From: Michael Catanzaro <mcatanzaro@redhat.com>
Subject: Properly encode untrusted data when injecting into trusted pages
Origin: https://gitlab.gnome.org/GNOME/epiphany/-/compare/c27a8180e12e6ec92292dcf53b9243815ad9aa2e...abac58c5191b7d653fbefa8d44e5c2bd4d002825?from_project_id=1906
Index: epiphany-browser/embed/ephy-about-handler.c
===================================================================
--- epiphany-browser.orig/embed/ephy-about-handler.c
+++ epiphany-browser/embed/ephy-about-handler.c
@@ -27,6 +27,7 @@
 #include "ephy-file-helpers.h"
 #include "ephy-flatpak-utils.h"
 #include "ephy-history-service.h"
+#include "ephy-output-encoding.h"
 #include "ephy-prefs.h"
 #include "ephy-settings.h"
 #include "ephy-smaps.h"
@@ -263,16 +264,34 @@ handle_applications_finished_cb (EphyAbo
 
     for (p = applications; p; p = p->next) {
       EphyWebApplication *app = (EphyWebApplication *)p->data;
-
+      g_autofree char *html_encoded_id = NULL;
+      g_autofree char *encoded_icon_url = NULL;
+      g_autofree char *encoded_name = NULL;
+      g_autofree char *encoded_url = NULL;
+      g_autofree char *js_encoded_id = NULL;
+      g_autofree char *encoded_install_date = NULL;
+
+      /* Most of these fields are untrusted. The web app suggests its own title,
+       * which gets used in the app ID and icon URL. The main URL could contain
+       * anything. Install date is the only trusted field here in that it's
+       * constructed by Epiphany, but it's a freeform string and we're encoding
+       * everything else here anyway, so might as well encode this too.
+       */
+      html_encoded_id = ephy_encode_for_html_attribute (app->id);
+      encoded_icon_url = ephy_encode_for_html_attribute (app->icon_url);
+      encoded_name = ephy_encode_for_html_entity (app->name);
+      encoded_url = ephy_encode_for_html_entity (app->url);
+      js_encoded_id = ephy_encode_for_javascript (app->id);
+      encoded_install_date = ephy_encode_for_html_entity (app->install_date);
       g_string_append_printf (data_str,
                               "<tbody><tr id =\"%s\">"
                               "<td class=\"icon\"><img width=64 height=64 src=\"file://%s\"></img></td>"
                               "<td class=\"data\"><div class=\"appname\">%s</div><div class=\"appurl\">%s</div></td>"
                               "<td class=\"input\"><input type=\"button\" value=\"%s\" onclick=\"deleteWebApp('%s');\"></td>"
                               "<td class=\"date\">%s <br /> %s</td></tr></tbody>",
-                              app->id, app->icon_url, app->name, app->url, _("Delete"), app->id,
+                              html_encoded_id, encoded_icon_url, encoded_name, encoded_url, _("Delete"), js_encoded_id,
                               /* Note for translators: this refers to the installation date. */
-                              _("Installed on:"), app->install_date);
+                              _("Installed on:"), encoded_install_date);
     }
 
     g_string_append (data_str, "</table></div></body></html>");
@@ -407,7 +426,9 @@ history_service_query_urls_cb (EphyHisto
     EphyHistoryURL *url = (EphyHistoryURL *)l->data;
     const char *snapshot;
     g_autofree char *thumbnail_style = NULL;
-    g_autofree char *markup = NULL;
+    g_autofree char *entity_encoded_title = NULL;
+    g_autofree char *attribute_encoded_title = NULL;
+    g_autofree char *encoded_url = NULL;
 
     snapshot = ephy_snapshot_service_lookup_cached_snapshot_path (snapshot_service, url->url);
     if (snapshot)
@@ -415,15 +436,19 @@ history_service_query_urls_cb (EphyHisto
     else
       ephy_embed_shell_schedule_thumbnail_update (shell, url);
 
