events: Escape plain bodies in replies

Replies generate an HTML body even if the reply itself only consists of
plain text. In order to convert the plain text to HTML, it has to be
escaped, which did not happen previously.

crates/ruma-common/CHANGELOG.md

@@ -1,5 +1,10 @@
 # [unreleased]
 
+Bug fixes:
+
+* HTML-relevant characters (`<`, `>`, etc) in plaintext replies are now escaped
+  during creation of the rich reply
+
 Breaking changes:
 
 * Remove deprecated `EventType` enum

crates/ruma-common/src/events/room/message/reply.rs

@@ -48,22 +48,8 @@ fn get_message_quote_fallbacks(original_message: &OriginalRoomMessageEvent) -> (
     }
 }
 
-fn formatted_or_plain_body(
-    formatted: Option<&FormattedBody>,
-    body: &str,
-    #[cfg(feature = "unstable-sanitize")] is_reply: bool,
-) -> String {
-    if let Some(formatted_body) = formatted {
-        #[cfg(feature = "unstable-sanitize")]
-        if is_reply {
-            sanitize_html(&formatted_body.body, HtmlSanitizerMode::Strict, RemoveReplyFallback::Yes)
-        } else {
-            formatted_body.body.clone()
-        }
-
-        #[cfg(not(feature = "unstable-sanitize"))]
-        formatted_body.body.clone()
-    } else {
+/// Converts a plaintext body to HTML, escaping any characters that would cause problems.
+fn escape_html_entities(body: &str) -> String {
     let mut escaped_body = String::with_capacity(body.len());
     for c in body.chars() {
         // Escape reserved HTML entities and new lines.
@@ -83,6 +69,25 @@ fn formatted_or_plain_body(
         }
     }
     escaped_body
+}
+
+fn formatted_or_plain_body(
+    formatted: Option<&FormattedBody>,
+    body: &str,
+    #[cfg(feature = "unstable-sanitize")] is_reply: bool,
+) -> String {
+    if let Some(formatted_body) = formatted {
+        #[cfg(feature = "unstable-sanitize")]
+        if is_reply {
+            sanitize_html(&formatted_body.body, HtmlSanitizerMode::Strict, RemoveReplyFallback::Yes)
+        } else {
+            formatted_body.body.clone()
+        }
+
+        #[cfg(not(feature = "unstable-sanitize"))]
+        formatted_body.body.clone()
+    } else {
+        escape_html_entities(body)
     }
 }
 
@@ -97,7 +102,7 @@ fn formatted_or_plain_body(
 /// [HTML tags and attributes]: https://spec.matrix.org/v1.4/client-server-api/#mroommessage-msgtypes
 /// [rich reply fallbacks]: https://spec.matrix.org/v1.4/client-server-api/#fallbacks-for-rich-replies
 pub fn plain_and_formatted_reply_body(
-    body: impl fmt::Display,
+    body: &str,
     formatted: Option<impl fmt::Display>,
     original_message: &OriginalRoomMessageEvent,
 ) -> (String, String) {
@@ -106,7 +111,7 @@ pub fn plain_and_formatted_reply_body(
     let plain = format!("{quoted}\n{body}");
     let html = match formatted {
         Some(formatted) => format!("{quoted_html}{formatted}"),
-        None => format!("{quoted_html}{body}"),
+        None => format!("{quoted_html}{}", escape_html_entities(body)),
     };
 
     (plain, html)

crates/ruma-common/tests/events/room_message.rs

@@ -8,8 +8,9 @@ use ruma_common::{
         key::verification::VerificationMethod,
         room::{
             message::{
-                AudioMessageEventContent, KeyVerificationRequestEventContent, MessageType,
-                OriginalRoomMessageEvent, RoomMessageEventContent, TextMessageEventContent,
+                AudioMessageEventContent, ForwardThread, KeyVerificationRequestEventContent,
+                MessageType, OriginalRoomMessageEvent, RoomMessageEventContent,
+                TextMessageEventContent,
             },
             MediaSource,
         },
@@ -434,6 +435,53 @@ fn content_deserialization_failure() {
     assert_matches!(from_json_value::<RoomMessageEventContent>(json_data), Err(_));
 }
 
+#[test]
+fn escape_tags_in_plain_reply_body() {
+    let first_message = OriginalRoomMessageEvent {
+        content: RoomMessageEventContent::text_plain("Usage: cp <source> <destination>"),
+        event_id: event_id!("$143273582443PhrSn:example.org").to_owned(),
+        origin_server_ts: MilliSecondsSinceUnixEpoch(uint!(10_000)),
+        room_id: room_id!("!testroomid:example.org").to_owned(),
+        sender: user_id!("@user:example.org").to_owned(),
+        unsigned: MessageLikeUnsigned::default(),
+    };
+    let second_message = RoomMessageEventContent::text_plain("Usage: rm <path>")
+        .make_reply_to(&first_message, ForwardThread::Yes);
+
+    let body = assert_matches!(
+        first_message.content.msgtype,
+        MessageType::Text(TextMessageEventContent { body, formatted: None, .. }) => body
+    );
+    assert_eq!(body, "Usage: cp <source> <destination>");
+
+    let (body, formatted) = assert_matches!(
+        second_message.msgtype,
+        MessageType::Text(TextMessageEventContent { body, formatted, .. }) => (body, formatted)
+    );
+    assert_eq!(
+        body,
+        "\
+        > <@user:example.org> Usage: cp <source> <destination>\n\
+        Usage: rm <path>\
+        "
+    );
+    let formatted = formatted.unwrap();
+    assert_eq!(
+        formatted.body,
+        "\
+        <mx-reply>\
+            <blockquote>\
+                <a href=\"https://matrix.to/#/!testroomid:example.org/$143273582443PhrSn:example.org\">In reply to</a> \
+                <a href=\"https://matrix.to/#/@user:example.org\">@user:example.org</a>\
+                <br>\
+                Usage: cp &lt;source&gt; &lt;destination&gt;\
+            </blockquote>\
+        </mx-reply>\
+        Usage: rm &lt;path&gt;\
+        "
+    );
+}
+
 #[test]
 #[cfg(feature = "unstable-sanitize")]
 fn reply_sanitize() {