Fix HtmlPreviewVisitor regressions and add sanitization tests (#813)

2026-02-17 22:12:27 +01:00
parent f49d276f5a
commit c8e1678e55
2 changed files with 279 additions and 78 deletions
@@ -13,8 +13,18 @@ namespace Wino.Core.Domain.Models.MailItem;
 /// </summary>
 public class HtmlPreviewVisitor : MimeVisitor
 {
-    List<MultipartRelated> stack = new List<MultipartRelated>();
-    List<MimeEntity> attachments = new List<MimeEntity>();
+    private static readonly HashSet<string> BlockedTags = new(StringComparer.OrdinalIgnoreCase)
+    {
+        "script", "iframe", "frame", "frameset", "object", "embed", "applet", "base", "meta", "form"
+    };
+
+    private static readonly HashSet<string> AllowedDataImageMimeTypes = new(StringComparer.OrdinalIgnoreCase)
+    {
+        "image/png", "image/jpeg", "image/jpg", "image/gif", "image/webp", "image/bmp", "image/x-icon", "image/avif", "image/svg+xml"
+    };
+
+    private readonly List<MultipartRelated> stack = [];
+    private readonly List<MimeEntity> attachments = [];

    readonly string tempDir;

@@ -89,9 +99,14 @@ public class HtmlPreviewVisitor : MimeVisitor
    // look up the image based on the img src url within our multipart/related stack
    bool TryGetImage(string url, out MimePart image)
    {
+        image = null;
+
+        if (string.IsNullOrWhiteSpace(url))
+            return false;
+
        UriKind kind;
        int index;
-        Uri uri;
+        Uri uri = null;

        if (Uri.IsWellFormedUriString(url, UriKind.Absolute))
            kind = UriKind.Absolute;
@@ -106,24 +121,50 @@ public class HtmlPreviewVisitor : MimeVisitor
        }
        catch
        {
-            image = null;
-            return false;
+            // noop: we still attempt CID/content-id lookup below.
        }

        for (int i = stack.Count - 1; i >= 0; i--)
        {
-            if ((index = stack[i].IndexOf(uri)) == -1)
-                continue;
-
+            if (uri != null && (index = stack[i].IndexOf(uri)) != -1)
+            {
                image = stack[i][index] as MimePart;
-            return image != null;
+
+                if (image != null)
+                    return true;
            }

-        image = null;
+            var normalizedContentId = NormalizeContentId(url);
+
+            if (string.IsNullOrEmpty(normalizedContentId))
+                continue;
+
+            foreach (var relatedPart in stack[i])
+            {
+                if (relatedPart is not MimePart candidate || string.IsNullOrEmpty(candidate.ContentId))
+                    continue;
+
+                if (string.Equals(candidate.ContentId.Trim('<', '>'), normalizedContentId, StringComparison.OrdinalIgnoreCase))
+                {
+                    image = candidate;
+                    return true;
+                }
+            }
+        }

        return false;
    }

+    private static string NormalizeContentId(string url)
+    {
+        var trimmed = url.Trim().Trim('\'', '"', '<', '>');
+
+        if (trimmed.StartsWith("cid:", StringComparison.OrdinalIgnoreCase))
+            trimmed = trimmed[4..];
+
+        return trimmed.Trim('<', '>');
+    }
+
    // Save the image to our temp directory and return a "file://" url suitable for
    // the browser control to load.
    // Note: if you'd rather embed the image data into the HTML, you can construct a
@@ -139,83 +180,173 @@ public class HtmlPreviewVisitor : MimeVisitor

            return string.Format("data:{0};base64,{1}", image.ContentType.MimeType, base64);
        }
-
-        //string fileName = url
-        //    .Replace(':', '_')
-        //    .Replace('\\', '_')
-        //    .Replace('/', '_');
-
-        //string path = Path.Combine(tempDir, fileName);
-
-        //if (!File.Exists(path))
-        //{
-        //    using (var output = File.Create(path))
-        //        image.Content.DecodeTo(output);
-        //}
-
-        //return "file://" + path.Replace('\\', '/');
    }

-    // Replaces <img src=...> urls that refer to images embedded within the message with
-    // "file://" urls that the browser control will actually be able to load.
+    // Replaces image references that refer to images embedded within the message with
+    // "data:" urls the browser control can load. Also sanitizes dangerous tags/attributes.
    void HtmlTagCallback(HtmlTagContext ctx, HtmlWriter htmlWriter)
    {
-        if (ctx.TagId == HtmlTagId.Image && !ctx.IsEndTag && stack.Count > 0)
-        {
-            ctx.WriteTag(htmlWriter, false);
+        var tagName = ctx.TagName;

