diff --git a/docs/docfx/articles/header-guidelines.md b/docs/docfx/articles/header-guidelines.md
index 36c5edead..c47376cdf 100644
--- a/docs/docfx/articles/header-guidelines.md
+++ b/docs/docfx/articles/header-guidelines.md
@@ -9,7 +9,7 @@ Headers are a very important part of processing HTTP requests and each have thei
 
 ## YARP header filtering
 
-YARP automatically removes request and response headers that could impact its ability to forward a request correctly, or that may be used maliciously to bypass features of the proxy. A complete list can be found [here](https://github.com/microsoft/reverse-proxy/blob/main/src/ReverseProxy/Forwarder/RequestUtilities.cs#L70), with some highlights described below.
+YARP automatically removes request and response headers that could impact its ability to forward a request correctly, or that may be used maliciously to bypass features of the proxy. A complete list can be found [here](https://github.com/microsoft/reverse-proxy/blob/main/src/ReverseProxy/Forwarder/RequestUtilities.cs#L71), with some highlights described below.
 
 ### Connection, KeepAlive, Close
 
diff --git a/eng/Versions.props b/eng/Versions.props
index e90859d3d..d6d29b142 100644
--- a/eng/Versions.props
+++ b/eng/Versions.props
@@ -12,6 +12,7 @@
   </PropertyGroup>
   <!--Package versions-->
   <PropertyGroup>
+    <SystemCollectionsImmutableVersion>8.0.0-rc.1.23419.4</SystemCollectionsImmutableVersion>
     <MicrosoftBclTimeProviderVersion>8.0.0-rc.1.23419.4</MicrosoftBclTimeProviderVersion>
     <MicrosoftDotNetXUnitExtensionsPackageVersion>8.0.0-beta.23463.1</MicrosoftDotNetXUnitExtensionsPackageVersion>
   </PropertyGroup>
diff --git a/src/ReverseProxy/Configuration/ConfigValidator.cs b/src/ReverseProxy/Configuration/ConfigValidator.cs
index 486cf8937..51d9c4da0 100644
--- a/src/ReverseProxy/Configuration/ConfigValidator.cs
+++ b/src/ReverseProxy/Configuration/ConfigValidator.cs
@@ -2,6 +2,7 @@
 // Licensed under the MIT License.
 
 using System;
+using System.Collections.Frozen;
 using System.Collections.Generic;
 using System.Linq;
 using System.Net;
@@ -31,11 +32,11 @@ internal sealed class ConfigValidator : IConfigValidator
     private readonly IAuthorizationPolicyProvider _authorizationPolicyProvider;
     private readonly IYarpRateLimiterPolicyProvider _rateLimiterPolicyProvider;
     private readonly ICorsPolicyProvider _corsPolicyProvider;
-    private readonly IDictionary<string, ILoadBalancingPolicy> _loadBalancingPolicies;
-    private readonly IDictionary<string, IAffinityFailurePolicy> _affinityFailurePolicies;
-    private readonly IDictionary<string, IAvailableDestinationsPolicy> _availableDestinationsPolicies;
-    private readonly IDictionary<string, IActiveHealthCheckPolicy> _activeHealthCheckPolicies;
-    private readonly IDictionary<string, IPassiveHealthCheckPolicy> _passiveHealthCheckPolicies;
+    private readonly FrozenDictionary<string, ILoadBalancingPolicy> _loadBalancingPolicies;
+    private readonly FrozenDictionary<string, IAffinityFailurePolicy> _affinityFailurePolicies;
+    private readonly FrozenDictionary<string, IAvailableDestinationsPolicy> _availableDestinationsPolicies;
+    private readonly FrozenDictionary<string, IActiveHealthCheckPolicy> _activeHealthCheckPolicies;
+    private readonly FrozenDictionary<string, IPassiveHealthCheckPolicy> _passiveHealthCheckPolicies;
     private readonly ILogger _logger;
 
 
diff --git a/src/ReverseProxy/Forwarder/RequestUtilities.cs b/src/ReverseProxy/Forwarder/RequestUtilities.cs
index da70a1688..e2914d4de 100644
--- a/src/ReverseProxy/Forwarder/RequestUtilities.cs
+++ b/src/ReverseProxy/Forwarder/RequestUtilities.cs
@@ -3,6 +3,7 @@
 
 using System;
 using System.Buffers;
+using System.Collections.Frozen;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
@@ -67,7 +68,7 @@ internal static bool ShouldSkipResponseHeader(string headerName)
         return _headersToExclude.Contains(headerName);
     }
 
