:       true,
+		},
+		{
+			name:       "loopback ignores port when registered with a different port",
+			registered: []uri.URI{"http://127.0.0.1:53682/callback"},
+			requested:  "http://127.0.0.1:8080/callback",
+			want:       true,
+		},
+		{
+			name:       "loopback host must match (localhost vs 127.0.0.1)",
+			registered: []uri.URI{"http://127.0.0.1/callback"},
+			requested:  "http://localhost:3118/callback",
+			want:       false,
+		},
+		{
+			name:       "loopback path must match",
+			registered: []uri.URI{"http://localhost/callback"},
+			requested:  "http://localhost:3118/other",
+			want:       false,
+		},
+		{
+			name:       "loopback scheme must match",
+			registered: []uri.URI{"http://localhost/callback"},
+			requested:  "https://localhost:3118/callback",
+			want:       false,
+		},
+		{
+			name:       "non-loopback host does not get port flexibility",
+			registered: []uri.URI{"https://app.example.com/callback"},
+			requested:  "https://app.example.com:8443/callback",
+			want:       false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(
+			tt.name,
+			func(t *testing.T) {
+				t.Parallel()
+
+				client := &coredata.OAuth2Client{RedirectURIs: tt.registered}
+
+				assert.Equal(t, tt.want, client.IsRedirectURIAllowed(tt.requested))
+			},
+		)
+	}
+}

pkg/iam/oauth2/cimd.go

@@ -33,7 +33,7 @@ import (
 	"go.probo.inc/probo/pkg/cachecontrol"
 	"go.probo.inc/probo/pkg/coredata"
 	"go.probo.inc/probo/pkg/gid"
-	"go.probo.inc/probo/pkg/net"
+	"go.probo.inc/probo/pkg/netx"
 )
 
 const (
@@ -239,7 +239,7 @@ func validateCIMDRedirectURI(redirectURI string) error {
 	switch parsed.Scheme {
 	case "https":
 	case "http":
-		if !net.IsLoopback(parsed.Hostname()) {
+		if !netx.IsLoopback(parsed.Hostname()) {
 			return NewError(
 				ErrInvalidClient,
 				WithDescription("client metadata document contains invalid redirect_uri"),
@@ -255,47 +255,6 @@ func validateCIMDRedirectURI(redirectURI string) error {
 	return nil
 }
 
-func cimdRedirectURIAllowed(doc *ClientMetadataDocument, redirectURI string) bool {
-	for _, allowed := range doc.RedirectURIs {
-		if redirectURI == allowed {
-			return true
-		}
-
-		if cimdLoopbackRedirectMatches(allowed, redirectURI) {
-			return true
-		}
-	}
-
-	return false
-}
-
-func cimdLoopbackRedirectMatches(registered, requested string) bool {
-	registeredURL, err := url.Parse(registered)
-	if err != nil {
-		return false
-	}
-
-	requestedURL, err := url.Parse(requested)
-	if err != nil {
-		return false
-	}
-
-	if registeredURL.Scheme != requestedURL.Scheme {
-		return false
-	}
-
-	if !net.IsLoopback(registeredURL.Hostname()) || !net.IsLoopback(requestedURL.Hostname()) {
-		return false
-	}
-
-	if registeredURL.Hostname() != requestedURL.Hostname() {
-		return false
-	}
-
-	return registeredURL.Path == requestedURL.Path &&
-		registeredURL.RawQuery == requestedURL.RawQuery
-}
-
 func (f *cimdFetcher) loadCache(clientIDURL string) (*ClientMetadataDocument, bool) {
 	raw, ok := f.cache.Load(clientIDURL)
 	if !ok {
@@ -338,7 +297,6 @@ func (s *Service) resolveClient(
 	ctx context.Context,
 	tx pg.Tx,
 	clientIDRaw string,
-	redirectURI string,
 ) (*coredata.OAuth2Client, error) {
 	if clientID, err := gid.ParseGID(clientIDRaw); err == nil {
 		if tx != nil {
@@ -373,10 +331,6 @@ func (s *Service) resolveClient(
 		return nil, err
 	}
 
