Sequence ']]>' not allowed in content

What it means

XML forbids the literal sequence ]]> in character data anywhere, not only inside a CDATA section. The rule exists so a parser scanning for the end of a section can never be fooled by content that merely looks like one.

The usual real-world version is worse than a stray literal: a CDATA section whose own content contains ]]>, which terminates the section early and leaves the remainder to be parsed as markup.

What usually causes it

• Embedding a code sample, a regular expression or serialized XML that happens to contain ]]>.
• Nesting CDATA — wrapping content that already contains a CDATA section, which cannot be done directly.
• A generator that wraps arbitrary text in CDATA without scanning it for the terminator first.
• Concatenating two CDATA-wrapped fragments so that one's terminator lands inside the other.

How to fix it

• Escape the closing bracket: write ]]> instead of ]]>.
• Split the sequence across two CDATA sections, ending the first after ]] and starting the next with >.
• Do not wrap in CDATA at all — escape the content normally with < and &, which is always safe.
• If a generator writes CDATA, make it replace ]]> in the payload before wrapping rather than after.

The same error elsewhere

Different parsers, same defect. If you arrived with one of these messages, you are in the right place.

• Xerces (Java)

  The character sequence "]]>" must not appear in content unless used to mark the end of a CDATA section.

• expat (Python)

  not well-formed (invalid token): line 2, column 7

• .NET System.Xml

  ']]>' is not allowed in character data.

• fast-xml-parser and similar

  (accepted — no error reported at all)