diff --git a/src/IdentityServer4/src/Validation/Default/StrictRedirectUriValidatorAppAuth.cs b/src/IdentityServer4/src/Validation/Default/StrictRedirectUriValidatorAppAuth.cs
index 22a4f0dacb..473ae1a998 100644
--- a/src/IdentityServer4/src/Validation/Default/StrictRedirectUriValidatorAppAuth.cs
+++ b/src/IdentityServer4/src/Validation/Default/StrictRedirectUriValidatorAppAuth.cs
@@ -6,6 +6,7 @@
 using Microsoft.Extensions.Logging;
 using System;
 using System.Threading.Tasks;
+using System.Text.RegularExpressions;
 
 namespace IdentityServer4.Validation
 {
@@ -84,7 +85,9 @@ internal bool IsLoopback(string requestedUri)
                 return false;
             }
 
-            if (int.TryParse(parts[2], out var port))
+            var portMatch = Regex.Match(parts[2], @"^(\d+)");
+
+            if (portMatch.Success && int.TryParse(portMatch.Value, out var port))
             {
                 if (port >= 0 && port <= 65536) return true;
             }
diff --git a/src/IdentityServer4/test/IdentityServer.UnitTests/Validation/StrictRedirectUriValidatorAppAuthValidation.cs b/src/IdentityServer4/test/IdentityServer.UnitTests/Validation/StrictRedirectUriValidatorAppAuthValidation.cs
new file mode 100644
index 0000000000..3e5a8486ed
--- /dev/null
+++ b/src/IdentityServer4/test/IdentityServer.UnitTests/Validation/StrictRedirectUriValidatorAppAuthValidation.cs
@@ -0,0 +1,75 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using System.Threading.Tasks;
+using FluentAssertions;
+using IdentityServer.UnitTests.Common;
+using IdentityServer4.Models;
+using IdentityServer4.Validation;
+using Xunit;
+
+namespace IdentityServer.UnitTests.Validation
+{
+    public class StrictRedirectUriValidatorAppAuthValidation
+    {
+        private const string Category = "Strict Redirect Uri Validator AppAuth Validation";
+
+        private Client clientWithValidLoopbackRedirectUri = new Client 
+        {
+            RequirePkce = true,
+            RedirectUris = new List<string>
+            {
+                "http://127.0.0.1"
+            }
+        };
+
+        private Client clientWithNoRedirectUris = new Client
+        {
+            RequirePkce = true
+        };
+
+        [Theory]
+        [Trait("Category", Category)]
+        [InlineData("http://127.0.0.1")] // This is in the clients redirect URIs
+        [InlineData("http://127.0.0.1:80")]
+        [InlineData("http://127.0.0.1:1")]
+        [InlineData("http://127.0.0.1:65536")]
+        [InlineData("http://127.0.0.1:65536?HelloWorld=1234")]
+        [InlineData("http://127.0.0.1:65536/hello/world")]
+        public async Task Loopback_Redirect_URIs_Should_Be_AllowedAsync(string requestedUri)
+        {
+            var strictRedirectUriValidatorAppAuthValidator = new StrictRedirectUriValidatorAppAuth(TestLogger.Create<StrictRedirectUriValidatorAppAuth>());
+
+            var result = await strictRedirectUriValidatorAppAuthValidator.IsRedirectUriValidAsync(requestedUri, clientWithValidLoopbackRedirectUri);
+
+            result.Should().BeTrue();
+        }
+
+        [Theory]
+        [Trait("Category", Category)]
+        [InlineData("127.0.0.1")]
+        [InlineData("//127.0.0.1")]
+        [InlineData("http://127.0.0.1:t65536")]
+        [InlineData("https://127.0.0.1")]
+        public async Task Loopback_Redirect_URIs_Should_Not_Be_AllowedAsync(string requestedUri)
+        {
+            var strictRedirectUriValidatorAppAuthValidator = new StrictRedirectUriValidatorAppAuth(TestLogger.Create<StrictRedirectUriValidatorAppAuth>());
+
+            var result = await strictRedirectUriValidatorAppAuthValidator.IsRedirectUriValidAsync(requestedUri, clientWithValidLoopbackRedirectUri);
+
+            result.Should().BeFalse();
+        }
+
+        [Fact]
+        [Trait("Category", Category)]
+        public async Task Client_With_No_Redirect_Uris_Should_Not_Be_AllowedAsync()
+        {
+            var strictRedirectUriValidatorAppAuthValidator = new StrictRedirectUriValidatorAppAuth(TestLogger.Create<StrictRedirectUriValidatorAppAuth>());
+
+            var result = await strictRedirectUriValidatorAppAuthValidator.IsRedirectUriValidAsync("http://127.0.0.1", clientWithNoRedirectUris);
+
+            result.Should().BeFalse();
+        }
+
+    }
+}