Burak Kaan Köse
443 lines
14 KiB
C#
443 lines
14 KiB
C#
using System;
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using System.Collections.Generic;
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using System.IO;
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using MimeKit;
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using MimeKit.Cryptography;
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using MimeKit.Text;
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using MimeKit.Tnef;
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namespace Wino.Core.Domain.Models.MailItem;
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/// <summary>
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/// Visits a MimeMessage and generates HTML suitable to be rendered by a browser control.
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/// </summary>
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public class HtmlPreviewVisitor : MimeVisitor
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{
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private static readonly HashSet<string> BlockedTags = new(StringComparer.OrdinalIgnoreCase)
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{
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"script", "iframe", "frame", "frameset", "object", "embed", "applet", "base", "meta", "form", "link"
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};
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private static readonly HashSet<string> AllowedDataImageMimeTypes = new(StringComparer.OrdinalIgnoreCase)
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{
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"image/png", "image/jpeg", "image/jpg", "image/gif", "image/webp", "image/bmp", "image/x-icon", "image/avif", "image/svg+xml"
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};
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private readonly List<MultipartRelated> stack = [];
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private readonly List<MimeEntity> attachments = [];
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readonly string tempDir;
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public string Body { get; set; }
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public Dictionary<IDigitalSignature, bool> Signatures = [];
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/// <summary>
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/// Creates a new HtmlPreviewVisitor.
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/// </summary>
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/// <param name="tempDirectory">A temporary directory used for storing image files.</param>
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public HtmlPreviewVisitor(string tempDirectory)
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{
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tempDir = tempDirectory;
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}
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/// <summary>
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/// The list of attachments that were in the MimeMessage.
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/// </summary>
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public IList<MimeEntity> Attachments
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{
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get { return attachments; }
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}
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/// <summary>
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/// The HTML string that can be set on the BrowserControl.
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/// </summary>
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public string HtmlBody
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{
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get { return Body ?? string.Empty; }
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}
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protected override void VisitMultipartAlternative(MultipartAlternative alternative)
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{
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// Prefer rich body alternatives first, and only fall back to calendar text if nothing else exists.
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for (int i = alternative.Count - 1; i >= 0 && Body == null; i--)
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{
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if (IsCalendarText(alternative[i]))
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continue;
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alternative[i].Accept(this);
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}
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for (int i = alternative.Count - 1; i >= 0 && Body == null; i--)
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{
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if (!IsCalendarText(alternative[i]))
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continue;
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alternative[i].Accept(this);
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}
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}
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protected override void VisitMultipartRelated(MultipartRelated related)
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{
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var root = related.Root;
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// push this multipart/related onto our stack
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stack.Add(related);
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// visit the root document
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root.Accept(this);
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// pop this multipart/related off our stack
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stack.RemoveAt(stack.Count - 1);
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}
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protected override void VisitMultipartSigned(MultipartSigned signed)
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{
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VerifySignatures(signed.Verify());
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VisitMultipart(signed);
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}
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// look up the image based on the img src url within our multipart/related stack
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bool TryGetImage(string url, out MimePart image)
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{
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image = null;
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if (string.IsNullOrWhiteSpace(url))
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return false;
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UriKind kind;
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int index;
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Uri uri = null;
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if (Uri.IsWellFormedUriString(url, UriKind.Absolute))
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kind = UriKind.Absolute;
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else if (Uri.IsWellFormedUriString(url, UriKind.Relative))
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kind = UriKind.Relative;
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else
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kind = UriKind.RelativeOrAbsolute;
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try
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{
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uri = new Uri(url, kind);
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}
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catch
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{
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// noop: we still attempt CID/content-id lookup below.
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}
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for (int i = stack.Count - 1; i >= 0; i--)
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{
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if (uri != null && (index = stack[i].IndexOf(uri)) != -1)
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{
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image = stack[i][index] as MimePart;
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if (image != null)
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return true;
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}
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var normalizedContentId = NormalizeContentId(url);
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if (string.IsNullOrEmpty(normalizedContentId))
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continue;
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foreach (var relatedPart in stack[i])
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{
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if (relatedPart is not MimePart candidate || string.IsNullOrEmpty(candidate.ContentId))
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continue;
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if (string.Equals(candidate.ContentId.Trim('<', '>'), normalizedContentId, StringComparison.OrdinalIgnoreCase))
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{
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image = candidate;
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return true;
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}
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}
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}
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return false;
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}
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private static string NormalizeContentId(string url)
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{
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var trimmed = url.Trim().Trim('\'', '"', '<', '>');
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if (trimmed.StartsWith("cid:", StringComparison.OrdinalIgnoreCase))
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trimmed = trimmed[4..];
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return trimmed.Trim('<', '>');
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}
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// Save the image to our temp directory and return a "file://" url suitable for
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// the browser control to load.
