make markdown slightly safer without the nohtml volflag

by running dompurify after marked.parse if plugins are not enabled;
adds no protection against the more practical approach of just
putting a malicious <script> in an html file and uploading that,
but one footgun less is one less footgun

README.md

@@ -1599,6 +1599,7 @@ some notes on hardening
 * set `--rproxy 0` if your copyparty is directly facing the internet (not through a reverse-proxy)
   * cors doesn't work right otherwise
 * if you allow anonymous uploads or otherwise don't trust the contents of a volume, you can prevent XSS with volflag `nohtml`
+  * this returns html documents as plaintext, and also disables markdown rendering
 
 safety profiles:
 

copyparty/web/browser.js

@@ -7123,7 +7123,12 @@ function show_md(md, name, div, url, depth) {
 
 	try {
 		clmod(div, 'mdo', 1);
-		if (sandbox(div, sb_md, 'mdo', marked.parse(md, marked_opts)))
+
+		var md_html = marked.parse(md, marked_opts);
+		if (!have_emp)
+			md_html = DOMPurify.sanitize(md_html);
+
+		if (sandbox(div, sb_md, 'mdo', md_html))
 			return;
 
 		ext = md_plug.post;

copyparty/web/md.js

@@ -212,6 +212,8 @@ function convert_markdown(md_text, dest_dom) {
 
     try {
         var md_html = marked.parse(md_text, marked_opts);
+        if (!have_emp)
+            md_html = DOMPurify.sanitize(md_html);
     }
     catch (ex) {
         if (ext)

scripts/deps-docker/Dockerfile

@@ -3,6 +3,7 @@ WORKDIR /z
 ENV     ver_asmcrypto=c72492f4a66e17a0e5dd8ad7874de354f3ccdaa5 \
         ver_hashwasm=4.9.0 \
         ver_marked=4.3.0 \
+        ver_dompf=3.0.5 \
         ver_mde=2.18.0 \
         ver_codemirror=5.65.12 \
         ver_fontawesome=5.13.0 \
@@ -13,6 +14,7 @@ ENV     ver_asmcrypto=c72492f4a66e17a0e5dd8ad7874de354f3ccdaa5 \
 # https://github.com/markedjs/marked/releases
 # https://github.com/Ionaru/easy-markdown-editor/tags
 # https://github.com/codemirror/codemirror5/releases
+# https://github.com/cure53/DOMPurify/releases
 # https://github.com/Daninet/hash-wasm/releases
 # https://github.com/openpgpjs/asmcrypto.js
 # https://github.com/google/zopfli/tags
@@ -27,6 +29,7 @@ RUN     mkdir -p /z/dist/no-pk \
         && wget https://github.com/markedjs/marked/archive/v$ver_marked.tar.gz -O marked.tgz \
         && wget https://github.com/Ionaru/easy-markdown-editor/archive/$ver_mde.tar.gz -O mde.tgz \
         && wget https://github.com/codemirror/codemirror5/archive/$ver_codemirror.tar.gz -O codemirror.tgz \
+        && wget https://github.com/cure53/DOMPurify/archive/refs/tags/$ver_dompf.tar.gz -O dompurify.tgz \
         && wget https://github.com/FortAwesome/Font-Awesome/releases/download/$ver_fontawesome/fontawesome-free-$ver_fontawesome-web.zip -O fontawesome.zip \
         && wget https://github.com/google/zopfli/archive/zopfli-$ver_zopfli.tar.gz -O zopfli.tgz \
         && wget https://github.com/Daninet/hash-wasm/releases/download/v$ver_hashwasm/hash-wasm@$ver_hashwasm.zip -O hash-wasm.zip \
@@ -48,6 +51,7 @@ RUN     mkdir -p /z/dist/no-pk \
             && cd easy-markdown-editor* \
             && npm install \
             && npm i gulp-cli -g ) \
+        && tar -xf dompurify.tgz \
         && tar -xf prism.tgz \
         && unzip fontawesome.zip \
         && tar -xf zopfli.tgz
@@ -120,6 +124,10 @@ RUN     cd easy-markdown-editor-$ver_mde \
         && cp -pv dist/easymde.min.js /z/dist/easymde.js
 
 
+# build dompurify
+RUN     (echo; cat DOMPurify-$ver_dompf/dist/purify.min.js) >> /z/dist/marked.js
+
+
 # build fontawesome and scp
 COPY    mini-fa.sh /z
 COPY    mini-fa.css /z