-    private static readonly HashSet<string> _headersToExclude = new(18, StringComparer.OrdinalIgnoreCase)
+    private static readonly FrozenSet<string> _headersToExclude = new HashSet<string>(18, StringComparer.OrdinalIgnoreCase)
     {
         HeaderNames.Connection,
         HeaderNames.TransferEncoding,
@@ -87,11 +88,11 @@ internal static bool ShouldSkipResponseHeader(string headerName)
         HeaderNames.TE,
         HeaderNames.AltSvc,
         HeaderNames.StrictTransportSecurity,
-    };
+    }.ToFrozenSet(StringComparer.OrdinalIgnoreCase);
 
     // Headers marked as HttpHeaderType.Content in
     // https://github.com/dotnet/runtime/blob/main/src/libraries/System.Net.Http/src/System/Net/Http/Headers/KnownHeaders.cs
-    private static readonly HashSet<string> _contentHeaders = new(11, StringComparer.OrdinalIgnoreCase)
+    private static readonly FrozenSet<string> _contentHeaders = new HashSet<string>(11, StringComparer.OrdinalIgnoreCase)
     {
         HeaderNames.Allow,
         HeaderNames.ContentDisposition,
@@ -104,7 +105,7 @@ internal static bool ShouldSkipResponseHeader(string headerName)
         HeaderNames.ContentType,
         HeaderNames.Expires,
         HeaderNames.LastModified
-    };
+    }.ToFrozenSet(StringComparer.OrdinalIgnoreCase);
 
     /// <summary>
     /// Appends the given path and query to the destination prefix while avoiding duplicate '/'.
diff --git a/src/ReverseProxy/Health/ActiveHealthCheckMonitor.cs b/src/ReverseProxy/Health/ActiveHealthCheckMonitor.cs
index 383ff619d..cd4f2e23c 100644
--- a/src/ReverseProxy/Health/ActiveHealthCheckMonitor.cs
+++ b/src/ReverseProxy/Health/ActiveHealthCheckMonitor.cs
@@ -2,6 +2,7 @@
 // Licensed under the MIT License.
 
 using System;
+using System.Collections.Frozen;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Net.Http;
@@ -17,7 +18,7 @@ namespace Yarp.ReverseProxy.Health;
 internal partial class ActiveHealthCheckMonitor : IActiveHealthCheckMonitor, IClusterChangeListener, IDisposable
 {
     private readonly ActiveHealthCheckMonitorOptions _monitorOptions;
-    private readonly IDictionary<string, IActiveHealthCheckPolicy> _policies;
+    private readonly FrozenDictionary<string, IActiveHealthCheckPolicy> _policies;
     private readonly IProbingRequestFactory _probingRequestFactory;
     private readonly ILogger<ActiveHealthCheckMonitor> _logger;
 
diff --git a/src/ReverseProxy/Health/ClusterDestinationsUpdater.cs b/src/ReverseProxy/Health/ClusterDestinationsUpdater.cs
index 7dd698a51..1b00c8923 100644
--- a/src/ReverseProxy/Health/ClusterDestinationsUpdater.cs
+++ b/src/ReverseProxy/Health/ClusterDestinationsUpdater.cs
@@ -2,6 +2,7 @@
 // Licensed under the MIT License.
 