-    markup = g_markup_escape_text (url->title, -1);
+    /* Title and URL are controlled by web content and could be malicious. */
+    entity_encoded_title = ephy_encode_for_html_entity (url->title);
+    attribute_encoded_title = ephy_encode_for_html_attribute (url->title);
+    encoded_url = ephy_encode_for_html_attribute (url->url);
     g_string_append_printf (data_str,
                             "<a class=\"overview-item\" title=\"%s\" href=\"%s\">"
                             "  <div class=\"overview-close-button\" title=\"%s\"></div>"
                             "  <span class=\"overview-thumbnail\"%s></span>"
                             "  <span class=\"overview-title\">%s</span>"
                             "</a>",
-                            markup, url->url, _("Remove from overview"),
-                            thumbnail_style ? thumbnail_style : "", url->title);
+                            attribute_encoded_title, encoded_url, _("Remove from overview"),
+                            thumbnail_style ? thumbnail_style : "",
+                            entity_encoded_title);
   }
 
   data_str = g_string_append (data_str,
Index: epiphany-browser/embed/ephy-pdf-handler.c
===================================================================
--- epiphany-browser.orig/embed/ephy-pdf-handler.c
+++ epiphany-browser/embed/ephy-pdf-handler.c
@@ -23,6 +23,7 @@
 
 #include "ephy-embed-container.h"
 #include "ephy-embed-shell.h"
+#include "ephy-output-encoding.h"
 #include "ephy-web-view.h"
 
 #include <gio/gio.h>
@@ -124,8 +125,9 @@ pdf_file_loaded (GObject      *source,
   GBytes *html_file;
   g_autoptr (GError) error = NULL;
   g_autoptr (GString) html = NULL;
-  g_autofree gchar *b64 = NULL;
   g_autofree char *file_data = NULL;
+  g_autofree char *encoded_file_data = NULL;
+  g_autofree char *encoded_filename = NULL;
   gsize len = 0;
 
   if (!g_file_load_contents_finish (G_FILE (source), res, &file_data, &len, NULL, &error)) {
@@ -134,13 +136,13 @@ pdf_file_loaded (GObject      *source,
     return;
   }
 
-  html_file = g_resources_lookup_data ("/org/gnome/epiphany/pdfjs/web/viewer.html", 0, NULL);
-
-  b64 = g_base64_encode ((const guchar *)file_data, len);
   g_file_delete_async (G_FILE (source), G_PRIORITY_DEFAULT, NULL, pdf_file_deleted, NULL);
 
-  html = g_string_new ("");
-  g_string_printf (html, g_bytes_get_data (html_file, NULL), b64, self->file_name ? self->file_name : "");
+  html = g_string_new (NULL);
+  html_file = g_resources_lookup_data ("/org/gnome/epiphany/pdfjs/web/viewer.html", 0, NULL);
+  encoded_file_data = g_base64_encode ((const guchar *)file_data, len);
+  encoded_filename = self->file_name ? ephy_encode_for_html_attribute (self->file_name) : g_strdup ("");
+  g_string_printf (html, g_bytes_get_data (html_file, NULL), encoded_file_data, encoded_filename);
 
   finish_uri_scheme_request (self, g_strdup (html->str), NULL);
 }
Index: epiphany-browser/embed/ephy-reader-handler.c
===================================================================
--- epiphany-browser.orig/embed/ephy-reader-handler.c
+++ epiphany-browser/embed/ephy-reader-handler.c
@@ -24,6 +24,7 @@
 #include "ephy-embed-container.h"
 #include "ephy-embed-shell.h"
 #include "ephy-lib-type-builtins.h"
+#include "ephy-output-encoding.h"
 #include "ephy-settings.h"
 #include "ephy-web-view.h"
 
@@ -156,7 +157,9 @@ readability_js_finish_cb (GObject      *
   g_autoptr (WebKitJavascriptResult) js_result = NULL;
   g_autoptr (GError) error = NULL;
   g_autofree gchar *byline = NULL;
+  g_autofree gchar *encoded_byline = NULL;
   g_autofree gchar *content = NULL;
+  g_autofree gchar *encoded_title = NULL;
   g_autoptr (GString) html = NULL;
   g_autoptr (GBytes) style_css = NULL;
   const gchar *title;
@@ -173,10 +176,14 @@ readability_js_finish_cb (GObject      *
 
   byline = readability_get_property_string (js_result, "byline");
   content = readability_get_property_string (js_result, "content");
+  title = webkit_web_view_get_title (web_view);
+
+  encoded_byline = byline ? ephy_encode_for_html_entity (byline) : g_strdup ("");
+  encoded_title = ephy_encode_for_html_entity (title);
 