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// Note: if you'd rather embed the image data into the HTML, you can construct a
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// "data:" url instead.
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string SaveImage(MimePart image)
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{
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using (var memory = new MemoryStream())
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{
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image.Content.DecodeTo(memory);
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var buffer = memory.GetBuffer();
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var length = (int)memory.Length;
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var base64 = Convert.ToBase64String(buffer, 0, length);
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return string.Format("data:{0};base64,{1}", image.ContentType.MimeType, base64);
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}
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}
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// Replaces image references that refer to images embedded within the message with
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// "data:" urls the browser control can load. Also sanitizes dangerous tags/attributes.
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void HtmlTagCallback(HtmlTagContext ctx, HtmlWriter htmlWriter)
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{
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var tagName = ctx.TagName;
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if (BlockedTags.Contains(tagName))
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{
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ctx.DeleteTag = true;
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ctx.DeleteEndTag = true;
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return;
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}
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if (ctx.IsEndTag)
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{
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ctx.WriteTag(htmlWriter, true);
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return;
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}
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ctx.WriteTag(htmlWriter, false);
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foreach (var attribute in ctx.Attributes)
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{
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var attributeName = attribute.Name;
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if (ShouldDropAttribute(tagName, attributeName))
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continue;
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if (TryResolveImageAttribute(tagName, attributeName, attribute.Value, out var resolvedValue))
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{
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htmlWriter.WriteAttributeName(attributeName);
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htmlWriter.WriteAttributeValue(resolvedValue);
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continue;
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}
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if (IsUrlAttribute(attributeName))
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{
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if (!TrySanitizeUrlValue(attribute.Value, out var sanitizedUrl))
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continue;
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htmlWriter.WriteAttributeName(attributeName);
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htmlWriter.WriteAttributeValue(sanitizedUrl);
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continue;
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}
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htmlWriter.WriteAttribute(attribute);
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}
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if (ctx.TagId == HtmlTagId.Body)
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htmlWriter.WriteAttribute("oncontextmenu", "return false;");
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}
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private bool TryResolveImageAttribute(string tagName, string attributeName, string value, out string resolvedValue)
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{
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resolvedValue = null;
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if (string.IsNullOrWhiteSpace(value))
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return false;
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var lowerAttributeName = attributeName.ToLowerInvariant();
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var isImageTag = string.Equals(tagName, "img", StringComparison.OrdinalIgnoreCase);
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if (isImageTag && lowerAttributeName == "srcset")
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{
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resolvedValue = ResolveSrcSet(value);
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return resolvedValue != value;
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}
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if (lowerAttributeName != "src" && lowerAttributeName != "background" && lowerAttributeName != "poster")
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return false;
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if (TryGetImage(value, out var image))
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{
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resolvedValue = SaveImage(image);
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return true;
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}
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return false;
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}
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private string ResolveSrcSet(string srcSetValue)
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{
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var candidates = srcSetValue.Split(',', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries);
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var updatedCandidates = new List<string>(candidates.Length);
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foreach (var candidate in candidates)
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{
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var parts = candidate.Split(' ', 2, StringSplitOptions.TrimEntries | StringSplitOptions.RemoveEmptyEntries);
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if (parts.Length == 0)
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continue;
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var imageSource = parts[0];
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if (TryGetImage(imageSource, out var image))
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imageSource = SaveImage(image);
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updatedCandidates.Add(parts.Length == 2 ? $"{imageSource} {parts[1]}" : imageSource);
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}
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return string.Join(", ", updatedCandidates);
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}
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private static bool ShouldDropAttribute(string tagName, string attributeName)
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{
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if (attributeName.StartsWith("on", StringComparison.OrdinalIgnoreCase))
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return true;
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if (string.Equals(tagName, "body", StringComparison.OrdinalIgnoreCase)
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&& string.Equals(attributeName, "oncontextmenu", StringComparison.OrdinalIgnoreCase))
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return true;
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if (string.Equals(attributeName, "srcdoc", StringComparison.OrdinalIgnoreCase))
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return true;
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return false;
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}
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private static bool IsUrlAttribute(string attributeName)
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=> string.Equals(attributeName, "href", StringComparison.OrdinalIgnoreCase)
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|| string.Equals(attributeName, "src", StringComparison.OrdinalIgnoreCase)
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|| string.Equals(attributeName, "action", StringComparison.OrdinalIgnoreCase)