 using System;
+using System.Collections.Frozen;
 using System.Collections.Generic;
 using System.Runtime.CompilerServices;
 using System.Threading;
@@ -13,7 +14,7 @@ namespace Yarp.ReverseProxy.Health;
 internal sealed class ClusterDestinationsUpdater : IClusterDestinationsUpdater
 {
     private readonly ConditionalWeakTable<ClusterState, SemaphoreSlim> _clusterLocks = new ConditionalWeakTable<ClusterState, SemaphoreSlim>();
-    private readonly IDictionary<string, IAvailableDestinationsPolicy> _destinationPolicies;
+    private readonly FrozenDictionary<string, IAvailableDestinationsPolicy> _destinationPolicies;
 
     public ClusterDestinationsUpdater(IEnumerable<IAvailableDestinationsPolicy> destinationPolicies)
     {
diff --git a/src/ReverseProxy/Health/PassiveHealthCheckMiddleware.cs b/src/ReverseProxy/Health/PassiveHealthCheckMiddleware.cs
index c8ff54a80..801c4c2dc 100644
--- a/src/ReverseProxy/Health/PassiveHealthCheckMiddleware.cs
+++ b/src/ReverseProxy/Health/PassiveHealthCheckMiddleware.cs
@@ -2,6 +2,7 @@
 // Licensed under the MIT License.
 
 using System;
+using System.Collections.Frozen;
 using System.Collections.Generic;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Http;
@@ -12,7 +13,7 @@ namespace Yarp.ReverseProxy.Health;
 public class PassiveHealthCheckMiddleware
 {
     private readonly RequestDelegate _next;
-    private readonly IDictionary<string, IPassiveHealthCheckPolicy> _policies;
+    private readonly FrozenDictionary<string, IPassiveHealthCheckPolicy> _policies;
 
     public PassiveHealthCheckMiddleware(RequestDelegate next, IEnumerable<IPassiveHealthCheckPolicy> policies)
     {
diff --git a/src/ReverseProxy/LoadBalancing/LoadBalancingMiddleware.cs b/src/ReverseProxy/LoadBalancing/LoadBalancingMiddleware.cs
index ec61b2c00..64830a2e2 100644
--- a/src/ReverseProxy/LoadBalancing/LoadBalancingMiddleware.cs
+++ b/src/ReverseProxy/LoadBalancing/LoadBalancingMiddleware.cs
@@ -2,6 +2,7 @@
 // Licensed under the MIT License.
 
 using System;
+using System.Collections.Frozen;
 using System.Collections.Generic;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Http;
@@ -17,7 +18,7 @@ namespace Yarp.ReverseProxy.LoadBalancing;
 internal sealed class LoadBalancingMiddleware
 {
     private readonly ILogger _logger;
-    private readonly IDictionary<string, ILoadBalancingPolicy> _loadBalancingPolicies;
+    private readonly FrozenDictionary<string, ILoadBalancingPolicy> _loadBalancingPolicies;
     private readonly RequestDelegate _next;
 
     public LoadBalancingMiddleware(
diff --git a/src/ReverseProxy/SessionAffinity/AffinitizeTransformProvider.cs b/src/ReverseProxy/SessionAffinity/AffinitizeTransformProvider.cs
index e33f6d22f..2934d8be0 100644
--- a/src/ReverseProxy/SessionAffinity/AffinitizeTransformProvider.cs
+++ b/src/ReverseProxy/SessionAffinity/AffinitizeTransformProvider.cs
@@ -2,6 +2,7 @@
 // Licensed under the MIT License.
 
 using System;
+using System.Collections.Frozen;
 using System.Collections.Generic;
 using Yarp.ReverseProxy.Transforms.Builder;
 using Yarp.ReverseProxy.Utilities;
@@ -10,7 +11,7 @@ namespace Yarp.ReverseProxy.SessionAffinity;
 
 internal sealed class AffinitizeTransformProvider : ITransformProvider
 {
-    private readonly IDictionary<string, ISessionAffinityPolicy> _sessionAffinityPolicies;
+    private readonly FrozenDictionary<string, ISessionAffinityPolicy> _sessionAffinityPolicies;
 
     public AffinitizeTransformProvider(IEnumerable<ISessionAffinityPolicy> sessionAffinityPolicies)
     {
diff --git a/src/ReverseProxy/SessionAffinity/SessionAffinityMiddleware.cs b/src/ReverseProxy/SessionAffinity/SessionAffinityMiddleware.cs
index 043bf2c18..c72b81f78 100644
--- a/src/ReverseProxy/SessionAffinity/SessionAffinityMiddleware.cs
+++ b/src/ReverseProxy/SessionAffinity/SessionAffinityMiddleware.cs
@@ -2,6 +2,7 @@
 // Licensed under the MIT License.
 