-  html = g_string_new ("");
+  html = g_string_new (NULL);
   style_css = g_resources_lookup_data ("/org/gnome/epiphany/readability/reader.css", G_RESOURCE_LOOKUP_FLAGS_NONE, NULL);
-  title = webkit_web_view_get_title (web_view);
+
   font_style = enum_nick (EPHY_TYPE_PREFS_READER_FONT_STYLE,
                           g_settings_get_enum (EPHY_SETTINGS_READER,
                                                EPHY_PREFS_READER_FONT_STYLE));
@@ -186,7 +193,8 @@ readability_js_finish_cb (GObject      *
 
   g_string_append_printf (html, "<style>%s</style>"
                           "<title>%s</title>"
-                          "<meta http-equiv=\"Content-Type\" content=\"text/html;\" charset=\"UTF-8\">" \
+                          "<meta http-equiv='Content-Type' content='text/html;' charset='UTF-8'>" \
+                          "<meta http-equiv='Content-Security-Policy' content=\"script-src 'none'\">" \
                           "<body class='%s %s'>"
                           "<article>"
                           "<h2>"
@@ -197,13 +205,27 @@ readability_js_finish_cb (GObject      *
                           "</i>"
                           "<hr>",
                           (gchar *)g_bytes_get_data (style_css, NULL),
-                          title,
+                          encoded_title,
                           font_style,
                           color_scheme,
-                          title,
-                          byline != NULL ? byline : "");
+                          encoded_title,
+                          encoded_byline);
+
+  /* We cannot encode the page content because it contains HTML tags inserted by
+   * Readability.js. Upstream recommends that we use an XSS sanitizer like
+   * DOMPurify plus Content-Security-Policy, but I'm not keen on adding more
+   * bundled JS dependencies, and we have an advantage over Firefox in that we
+   * don't need scripts to work at this point. So instead the above CSP
+   * completely blocks all scripts, which should hopefully obviate the need for
+   * a DOM purifier.
+   *
+   * Note the encoding for page title and byline is still required, as they're
+   * not supposed to contain markup, and Readability.js unescapes them before
+   * returning them to us.
+   */
   g_string_append (html, content);
   g_string_append (html, "</article>");
+  g_string_append (html, "</body>");
 
   finish_uri_scheme_request (request, g_strdup (html->str), NULL);
 }
Index: epiphany-browser/embed/ephy-view-source-handler.c
===================================================================
--- epiphany-browser.orig/embed/ephy-view-source-handler.c
+++ epiphany-browser/embed/ephy-view-source-handler.c
@@ -23,6 +23,7 @@
 
 #include "ephy-embed-container.h"
 #include "ephy-embed-shell.h"
+#include "ephy-output-encoding.h"
 #include "ephy-web-view.h"
 
 #include <gio/gio.h>
@@ -109,7 +110,9 @@ web_resource_data_cb (WebKitWebResource
                       EphyViewSourceRequest *request)
 {
   g_autofree guchar *data = NULL;
-  g_autofree char *escaped_str = NULL;
+  g_autofree char *data_str = NULL;
+  g_autofree char *encoded_str = NULL;
+  g_autofree char *encoded_uri = NULL;
   g_autoptr (GError) error = NULL;
   g_autofree char *html = NULL;
   gsize length;
@@ -120,8 +123,13 @@ web_resource_data_cb (WebKitWebResource
     return;
   }
 
-  /* Warning: data is not a string, so we pass length here because it's not NUL-terminated. */
-  escaped_str = g_markup_escape_text ((const char *)data, length);
+  /* Convert data to a string */
+  data_str = g_malloc (length + 1);
+  memcpy (data_str, data, length);
+  data_str[length] = '\0';
+
+  encoded_str = ephy_encode_for_html_entity (data_str);
+  encoded_uri = ephy_encode_for_html_entity (webkit_web_resource_get_uri (resource));
 
