Do not reject end session requests that don't include an explicit client_id when request caching is used

src/OpenIddict.Server/OpenIddictServerHandlers.Session.cs

@@ -404,17 +404,8 @@ public static partial class OpenIddictServerHandlers
                     return default;
                 }
 
-                // For consistency with authorization requests, the client_id parameter
-                // is also required when using a request_uri parameter is present.
-                if (string.IsNullOrEmpty(context.Request.ClientId))
-                {
-                    context.Reject(
-                        error: Errors.InvalidRequest,
-                        description: SR.FormatID2037(Parameters.RequestUri, Parameters.ClientId),
-                        uri: SR.FormatID8000(SR.ID2037));
-
-                    return default;
-                }
+                // Note: unlike authorization requests, the client_id parameter is not required for end
+                // session requests and may not be present in the original request before it is cached.
 
                 return default;
             }

test/OpenIddict.Server.IntegrationTests/OpenIddictServerIntegrationTests.Session.cs

@@ -206,6 +206,62 @@ public abstract partial class OpenIddictServerIntegrationTests
         Assert.Equal("af0ifjsldkj", response.State);
     }
 
+    [Fact]
+    public async Task ValidateEndSession_MissingClientIdDoesNotCauseAnError()
+    {
+        // Arrange
+        await using var server = await CreateServerAsync(options =>
+        {
+            options.EnableDegradedMode();
+
+            options.AddEventHandler<HandleEndSessionRequestContext>(builder =>
+                builder.UseInlineHandler(context =>
+                {
+                    context.SignOut();
+
+                    return default;
+                }));
+
+            options.AddEventHandler<ValidateTokenContext>(builder =>
+            {
+                builder.UseInlineHandler(context =>
+                {
+                    Assert.Equal("6esc_11ACC5bwc014ltc14eY22c", context.Token);
+                    Assert.Equal([TokenTypeHints.Private.RequestToken], context.ValidTokenTypes);
+
+                    context.Principal = new ClaimsPrincipal(new ClaimsIdentity("Bearer"))
+                        .SetTokenType(TokenTypeHints.Private.RequestToken)
+                        .SetClaim(Claims.Private.RequestTokenType, RequestTokenTypes.Private.CachedEndSessionRequest)
+                        .SetClaim(Claims.Private.RequestParameters, $$"""
+                        {
+                          "post_logout_redirect_uri": "http://www.fabrikam.com/path",
+                          "state": "af0ifjsldkj"
+                        }
+                        """);
+
+                    return default;
+                });
+
+                builder.SetOrder(ValidateIdentityModelToken.Descriptor.Order - 500);
+            });
+        });
+
+        await using var client = await server.CreateClientAsync();
+
+        // Act
+        var response = await client.GetAsync("/connect/endsession", new OpenIddictRequest
+        {
+            ClientId = null,
+            RequestUri = RequestUris.Prefixes.Generic + "6esc_11ACC5bwc014ltc14eY22c"
+        });
+
+        // Assert
+        Assert.Null(response.Error);
+        Assert.Null(response.ErrorDescription);
+        Assert.Null(response.ErrorUri);
+        Assert.Equal("af0ifjsldkj", response.State);
+    }
+
     [Theory]
     [InlineData("/path", SR.ID2030)]
     [InlineData("/tmp/file.xml", SR.ID2030)]