 using System;
+using System.Collections.Frozen;
 using System.Collections.Generic;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Http;
@@ -18,8 +19,8 @@ namespace Yarp.ReverseProxy.SessionAffinity;
 internal sealed class SessionAffinityMiddleware
 {
     private readonly RequestDelegate _next;
-    private readonly IDictionary<string, ISessionAffinityPolicy> _sessionAffinityPolicies;
-    private readonly IDictionary<string, IAffinityFailurePolicy> _affinityFailurePolicies;
+    private readonly FrozenDictionary<string, ISessionAffinityPolicy> _sessionAffinityPolicies;
+    private readonly FrozenDictionary<string, IAffinityFailurePolicy> _affinityFailurePolicies;
     private readonly ILogger _logger;
 
     public SessionAffinityMiddleware(
diff --git a/src/ReverseProxy/Transforms/RequestHeadersAllowedTransform.cs b/src/ReverseProxy/Transforms/RequestHeadersAllowedTransform.cs
index 6385d5088..f02630e96 100644
--- a/src/ReverseProxy/Transforms/RequestHeadersAllowedTransform.cs
+++ b/src/ReverseProxy/Transforms/RequestHeadersAllowedTransform.cs
@@ -2,6 +2,7 @@
 // Licensed under the MIT License.
 
 using System;
+using System.Collections.Frozen;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Threading.Tasks;
@@ -22,12 +23,12 @@ public RequestHeadersAllowedTransform(string[] allowedHeaders)
         }
 
         AllowedHeaders = allowedHeaders;
-        AllowedHeadersSet = new HashSet<string>(allowedHeaders, StringComparer.OrdinalIgnoreCase);
+        AllowedHeadersSet = new HashSet<string>(allowedHeaders, StringComparer.OrdinalIgnoreCase).ToFrozenSet(StringComparer.OrdinalIgnoreCase);
     }
 
     internal string[] AllowedHeaders { get; }
 
-    private HashSet<string> AllowedHeadersSet { get; }
+    private FrozenSet<string> AllowedHeadersSet { get; }
 
     /// <inheritdoc/>
     public override ValueTask ApplyAsync(RequestTransformContext context)
diff --git a/src/ReverseProxy/Transforms/ResponseHeadersAllowedTransform.cs b/src/ReverseProxy/Transforms/ResponseHeadersAllowedTransform.cs
index d6334030d..f7ab2b7f9 100644
--- a/src/ReverseProxy/Transforms/ResponseHeadersAllowedTransform.cs
+++ b/src/ReverseProxy/Transforms/ResponseHeadersAllowedTransform.cs
@@ -2,6 +2,7 @@
 // Licensed under the MIT License.
 
 using System;
+using System.Collections.Frozen;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Net.Http.Headers;
@@ -24,12 +25,12 @@ public ResponseHeadersAllowedTransform(string[] allowedHeaders)
         }
 
         AllowedHeaders = allowedHeaders;
-        AllowedHeadersSet = new HashSet<string>(allowedHeaders, StringComparer.OrdinalIgnoreCase);
+        AllowedHeadersSet = new HashSet<string>(allowedHeaders, StringComparer.OrdinalIgnoreCase).ToFrozenSet(StringComparer.OrdinalIgnoreCase);
     }
 
     internal string[] AllowedHeaders { get; }
 
-    private HashSet<string> AllowedHeadersSet { get; }
+    private FrozenSet<string> AllowedHeadersSet { get; }
 