-            // replace the src attribute with a file:// URL
-            foreach (var attribute in ctx.Attributes)
-            {
-                if (attribute.Id == HtmlAttributeId.Src)
-                {
-                    MimePart image;
-                    string url;
-
-                    if (!TryGetImage(attribute.Value, out image))
-                    {
-                        htmlWriter.WriteAttribute(attribute);
-                        continue;
-                    }
-
-                    url = SaveImage(image);
-
-                    htmlWriter.WriteAttributeName(attribute.Name);
-                    htmlWriter.WriteAttributeValue(url);
-                }
-                else
-                {
-                    htmlWriter.WriteAttribute(attribute);
-                }
-            }
-        }
-        else if (ctx.TagId == HtmlTagId.Body && !ctx.IsEndTag)
-        {
-            ctx.WriteTag(htmlWriter, false);
-
-            // add and/or replace oncontextmenu="return false;"
-            foreach (var attribute in ctx.Attributes)
-            {
-                if (attribute.Name.ToLowerInvariant() == "oncontextmenu")
-                    continue;
-
-                htmlWriter.WriteAttribute(attribute);
-            }
-
-            htmlWriter.WriteAttribute("oncontextmenu", "return false;");
-        }
-        else
-        {
-            if (ctx.TagId == HtmlTagId.Unknown)
+        if (BlockedTags.Contains(tagName))
        {
            ctx.DeleteTag = true;
            ctx.DeleteEndTag = true;
+            return;
        }
-            else
+
+        if (ctx.IsEndTag)
        {
            ctx.WriteTag(htmlWriter, true);
+            return;
        }
+
+        ctx.WriteTag(htmlWriter, false);
+
+        foreach (var attribute in ctx.Attributes)
+        {
+            var attributeName = attribute.Name;
+
+            if (ShouldDropAttribute(tagName, attributeName))
+                continue;
+
+            if (TryResolveImageAttribute(tagName, attributeName, attribute.Value, out var resolvedValue))
+            {
+                htmlWriter.WriteAttributeName(attributeName);
+                htmlWriter.WriteAttributeValue(resolvedValue);
+                continue;
            }
+
+            if (IsUrlAttribute(attributeName))
+            {
+                if (!TrySanitizeUrlValue(attribute.Value, out var sanitizedUrl))
+                    continue;
+
+                htmlWriter.WriteAttributeName(attributeName);
+                htmlWriter.WriteAttributeValue(sanitizedUrl);
+                continue;
+            }
+
+            htmlWriter.WriteAttribute(attribute);
+        }
+
+        if (ctx.TagId == HtmlTagId.Body)
+            htmlWriter.WriteAttribute("oncontextmenu", "return false;");
+    }
+
+    private bool TryResolveImageAttribute(string tagName, string attributeName, string value, out string resolvedValue)
+    {
+        resolvedValue = null;
+
+        if (string.IsNullOrWhiteSpace(value))
+            return false;
+
+        var lowerAttributeName = attributeName.ToLowerInvariant();
+        var isImageTag = string.Equals(tagName, "img", StringComparison.OrdinalIgnoreCase);
+
+        if (isImageTag && lowerAttributeName == "srcset")
+        {
+            resolvedValue = ResolveSrcSet(value);
+            return resolvedValue != value;
+        }
+
+        if (lowerAttributeName != "src" && lowerAttributeName != "background" && lowerAttributeName != "poster")
+            return false;
+
+        if (TryGetImage(value, out var image))
+        {
+            resolvedValue = SaveImage(image);
+            return true;
+        }
+
+        return false;
+    }
+
+    private string ResolveSrcSet(string srcSetValue)
+    {
+        var candidates = srcSetValue.Split(',', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries);
+        var updatedCandidates = new List<string>(candidates.Length);
+
+        foreach (var candidate in candidates)
+        {
+            var parts = candidate.Split(' ', 2, StringSplitOptions.TrimEntries | StringSplitOptions.RemoveEmptyEntries);
+
+            if (parts.Length == 0)
+                continue;
+
+            var imageSource = parts[0];
+
+            if (TryGetImage(imageSource, out var image))
+                imageSource = SaveImage(image);
+
+            updatedCandidates.Add(parts.Length == 2 ? $"{imageSource} {parts[1]}" : imageSource);
+        }
+
+        return string.Join(", ", updatedCandidates);
+    }
+
+    private static bool ShouldDropAttribute(string tagName, string attributeName)
+    {
+        if (attributeName.StartsWith("on", StringComparison.OrdinalIgnoreCase))
+            return true;
+
+        if (string.Equals(tagName, "body", StringComparison.OrdinalIgnoreCase)
+            && string.Equals(attributeName, "oncontextmenu", StringComparison.OrdinalIgnoreCase))
+            return true;
+
+        if (string.Equals(attributeName, "srcdoc", StringComparison.OrdinalIgnoreCase))
+            return true;
+
+        return false;
+    }
+
+    private static bool IsUrlAttribute(string attributeName)
+        => string.Equals(attributeName, "href", StringComparison.OrdinalIgnoreCase)
+           || string.Equals(attributeName, "src", StringComparison.OrdinalIgnoreCase)
+           || string.Equals(attributeName, "action", StringComparison.OrdinalIgnoreCase)
+           || string.Equals(attributeName, "xlink:href", StringComparison.OrdinalIgnoreCase)
+           || string.Equals(attributeName, "background", StringComparison.OrdinalIgnoreCase)
+           || string.Equals(attributeName, "poster", StringComparison.OrdinalIgnoreCase);
+
+    private static bool TrySanitizeUrlValue(string rawValue, out string sanitizedValue)
+    {
+        sanitizedValue = null;
+
+        if (string.IsNullOrWhiteSpace(rawValue))
+            return false;
+
+        var value = rawValue.Trim().Trim('"', '\'');
+
+        if (value.StartsWith("javascript:", StringComparison.OrdinalIgnoreCase)
+            || value.StartsWith("vbscript:", StringComparison.OrdinalIgnoreCase))
+            return false;
+
+        if (value.StartsWith("data:", StringComparison.OrdinalIgnoreCase) && !IsAllowedImageDataUrl(value))
+            return false;
+
+        sanitizedValue = value;
+        return true;
+    }
+
+    private static bool IsAllowedImageDataUrl(string value)
+    {
+        const string dataPrefix = "data:";
+
+        if (!value.StartsWith(dataPrefix, StringComparison.OrdinalIgnoreCase))
+            return false;
+
+        var payloadStart = value.IndexOf(',', StringComparison.Ordinal);
+
+        if (payloadStart <= dataPrefix.Length)
+            return false;
+
+        var metadata = value[dataPrefix.Length..payloadStart];
+        var metadataParts = metadata.Split(';', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries);
+
+        if (metadataParts.Length == 0)
+            return false;
+
+        return AllowedDataImageMimeTypes.Contains(metadataParts[0]);
    }

