ASP.NET Web Forms でリッチテキストエディターから画像をアップロードする

<link rel="stylesheet" href="<%= ResolveUrl("~/richtexteditor/rte_theme_default.css") %>" />
<script src="<%= ResolveUrl("~/richtexteditor/rte.js") %>"></script>
<script src="<%= ResolveUrl("~/richtexteditor/plugins/all_plugins.js") %>"></script>
<script>window.RTE_FILES_URL = "<%= ResolveUrl("~/rte") %>";</script>
<script src="<%= ResolveUrl("~/rte-files.js") %>"></script>

<handlers>
  <!-- The two endpoints. The classes are in App_Code; ASP.NET compiles them on first use. -->
  <add name="RteGallery" path="rte/gallery" verb="*" type="RteGalleryHandler" preCondition="integratedMode" />
  <add name="RteUpload" path="rte/upload" verb="*" type="RteUploadHandler" preCondition="integratedMode" />
</handlers>

public void ProcessRequest(HttpContext context)
{
    try
    {
        if (context.Request.HttpMethod != "POST")
        {
            context.Response.AppendHeader("Allow", "POST");
            throw new RteException(405, "Use POST.");
        }

        HttpPostedFile file;
        string folder;
        try
        {
            // Reading the body is where ASP.NET enforces <httpRuntime maxRequestLength>.
            // Unvalidated: request validation also covers Request.Files, and a file
            // named "<b>.png" would otherwise fail with an HTML 500 page.
            file = context.Request.Unvalidated.Files["file"];
            folder = context.Request.Unvalidated.Form["folder"];
        }
        catch (HttpException ex)
        {
            if (ex.WebEventCode != System.Web.Management.WebEventCodes.RuntimeErrorPostTooLarge) throw;
            throw new RteException(413, "The upload is larger than the server allows.");
        }

        string url = RteFiles.SaveUpload(context, file, folder);
        RteFiles.WriteJson(context, 200, new { url = url });
    }
    catch (RteException ex)
    {
        RteFiles.WriteError(context, ex.Status, ex.Message);
    }
}

StorageRoot root = GetRoot(context);   // 401 here first in per-user mode
if (file == null || file.ContentLength == 0)
    throw new RteException(400, "No file was uploaded.");

// FileName is whatever the browser sent (old IE sends a full path). Don't feed it
// to Path.GetFileName: on .NET Framework that throws on characters like " < > |.
string clientName = file.FileName ?? "";
clientName = clientName.Substring(Math.Max(clientName.LastIndexOf('/'), clientName.LastIndexOf('\\')) + 1);
int dot = clientName.LastIndexOf('.');
string ext = dot >= 0 ? clientName.Substring(dot).ToLowerInvariant() : "";
if (Array.IndexOf(ImageExtensions, ext) < 0)
    throw new RteException(400, "Only .jpg, .jpeg, .png, .gif and .webp images can be uploaded.");
if (file.ContentLength > MaxUploadBytes)
    throw new RteException(413, "The image is larger than the 4 MB (4,000,000 bytes) limit.");

string dir = ResolveFolder(root, folder ?? "");

byte[] data = new byte[file.ContentLength];
Stream input = file.InputStream;
int read = 0;
while (read < data.Length)
{
    int n = input.Read(data, read, data.Length - read);
    if (n <= 0) break;
    read += n;
}
// file.ContentType is chosen by the browser (or the attacker), so look at the bytes.
if (read != data.Length || !SignatureMatches(ext, data))
    throw new RteException(400, "The file is not a valid " + ext.Substring(1).ToUpperInvariant() + " image.");

string baseName = SanitizeBaseName(dot >= 0 ? clientName.Substring(0, dot) : clientName);
for (int attempt = 0; ; attempt++)
{
    string stored = baseName + "-" + RandomHex(4) + ext;
    string path = Path.Combine(dir, stored);
    try
    {
        // CreateNew: never overwrite an existing file, even on a random-name collision.
        using (FileStream output = new FileStream(path, FileMode.CreateNew, FileAccess.Write))
            output.Write(data, 0, data.Length);
        return UrlFor(root, folder ?? "", stored);
    }
    catch (IOException)
    {
        if (attempt >= 4 || !File.Exists(path)) throw;
    }
}

curl -F "file=@harbor.jpg" http://localhost:5401/rte/upload
{"url":"/uploads/harbor-2e576eda.jpg"}

echo "<script>alert(1)</script>" > fake.png
curl -F "file=@fake.png" http://localhost:5401/rte/upload
{"error":"The file is not a valid PNG image."}