   html = g_strdup_printf ("<head>"
                           "  <link rel='stylesheet' href='ephy-resource:///org/gnome/epiphany/highlightjs/nnfx.css' media='(prefers-color-scheme: no-preference), (prefers-color-scheme: light)'>"
@@ -136,8 +144,8 @@ web_resource_data_cb (WebKitWebResource
                           "          hljs.initLineNumbersOnLoad();</script>"
                           "  <pre><code class='html'>%s</code></pre>"
                           "</body>",
-                          webkit_web_resource_get_uri (resource),
-                          escaped_str);
+                          encoded_uri,
+                          encoded_str);
 
   finish_uri_scheme_request (request, g_steal_pointer (&html), NULL);
 }
Index: epiphany-browser/embed/ephy-web-view.c
===================================================================
--- epiphany-browser.orig/embed/ephy-web-view.c
+++ epiphany-browser/embed/ephy-web-view.c
@@ -38,6 +38,7 @@
 #include "ephy-gsb-utils.h"
 #include "ephy-history-service.h"
 #include "ephy-lib-type-builtins.h"
+#include "ephy-output-encoding.h"
 #include "ephy-permissions-manager.h"
 #include "ephy-prefs.h"
 #include "ephy-reader-handler.h"
@@ -1772,9 +1773,11 @@ format_network_error_page (const char  *
                            const char **icon_name,
                            const char **style)
 {
-  char *formatted_origin;
-  char *formatted_reason;
-  char *first_paragraph;
+  g_autofree char *encoded_uri = NULL;
+  g_autofree char *encoded_origin = NULL;
+  g_autofree char *formatted_origin = NULL;
+  g_autofree char *formatted_reason = NULL;
+  g_autofree char *first_paragraph = NULL;
   const char *second_paragraph;
 
   /* Page title when a site cannot be loaded due to a network error. */
@@ -1783,7 +1786,8 @@ format_network_error_page (const char  *
   /* Message title when a site cannot be loaded due to a network error. */
   *message_title = g_strdup (_("Unable to display this website"));
 
-  formatted_origin = g_strdup_printf ("<strong>%s</strong>", origin);
+  encoded_origin = ephy_encode_for_html_entity (origin);
+  formatted_origin = g_strdup_printf ("<strong>%s</strong>", encoded_origin);
   /* Error details when a site cannot be loaded due to a network error. */
   first_paragraph = g_strdup_printf (_("The site at %s seems to be "
                                        "unavailable."),
@@ -1805,16 +1809,13 @@ format_network_error_page (const char  *
 
   /* The button on the network error page. DO NOT ADD MNEMONICS HERE. */
   *button_label = g_strdup (_("Reload"));
-  *button_action = g_strdup_printf ("window.location = '%s';", uri);
+  encoded_uri = ephy_encode_for_javascript (uri);
+  *button_action = g_strdup_printf ("window.location = '%s';", encoded_uri);
   /* Mnemonic for the Reload button on browser error pages. */
   *button_accesskey = C_("reload-access-key", "R");
 
   *icon_name = "network-error-symbolic.svg";
   *style = "default";
-
-  g_free (formatted_origin);
-  g_free (formatted_reason);
-  g_free (first_paragraph);
 }
 
 static void
@@ -1828,10 +1829,12 @@ format_crash_error_page (const char  *ur
                          const char **icon_name,
                          const char **style)
 {
-  char *formatted_uri;
-  char *formatted_distributor;
-  char *first_paragraph;
-  char *second_paragraph;
+  g_autofree char *html_encoded_uri = NULL;
+  g_autofree char *js_encoded_uri = NULL;
+  g_autofree char *formatted_uri = NULL;
+  g_autofree char *formatted_distributor = NULL;
+  g_autofree char *first_paragraph = NULL;
+  g_autofree char *second_paragraph = NULL;
 
   /* Page title when a site cannot be loaded due to a page crash error. */
   *page_title = g_strdup_printf (_("Problem Loading Page"));
@@ -1839,7 +1842,8 @@ format_crash_error_page (const char  *ur
   /* Message title when a site cannot be loaded due to a page crash error. */
   *message_title = g_strdup (_("Oops! There may be a problem"));
 
-  formatted_uri = g_strdup_printf ("<strong>%s</strong>", uri);
+  html_encoded_uri = ephy_encode_for_html_entity (uri);
+  formatted_uri = g_strdup_printf ("<strong>%s</strong>", html_encoded_uri);
   /* Error details when a site cannot be loaded due to a page crash error. */
   first_paragraph = g_strdup_printf (_("The page %s may have caused Web to "
                                        "close unexpectedly."),
@@ -1858,17 +1862,13 @@ format_crash_error_page (const char  *ur
 
