Only uppercase <![CDATA[ starts CDATA section

Author: Mingun

Changelog.md

@@ -17,8 +17,13 @@
 
 ### Bug Fixes
 
+- [#781]: Fix conditions to start CDATA section. Only uppercase `<![CDATA[` can start it.
+  Previously any case was allowed.
+
 ### Misc Changes
 
+[#781]: https://github.com/tafia/quick-xml/pull/781
+
 
 ## 0.35.0 -- 2024-06-29
 

src/reader/state.rs

@@ -128,14 +128,22 @@ impl ReaderState {
                     self.decoder(),
                 )))
             }
-            BangType::CData if uncased_starts_with(buf, b"![CDATA[") => {
+            // XML requires uppercase only:
+            // https://www.w3.org/TR/xml11/#sec-cdata-sect
+            // Even HTML5 required uppercase only:
+            // https://html.spec.whatwg.org/multipage/parsing.html#markup-declaration-open-state
+            BangType::CData if buf.starts_with(b"![CDATA[") => {
                 debug_assert!(buf.ends_with(b"]]"));
                 Ok(Event::CData(BytesCData::wrap(
                     // Cut of `![CDATA[` and `]]` from start and end
                     &buf[8..len - 2],
                     self.decoder(),
                 )))
             }
+            // XML requires uppercase only, but we will check that on validation stage:
+            // https://www.w3.org/TR/xml11/#sec-prolog-dtd
+            // HTML5 allows mixed case for doctype declarations:
+            // https://html.spec.whatwg.org/multipage/parsing.html#markup-declaration-open-state
             BangType::DocType if uncased_starts_with(buf, b"!DOCTYPE") => {
                 match buf[8..].iter().position(|&b| !is_whitespace(b)) {
                     Some(start) => Ok(Event::DocType(BytesText::wrap(