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|| string.Equals(attributeName, "xlink:href", StringComparison.OrdinalIgnoreCase)
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|| string.Equals(attributeName, "background", StringComparison.OrdinalIgnoreCase)
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|| string.Equals(attributeName, "poster", StringComparison.OrdinalIgnoreCase);
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private static bool TrySanitizeUrlValue(string rawValue, out string sanitizedValue)
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{
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sanitizedValue = null;
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if (string.IsNullOrWhiteSpace(rawValue))
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return false;
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var value = rawValue.Trim().Trim('"', '\'');
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if (value.StartsWith("javascript:", StringComparison.OrdinalIgnoreCase)
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|| value.StartsWith("vbscript:", StringComparison.OrdinalIgnoreCase))
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return false;
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if (value.StartsWith("data:", StringComparison.OrdinalIgnoreCase) && !IsAllowedImageDataUrl(value))
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return false;
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sanitizedValue = value;
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return true;
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}
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private static bool IsAllowedImageDataUrl(string value)
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{
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const string dataPrefix = "data:";
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if (!value.StartsWith(dataPrefix, StringComparison.OrdinalIgnoreCase))
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return false;
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var payloadStart = value.IndexOf(',', StringComparison.Ordinal);
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if (payloadStart <= dataPrefix.Length)
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return false;
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var metadata = value[dataPrefix.Length..payloadStart];
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var metadataParts = metadata.Split(';', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries);
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if (metadataParts.Length == 0)
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return false;
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return AllowedDataImageMimeTypes.Contains(metadataParts[0]);
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}
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protected override void VisitTextPart(TextPart entity)
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{
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TextConverter converter;
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if (Body != null)
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{
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// since we've already found the body, treat this as an attachment
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attachments.Add(entity);
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return;
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}
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if (entity.IsHtml)
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{
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converter = new HtmlToHtml
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{
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HtmlTagCallback = HtmlTagCallback
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};
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}
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else if (entity.IsFlowed)
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{
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var flowed = new FlowedToHtml();
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string delsp;
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if (entity.ContentType.Parameters.TryGetValue("delsp", out delsp))
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flowed.DeleteSpace = delsp.ToLowerInvariant() == "yes";
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converter = flowed;
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}
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else
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{
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converter = new TextToHtml();
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}
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Body = converter.Convert(entity.Text);
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}
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private static bool IsCalendarText(MimeEntity entity)
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=> entity is TextPart textPart &&
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textPart.ContentType?.MimeType?.Equals("text/calendar", StringComparison.OrdinalIgnoreCase) == true;
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protected override void VisitTnefPart(TnefPart entity)
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{
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// extract any attachments in the MS-TNEF part
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attachments.AddRange(entity.ExtractAttachments());
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}
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protected override void VisitMessagePart(MessagePart entity)
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{
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// treat message/rfc822 parts as attachments
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attachments.Add(entity);
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}
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protected override void VisitMimePart(MimePart entity)
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{
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if (entity is ApplicationPkcs7Mime { SecureMimeType: SecureMimeType.EnvelopedData } encrypted)
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{
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encrypted.Decrypt().Accept(this);
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}
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else if (entity is ApplicationPkcs7Mime { SecureMimeType: SecureMimeType.SignedData } signed)
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{
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MimeEntity extracted;
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VerifySignatures(signed.Verify(out extracted));
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extracted.Accept(this);
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}
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else
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{
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// realistically, if we've gotten this far, then we can treat this as an attachment
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// even if the IsAttachment property is false.
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attachments.Add(entity);
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}
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}
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private void VerifySignatures(DigitalSignatureCollection signatures)
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{
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foreach (var signature in signatures)
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{
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try
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{
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bool valid = signature.Verify();
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Signatures.Add(signature, valid);
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}
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catch (DigitalSignatureVerifyException)
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{
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// There was an error verifying the signature.
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Signatures.Add(signature, false);
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}
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}
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}
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}
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