   /* The button on the page crash error page. DO NOT ADD MNEMONICS HERE. */
   *button_label = g_strdup (_("Reload"));
-  *button_action = g_strdup_printf ("window.location = '%s';", uri);
+  js_encoded_uri = ephy_encode_for_javascript (uri);
+  *button_action = g_strdup_printf ("window.location = '%s';", js_encoded_uri);
   /* Mnemonic for the Reload button on browser error pages. */
   *button_accesskey = C_("reload-access-key", "R");
 
   *icon_name = "computer-fail-symbolic.svg";
   *style = "default";
-
-  g_free (formatted_uri);
-  g_free (formatted_distributor);
-  g_free (first_paragraph);
-  g_free (second_paragraph);
 }
 
 static void
@@ -1882,6 +1882,7 @@ format_process_crash_error_page (const c
                                  const char **icon_name,
                                  const char **style)
 {
+  g_autofree char *encoded_uri = NULL;
   const char *first_paragraph;
 
   /* Page title when a site cannot be loaded due to a process crash error. */
@@ -1897,7 +1898,8 @@ format_process_crash_error_page (const c
 
   /* The button on the process crash error page. DO NOT ADD MNEMONICS HERE. */
   *button_label = g_strdup (_("Reload"));
-  *button_action = g_strdup_printf ("window.location = '%s';", uri);
+  encoded_uri = ephy_encode_for_javascript (uri);
+  *button_action = g_strdup_printf ("window.location = '%s';", encoded_uri);
   /* Mnemonic for the Reload button on browser error pages. */
   *button_accesskey = C_("reload-access-key", "R");
 
@@ -1921,8 +1923,9 @@ format_tls_error_page (EphyWebView  *vie
                        const char  **icon_name,
                        const char  **style)
 {
-  char *formatted_origin;
-  char *first_paragraph;
+  g_autofree char *encoded_origin = NULL;
+  g_autofree char *formatted_origin = NULL;
+  g_autofree char *first_paragraph = NULL;
 
   /* Page title when a site is not loaded due to an invalid TLS certificate. */
   *page_title = g_strdup_printf (_("Security Violation"));
@@ -1930,7 +1933,8 @@ format_tls_error_page (EphyWebView  *vie
   /* Message title when a site is not loaded due to an invalid TLS certificate. */
   *message_title = g_strdup (_("This Connection is Not Secure"));
 
-  formatted_origin = g_strdup_printf ("<strong>%s</strong>", origin);
+  encoded_origin = ephy_encode_for_html_entity (origin);
+  formatted_origin = g_strdup_printf ("<strong>%s</strong>", encoded_origin);
   /* Error details when a site is not loaded due to an invalid TLS certificate. */
   first_paragraph = g_strdup_printf (_("This does not look like the real %s. "
                                        "Attackers might be trying to steal or "
@@ -1956,9 +1960,6 @@ format_tls_error_page (EphyWebView  *vie
 
   *icon_name = "channel-insecure-symbolic.svg";
   *style = "danger";
-
-  g_free (formatted_origin);
-  g_free (first_paragraph);
 }
 
 static void
@@ -1978,8 +1979,9 @@ format_unsafe_browsing_error_page (EphyW
                                    const char  **icon_name,
                                    const char  **style)
 {
-  char *formatted_origin;
-  char *first_paragraph;
+  g_autofree char *encoded_origin = NULL;
+  g_autofree char *formatted_origin = NULL;
+  g_autofree char *first_paragraph = NULL;
 
   /* Page title when a site is flagged by Google Safe Browsing verification. */
   *page_title = g_strdup_printf (_("Security Warning"));
@@ -1987,7 +1989,8 @@ format_unsafe_browsing_error_page (EphyW
   /* Message title on the unsafe browsing error page. */
   *message_title = g_strdup (_("Unsafe website detected!"));
 
-  formatted_origin = g_strdup_printf ("<strong>%s</strong>", origin);
+  encoded_origin = ephy_encode_for_html_entity (origin);
+  formatted_origin = g_strdup_printf ("<strong>%s</strong>", encoded_origin);
   /* Error details on the unsafe browsing error page.
    * https://developers.google.com/safe-browsing/v4/usage-limits#UserWarnings
    */
@@ -2045,9 +2048,6 @@ format_unsafe_browsing_error_page (EphyW
 