-	if redirectURI != "" && !cimdRedirectURIAllowed(doc, redirectURI) {
-		return nil, ErrInvalidRedirectURI
-	}
-
 	client, err := s.upsertCIMDClient(ctx, tx, clientIDRaw, doc)
 	if err != nil {
 		return nil, err

pkg/iam/oauth2/cimd_test.go

@@ -141,30 +141,6 @@ func TestValidateClientMetadataDocument(t *testing.T) {
 	)
 }
 
-func TestCIMDRedirectURIAllowed(t *testing.T) {
-	t.Parallel()
-
-	doc := &ClientMetadataDocument{
-		RedirectURIs: []string{
-			"https://chatgpt.com/connector/oauth/callback",
-			"http://127.0.0.1:53682/callback",
-		},
-	}
-
-	assert.True(
-		t,
-		cimdRedirectURIAllowed(doc, "https://chatgpt.com/connector/oauth/callback"),
-	)
-	assert.True(
-		t,
-		cimdRedirectURIAllowed(doc, "http://127.0.0.1:8080/callback"),
-	)
-	assert.False(
-		t,
-		cimdRedirectURIAllowed(doc, "https://evil.example/callback"),
-	)
-}
-
 func TestCIMDFetcherFetch(t *testing.T) {
 	t.Parallel()
 

pkg/iam/oauth2/service.go

@@ -32,7 +32,7 @@ import (
 	"go.probo.inc/probo/pkg/crypto/rand"
 	"go.probo.inc/probo/pkg/gid"
 	"go.probo.inc/probo/pkg/iam/oauth2scope"
-	"go.probo.inc/probo/pkg/net"
+	"go.probo.inc/probo/pkg/netx"
 	"go.probo.inc/probo/pkg/page"
 	"go.probo.inc/probo/pkg/uri"
 )
@@ -1060,7 +1060,7 @@ func (s *Service) RegisterClient(
 			}
 		case coredata.OAuth2ClientVisibilityPrivate:
 			if parsed.Scheme == "http" {
-				if !net.IsLoopback(parsed.Hostname()) {
+				if !netx.IsLoopback(parsed.Hostname()) {
 					return gid.Nil,
 						"",
 						NewError(
@@ -1433,13 +1433,11 @@ func (s *Service) Authorize(
 	if err := s.pg.WithTx(
 		ctx,
 		func(ctx context.Context, tx pg.Tx) error {
-			client, err := s.resolveClient(ctx, tx, req.ClientIDRaw, req.RedirectURI)
+			client, err := s.resolveClient(ctx, tx, req.ClientIDRaw)
 			if err != nil {
 				return err
 			}
 
-			fmt.Printf("X: %+v\n", client)
-
 			if !client.IsRedirectURIAllowed(req.RedirectURI) {
 				return ErrInvalidRedirectURI
 			}
@@ -1738,7 +1736,7 @@ func (s *Service) AuthenticateClient(
 	clientIDRaw string,
 	clientSecret string,
 ) (*coredata.OAuth2Client, error) {
-	client, err := s.resolveClient(ctx, nil, clientIDRaw, "")
+	client, err := s.resolveClient(ctx, nil, clientIDRaw)
 	if err != nil {
 		return nil, err
 	}

pkg/netx/netx.go

@@ -12,7 +12,7 @@
 // OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
 // PERFORMANCE OF THIS SOFTWARE.
 
-package net
+package netx
 
 import "net"
 

pkg/netx/netx_test.go

@@ -12,13 +12,13 @@
 // OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
 // PERFORMANCE OF THIS SOFTWARE.
 
-package net_test
+package netx_test
 
 import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
-	"go.probo.inc/probo/pkg/net"
+	"go.probo.inc/probo/pkg/netx"
 )
 
 func TestIsLoopback(t *testing.T) {
@@ -77,7 +77,7 @@ func TestIsLoopback(t *testing.T) {
 			func(t *testing.T) {
 				t.Parallel()
 
-				assert.Equal(t, tt.want, net.IsLoopback(tt.host))
+				assert.Equal(t, tt.want, netx.IsLoopback(tt.host))
 			},
 		)
 	}