HrynCo.NotificationService.Services.Tests/EmailProcessing/EmailTemplateRenderingServiceTests.cs

@@ -40,6 +40,27 @@ public sealed class EmailTemplateRenderingServiceTests
         Assert.Equal("Required template variables are missing: VerificationUrl.", exception.Message);
     }
 
+    [Fact]
+    public void Render_HtmlEncodesVariablesWithoutChangingSubjectOrTextBody()
+    {
+        EmailTemplate template = CreateTemplate();
+        SendEmailMessageData data = CreateData(new Dictionary<string, string>
+        {
+            ["AppName"] = "Invemory <script>alert('xss')</script>",
+            ["VerificationUrl"] = "https://example.invalid/verify?next=\" onclick=\"alert('xss')"
+        });
+
+        RenderedEmail result = _service.Render(template, data);
+
+        Assert.Equal("Verify Invemory <script>alert('xss')</script>", result.Subject);
+        Assert.Equal(
+            "<a href=\"https://example.invalid/verify?next=&quot; onclick=&quot;alert(&#39;xss&#39;)\">Verify</a>",
+            result.HtmlBody);
+        Assert.Equal(
+            "Verify at https://example.invalid/verify?next=\" onclick=\"alert('xss')",
+            result.TextBody);
+    }
+
     private static EmailTemplate CreateTemplate() => new()
     {
         ServiceName = "StoreMate-Prod",

HrynCo.NotificationService.Worker.Services/EmailProcessing/EmailTemplateRenderingService.cs

@@ -1,5 +1,6 @@
 namespace HrynCo.NotificationService.Worker.Services.EmailProcessing;
 
+using System.Net;
 using System.Text;
 using HrynCo.NotificationService.Contracts.Messages;
 using HrynCo.NotificationService.DAL.Abstract.Templates;
@@ -22,15 +23,28 @@ internal sealed class EmailTemplateRenderingService : IEmailTemplateRenderingSer
 
         return new RenderedEmail(
             Interpolate(template.Subject, data.Variables),
-            Interpolate(template.HtmlBody, data.Variables),
+            InterpolateHtml(template.HtmlBody, data.Variables),
             Interpolate(template.TextBody, data.Variables));
     }
 
+    private static string InterpolateHtml(string text, IReadOnlyDictionary<string, string> variables)
+    {
+        return Interpolate(text, variables, WebUtility.HtmlEncode);
+    }
+
     private static string Interpolate(string text, IReadOnlyDictionary<string, string> variables)
+    {
+        return Interpolate(text, variables, static value => value);
+    }
+
+    private static string Interpolate(
+        string text,
+        IReadOnlyDictionary<string, string> variables,
+        Func<string, string> encodeValue)
     {
         var sb = new StringBuilder(text);
         foreach (var (key, value) in variables)
-            sb.Replace($"{{{{{key}}}}}", value);
+            sb.Replace($"{{{{{key}}}}}", encodeValue(value));
         return sb.ToString();
     }
 }

README.md

@@ -112,3 +112,7 @@ flowchart TD
     M -->|retries exhausted| N[Publish terminal failure result]
     N --> O[Nack original delivery without requeue]
 ```
+
+Template variables are treated as plain text. The worker HTML-encodes every variable while
+rendering `HtmlBody`; subject and plain-text body interpolation preserve the original value.
+Templates must express markup in `body.html` instead of supplying HTML through variables.