   *icon_name = "security-high-symbolic.svg";
   *style = "danger";
-
-  g_free (formatted_origin);
-  g_free (first_paragraph);
 }
 
 static void
@@ -2061,7 +2061,8 @@ format_no_such_file_error_page (EphyWebV
                                 const char  **icon_name,
                                 const char  **style)
 {
-  g_autofree gchar *formatted_origin = NULL;
+  g_autofree gchar *encoded_address = NULL;
+  g_autofree gchar *formatted_address = NULL;
   g_autofree gchar *first_paragraph = NULL;
   g_autofree gchar *second_paragraph = NULL;
 
@@ -2071,10 +2072,11 @@ format_no_such_file_error_page (EphyWebV
   /* Message title on the no such file error page. */
   *message_title = g_strdup (_("File not found"));
 
-  formatted_origin = g_strdup_printf ("<strong>%s</strong>", view->address);
+  encoded_address = ephy_encode_for_html_entity (view->address);
+  formatted_address = g_strdup_printf ("<strong>%s</strong>", encoded_address);
 
   first_paragraph = g_strdup_printf (_("%s could not be found."),
-                                     formatted_origin);
+                                     formatted_address);
   second_paragraph = g_strdup_printf (_("Please check the file name for "
                                         "capitalization or other typing errors. Also check if "
                                         "it has been moved, renamed, or deleted."));
@@ -2109,19 +2111,19 @@ ephy_web_view_load_error_page (EphyWebVi
                                GError               *error,
                                gpointer              user_data)
 {
-  GBytes *html_file;
-  GString *html = g_string_new ("");
-  char *origin = NULL;
-  char *lang = NULL;
-  char *page_title = NULL;
-  char *msg_title = NULL;
-  char *msg_body = NULL;
-  char *msg_details = NULL;
-  char *button_label = NULL;
-  char *hidden_button_label = NULL;
-  char *button_action = NULL;
-  char *hidden_button_action = NULL;
-  char *style_sheet = NULL;
+  g_autoptr (GBytes) html_file = NULL;
+  g_autoptr (GString) html = g_string_new (NULL);
+  g_autofree char *origin = NULL;
+  g_autofree char *lang = NULL;
+  g_autofree char *page_title = NULL;
+  g_autofree char *msg_title = NULL;
+  g_autofree char *msg_body = NULL;
+  g_autofree char *msg_details = NULL;
+  g_autofree char *button_label = NULL;
+  g_autofree char *hidden_button_label = NULL;
+  g_autofree char *button_action = NULL;
+  g_autofree char *hidden_button_action = NULL;
+  g_autofree char *style_sheet = NULL;
   const char *button_accesskey = NULL;
   const char *hidden_button_accesskey = NULL;
   const char *icon_name = NULL;
@@ -2261,23 +2263,9 @@ ephy_web_view_load_error_page (EphyWebVi
                    button_accesskey, button_label);
 #pragma GCC diagnostic pop
 
-  g_bytes_unref (html_file);
-  g_free (origin);
-  g_free (lang);
-  g_free (page_title);
-  g_free (msg_title);
-  g_free (msg_body);
-  g_free (msg_details);
-  g_free (button_label);
-  g_free (button_action);
-  g_free (hidden_button_label);
-  g_free (hidden_button_action);
-  g_free (style_sheet);
-
   /* Make our history backend ignore the next page load, since it will be an error page. */
   ephy_web_view_freeze_history (view);
   webkit_web_view_load_alternate_html (WEBKIT_WEB_VIEW (view), html->str, uri, 0);
-  g_string_free (html, TRUE);
 }
 