    protected override void VisitTextPart(TextPart entity)
@@ -300,12 +431,6 @@ public class HtmlPreviewVisitor : MimeVisitor
            {
                bool valid = signature.Verify();
                Signatures.Add(signature, valid);
-
-                // If valid is true, then it signifies that the signed content has not
-                // been modified since this particular signer signed the content.
-                //
-                // However, if it is false, then it indicates that the signed content
-                // has been modified.
            }
            catch (DigitalSignatureVerifyException)
            {
@@ -0,0 +1,76 @@
+using Xunit;
+using FluentAssertions;
+using MimeKit;
+using Wino.Core.Domain.Models.MailItem;
+
+namespace Wino.Core.Tests.Models;
+
+public class HtmlPreviewVisitorTests
+{
+    [Fact]
+    public void HtmlPreviewVisitor_Should_Remove_Blocked_Tags_And_Event_Attributes()
+    {
+        // Arrange
+        var html = """
+            <html>
+                <body onload="alert('x')">
+                    <h1 onclick="evil()">hello</h1>
+                    <script>alert('xss')</script>
+                    <iframe src="https://malicious.example"></iframe>
+                    <object data="https://malicious.example/file.swf"></object>
+                    <img src="https://cdn.example/image.png" onerror="steal()" />
+                </body>
+            </html>
+            """;
+
+        var message = new MimeMessage();
+        message.Body = new TextPart("html") { Text = html };
+
+        var visitor = new HtmlPreviewVisitor(Path.GetTempPath());
+
+        // Act
+        message.Accept(visitor);
+        var output = visitor.HtmlBody;
+
+        // Assert
+        output.Should().NotContain("<script", "script tags must be blocked in rendered html");
+        output.Should().NotContain("<iframe", "iframe tags must be blocked in rendered html");
+        output.Should().NotContain("<object", "object tags must be blocked in rendered html");
+        output.Should().NotContain("onload=", "event handler attributes must be stripped");
+        output.Should().NotContain("onclick=", "event handler attributes must be stripped");
+        output.Should().NotContain("onerror=", "event handler attributes must be stripped");
+        output.Should().Contain("oncontextmenu=\"return false;\"", "body context-menu suppression should be kept");
+    }
+
+    [Fact]
+    public void HtmlPreviewVisitor_Should_Sanitize_Dangerous_Url_Attributes()
+    {
+        // Arrange
+        var html = """
+            <html>
+                <body>
+                    <a id="safe-link" href="https://contoso.com/path">safe</a>
+                    <a id="js-link" href="javascript:alert('xss')">bad</a>
+                    <img id="svg-script" src="data:text/html;base64,PHNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==" />
+                    <img id="allowed" src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAAB" />
+                </body>
+            </html>
+            """;
+
+        var message = new MimeMessage();
+        message.Body = new TextPart("html") { Text = html };
+
+        var visitor = new HtmlPreviewVisitor(Path.GetTempPath());
+
+        // Act
+        message.Accept(visitor);
+        var output = visitor.HtmlBody;
+
+        // Assert
+        output.Should().Contain("id=\"safe-link\" href=\"https://contoso.com/path\"", "http/https links should be preserved");
+        output.Should().Contain("id=\"js-link\"", "the element should remain");
+        output.Should().NotContain("href=\"javascript:", "javascript URLs must be removed");
+        output.Should().Contain("id=\"allowed\" src=\"data:image/png;base64", "safe image data URLs should be preserved");
+        output.Should().NotContain("id=\"svg-script\" src=\"data:text/html", "non-image data URLs should be removed");
+    }
+}