XML comment injection in Saxy encoder via unescaped double hyphen

XML injection in the encoder of Saxy. Saxy.Encoder does not neutralize the -- sequence in XML comment content: escape_comment/2 returns the comment binary unchanged in every clause and only appends a space when the comment ends in a single -, so neither an interior -- nor a full --> is escaped or rejected before the encoder wraps the content in the <!-- and --> delimiters.

When an application serializes attacker-controlled text as a comment, through Saxy.XML.comment/1 or a raw {:comment, text} simple form child, the attacker can close the comment early with --> and inject arbitrary sibling markup into the serialized document. A downstream parser then sees elements, attributes, and text that the application never built. Where the document is rendered as HTML or SVG, the injected markup executes as stored cross-site scripting; where it is consumed by another XML processor, the attacker can forge data that the producing application never emitted. Documents that Saxy only parses are not a vector.

This issue affects saxy: from 0.7.0 before 1.7.0.

Reject or rewrite the -- sequence in untrusted text before passing it to Saxy.XML.comment/1, for example by replacing -- with - -.

Alternatively, stop emitting untrusted data as XML comments. Comments carry no semantic payload for consumers, so the text can usually be moved into an element or a CDATA section instead.

Only applications that place untrusted text into an XML comment node are affected. The untrusted value must reach Saxy.XML.comment/1 or a raw {:comment, text} simple form child that is then serialized with Saxy.encode!/2 or Saxy.encode_to_iodata!/2. Documents that are only parsed by Saxy never reach the encoder and are not a vector.