 static gboolean
Index: epiphany-browser/lib/ephy-output-encoding.c
===================================================================
--- /dev/null
+++ epiphany-browser/lib/ephy-output-encoding.c
@@ -0,0 +1,117 @@
+/* -*- Mode: C; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/*
+ *  Copyright © Red Hat Inc.
+ *
+ *  This file is part of Epiphany.
+ *
+ *  Epiphany is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  Epiphany is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with Epiphany.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "config.h"
+#include "ephy-output-encoding.h"
+
+#include <glib.h>
+
+#if !GLIB_CHECK_VERSION(2, 68, 0)
+static guint
+g_string_replace (GString     *string,
+                  const gchar *find,
+                  const gchar *replace,
+                  guint        limit)
+{
+  gsize f_len, r_len, pos;
+  gchar *cur, *next;
+  guint n = 0;
+
+  g_return_val_if_fail (string != NULL, 0);
+  g_return_val_if_fail (find != NULL, 0);
+  g_return_val_if_fail (replace != NULL, 0);
+
+  f_len = strlen (find);
+  r_len = strlen (replace);
+  cur = string->str;
+
+  while ((next = strstr (cur, find)) != NULL)
+    {
+      pos = next - string->str;
+      g_string_erase (string, pos, f_len);
+      g_string_insert (string, pos, replace);
+      cur = string->str + pos + r_len;
+      n++;
+      /* Only match the empty string once at any given position, to
+       * avoid infinite loops */
+      if (f_len == 0)
+        {
+          if (cur[0] == '\0')
+            break;
+          else
+            cur++;
+        }
+      if (n == limit)
+        break;
+    }
+
+  return n;
+}
+#endif
+
+char *
+ephy_encode_for_html_entity (const char *input)
+{
+  GString *str = g_string_new (input);
+
+  g_string_replace (str, "&", "&amp;", 0);
+  g_string_replace (str, "<", "&lt;", 0);
+  g_string_replace (str, ">", "&gt;", 0);
+  g_string_replace (str, "\"", "&quot;", 0);
+  g_string_replace (str, "'", "&#x27;", 0);
+  g_string_replace (str, "/", "&#x2F;", 0);
+
+  return g_string_free (str, FALSE);
+}
+
+static char *
+encode_all_except_alnum (const char *input,
+                         const char *format)
+{
+  GString *str;
+  const char *c = input;
+
+  if (!g_utf8_validate (input, -1, NULL))
+    return g_strdup ("");
+
+  str = g_string_new (NULL);
+  do {
+    gunichar u = g_utf8_get_char (c);
+    if (g_unichar_isalnum (u))
+      g_string_append_unichar (str, u);
+    else
+      g_string_append_printf (str, format, u);
+    c = g_utf8_next_char (c);
+  } while (*c);
+
+  return g_string_free (str, FALSE);
+}
+
+char *
+ephy_encode_for_html_attribute (const char *input)
+{
+  return encode_all_except_alnum (input, "&#x%02x;");
+}
+
+char *
+ephy_encode_for_javascript (const char *input)
+{
+  return encode_all_except_alnum (input, "\\u%04u;");
+}
Index: epiphany-browser/lib/ephy-output-encoding.h
===================================================================
--- /dev/null
+++ epiphany-browser/lib/ephy-output-encoding.h
@@ -0,0 +1,38 @@
+/* -*- Mode: C; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/*
+ *  Copyright © 2021 Red Hat Inc.
+ *
+ *  This file is part of Epiphany.
+ *
+ *  Epiphany is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  Epiphany is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with Epiphany.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include <glib.h>
+
+G_BEGIN_DECLS
+
+/* These functions implement the OWASP XSS prevention output encoding rules:
+ * https://cheatsheetseries.owasp.org/cheatsheets/Cross_Site_Scripting_Prevention_Cheat_Sheet.html#output-encoding-rules-summary
+ *
+ * You must *carefully* read that document to safely inject untrusted data into
+ * web content. Here be dragons.
+ */
+
+char *ephy_encode_for_html_entity    (const char *input);
+char *ephy_encode_for_html_attribute (const char *input);
+char *ephy_encode_for_javascript     (const char *input);
+
+G_END_DECLS
Index: epiphany-browser/lib/meson.build
===================================================================
--- epiphany-browser.orig/lib/meson.build
+++ epiphany-browser/lib/meson.build
@@ -21,6 +21,7 @@ libephymisc_sources = [
   'ephy-langs.c',
   'ephy-notification.c',
   'ephy-notification-container.c',
+  'ephy-output-encoding.c',
   'ephy-permissions-manager.c',
   'ephy-profile-utils.c',
   'ephy-search-engine-manager.c',