diff --git a/dotnet/src/Plugins/Plugins.Core/CodeInterpreter/SessionsPythonPlugin.cs b/dotnet/src/Plugins/Plugins.Core/CodeInterpreter/SessionsPythonPlugin.cs
index fc43e1ba7299..f0b88b65bf0c 100644
--- a/dotnet/src/Plugins/Plugins.Core/CodeInterpreter/SessionsPythonPlugin.cs
+++ b/dotnet/src/Plugins/Plugins.Core/CodeInterpreter/SessionsPythonPlugin.cs
@@ -271,9 +271,25 @@ private async Task<HttpResponseMessage> SendAsync(HttpClient httpClient, HttpMet
 
         var uri = new Uri(this._poolManagementEndpoint, pathWithQueryString);
 
-        // If a list of allowed domains has been provided, the host of the provided
-        // uri is checked to verify it is in the allowed domain list.
-        if (!this._settings.AllowedDomains?.Contains(uri.Host) ?? false)
+        // Validate the request host against the allowed domains to prevent SSRF.
+        // When AllowedDomains is explicitly configured, only those hosts are permitted:
+        // a request whose host is not on the list (including the configured endpoint's
+        // own host, e.g. IMDS at 169.254.169.254) is rejected. This is the effective
+        // SSRF guard and it now works correctly regardless of casing.
+        //
+        // When AllowedDomains is null (not configured) the plugin defaults to permitting
+        // only the configured endpoint's own host. Since every request in this type is
+        // built as a relative path against that endpoint, this is a readable,
+        // deterministic replacement for the previous nullable-bool expression rather than
+        // an additional restriction — it preserves backward-compatible behavior while
+        // removing the confusing "(!x) ?? false" fail-open construct. To actually
+        // restrict which host the plugin talks to, configure AllowedDomains explicitly.
+        // Host comparison is ordinal-ignore-case to match DNS semantics and the
+        // convention used by other plugins in this repository.
+        var effectiveAllowedDomains = this._settings.AllowedDomains
+            ?? new[] { this._poolManagementEndpoint.Host };
+
+        if (!effectiveAllowedDomains.Contains(uri.Host, StringComparer.OrdinalIgnoreCase))
         {
             throw new InvalidOperationException("Sending requests to the provided location is not allowed.");
         }
diff --git a/dotnet/src/Plugins/Plugins.UnitTests/Core/SessionsPythonPluginTests.cs b/dotnet/src/Plugins/Plugins.UnitTests/Core/SessionsPythonPluginTests.cs
index e4fc1cf2de12..ddfcbe3c1f5f 100644
--- a/dotnet/src/Plugins/Plugins.UnitTests/Core/SessionsPythonPluginTests.cs
+++ b/dotnet/src/Plugins/Plugins.UnitTests/Core/SessionsPythonPluginTests.cs
@@ -338,6 +338,7 @@ public async Task ItShouldDownloadFileSavingInDiskAsync()
     [InlineData("fake-test-host.io", "https://fake-test-host-1.io/subscriptions/123/rg/456/sps/test-pool", false)]
     [InlineData("fake-test-host.io", "https://www.fake-test-host.io/subscriptions/123/rg/456/sps/test-pool", false)]
     [InlineData("www.fake-test-host.io", "https://fake-test-host.io/subscriptions/123/rg/456/sps/test-pool", false)]
+    [InlineData("FAKE-TEST-HOST.io", "https://fake-test-host.io/subscriptions/123/rg/456/sps/test-pool", true)]
     public async Task ItShouldRespectAllowedDomainsAsync(string allowedDomain, string actualEndpoint, bool isAllowed)
     {
         // Arrange
@@ -364,6 +365,61 @@ public async Task ItShouldRespectAllowedDomainsAsync(string allowedDomain, strin
 #pragma warning restore CA1031 // Do not catch general exception types
     }
 
+    /// <summary>
+    /// When AllowedDomains is null (not configured), requests to the configured endpoint's
+    /// own host should succeed — the plugin defaults to allowing only that host.
+    /// </summary>
+    [Fact]
+    public async Task ItShouldAllowConfiguredEndpointHostWhenAllowedDomainsIsNullAsync()
+    {
+        // Arrange — AllowedDomains is null (default)
+        var settings = new SessionsPythonSettings(
+            sessionId: Guid.NewGuid().ToString(),
+            endpoint: new Uri("https://eastus.acasessions.io/subscriptions/123/rg/456/sps/test-pool"))
+        {
+            AllowedDomains = null
+        };
+
+        this._messageHandlerStub.ResponseToReturn = new HttpResponseMessage(HttpStatusCode.OK)
+        {
+            Content = new StringContent(File.ReadAllText(ListFilesTestDataFilePath)),
+        };
+
+        var sut = new SessionsPythonPlugin(settings, this._httpClientFactory);
+
+        // Act & Assert — should NOT throw; the endpoint host is implicitly allowed
+        var files = await sut.ListFilesAsync();
+        Assert.NotNull(files);
+
+        // The request must actually be sent to the configured endpoint host.
+        Assert.Equal("eastus.acasessions.io", this._messageHandlerStub.RequestUri?.Host);
+    }
+
+    /// <summary>
+    /// When AllowedDomains is explicitly configured, a request whose host is not in the
+    /// allowlist must be blocked — even when that host is the configured endpoint itself.
+    /// This prevents SSRF to arbitrary targets such as IMDS (169.254.169.254).
+    /// </summary>
+    [Fact]
+    public async Task ItShouldBlockEndpointHostNotInAllowedDomainsAsync()
+    {
+        // Arrange — endpoint points at IMDS, but the allowlist only permits a legitimate host.
+        var settings = new SessionsPythonSettings(
+            sessionId: Guid.NewGuid().ToString(),
+            endpoint: new Uri("http://169.254.169.254/metadata/instance"))
+        {
+            AllowedDomains = new[] { "eastus.acasessions.io" }
+        };
+
+        var sut = new SessionsPythonPlugin(settings, this._httpClientFactory);
+
+        // Act & Assert — should throw because 169.254.169.254 is not in AllowedDomains.
+        var exception = await Assert.ThrowsAsync<InvalidOperationException>(
+            () => sut.ListFilesAsync());
+
+        Assert.Contains("not allowed", exception.Message);
+    }
+
     [Fact]
     public async Task ItShouldAddHeadersAsync()
     {