     /// <inheritdoc/>
     public override ValueTask ApplyAsync(ResponseTransformContext context)
diff --git a/src/ReverseProxy/Transforms/ResponseTrailersAllowedTransform.cs b/src/ReverseProxy/Transforms/ResponseTrailersAllowedTransform.cs
index 0ad794798..0c072227d 100644
--- a/src/ReverseProxy/Transforms/ResponseTrailersAllowedTransform.cs
+++ b/src/ReverseProxy/Transforms/ResponseTrailersAllowedTransform.cs
@@ -2,6 +2,7 @@
 // Licensed under the MIT License.
 
 using System;
+using System.Collections.Frozen;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Net.Http.Headers;
@@ -25,12 +26,12 @@ public ResponseTrailersAllowedTransform(string[] allowedHeaders)
         }
 
         AllowedHeaders = allowedHeaders;
-        AllowedHeadersSet = new HashSet<string>(allowedHeaders, StringComparer.OrdinalIgnoreCase);
+        AllowedHeadersSet = new HashSet<string>(allowedHeaders, StringComparer.OrdinalIgnoreCase).ToFrozenSet(StringComparer.OrdinalIgnoreCase);
     }
 
     internal string[] AllowedHeaders { get; }
 
-    private HashSet<string> AllowedHeadersSet { get; }
+    private FrozenSet<string> AllowedHeadersSet { get; }
 
     /// <inheritdoc/>
     public override ValueTask ApplyAsync(ResponseTrailersTransformContext context)
diff --git a/src/ReverseProxy/Utilities/ServiceLookupHelper.cs b/src/ReverseProxy/Utilities/ServiceLookupHelper.cs
index 264c0a881..95de05354 100644
--- a/src/ReverseProxy/Utilities/ServiceLookupHelper.cs
+++ b/src/ReverseProxy/Utilities/ServiceLookupHelper.cs
@@ -2,13 +2,14 @@
 // Licensed under the MIT License.
 
 using System;
+using System.Collections.Frozen;
 using System.Collections.Generic;
 
 namespace Yarp.ReverseProxy.Utilities;
 
 internal static class ServiceLookupHelper
 {
-    public static IDictionary<string, T> ToDictionaryByUniqueId<T>(this IEnumerable<T> services, Func<T, string> idSelector)
+    public static FrozenDictionary<string, T> ToDictionaryByUniqueId<T>(this IEnumerable<T> services, Func<T, string> idSelector)
     {
         if (services is null)
         {
@@ -25,10 +26,10 @@ public static IDictionary<string, T> ToDictionaryByUniqueId<T>(this IEnumerable<
             }
         }
 
-        return result;
+        return result.ToFrozenDictionary(StringComparer.OrdinalIgnoreCase);
     }
 
-    public static T GetRequiredServiceById<T>(this IDictionary<string, T> services, string? id, string defaultId)
+    public static T GetRequiredServiceById<T>(this FrozenDictionary<string, T> services, string? id, string defaultId)
     {
         var lookup = id;
         if (string.IsNullOrEmpty(lookup))
diff --git a/src/ReverseProxy/Yarp.ReverseProxy.csproj b/src/ReverseProxy/Yarp.ReverseProxy.csproj
index b6f447e10..e45cf4b00 100644
--- a/src/ReverseProxy/Yarp.ReverseProxy.csproj
+++ b/src/ReverseProxy/Yarp.ReverseProxy.csproj
@@ -13,6 +13,7 @@
 
   <ItemGroup>
     <PackageReference Include="System.IO.Hashing" Version="7.0.0" />
+    <PackageReference Include="System.Collections.Immutable" Version="$(SystemCollectionsImmutableVersion)" Condition="'$(TargetFramework)' != 'net8.0'" />
     <PackageReference Include="Microsoft.Bcl.TimeProvider" Version="$(MicrosoftBclTimeProviderVersion)" Condition="'$(TargetFramework)' != 'net8.0'" />
   </ItemGroup>