Go standard library Html/template vulnerabilities

A trusted template author may have previously written a valid template wherein the use of the 'yield' keyword would not be correctly escaped. We now ensure that valid keyword uses are escaped and non-keyword uses are not escaped.

When a JavaScript template literal contains consecutive expressions, the context tracking state was not properly reset upon entering a new expression. We now ensure that template-literal expression entries correctly reset context variables so all subsequent regular expression literals are accurately recognized and escaped.

Previously, pathological inputs could close an unescaped '/' early, allowing for attack-controlled data to inject arbitrary content, potentially leading to XSS.

If a trusted template author were to write a <script> tag containing an empty 'type' attribute or a 'type' attribute with an ASCII whitespace, the execution of the template would incorrectly escape any data passed into the <script> block.

CVE-2026-27142 fixed a vulnerability in which URLs were not correctly escaped inside of a <meta> tag's <content> attribute. If the URL content were to insert ASCII whitespaces around the '=' rune inside of the <content> attribute, the escaper would fail to similarly escape it, leading to XSS.

Context was not properly tracked across template branches for JS template literals, leading to possibly incorrect escaping of content when branches were used. Additionally template actions within JS template literals did not properly track the brace depth, leading to incorrect escaping being applied. These issues could cause actions within JS template literals to be incorrectly or improperly escaped, leading to XSS vulnerabilities.

Actions which insert URLs into the content attribute of HTML meta tags are not escaped. This can allow XSS if the meta tag also has an http-equiv attribute with the value "refresh". A new GODEBUG setting has been added, htmlmetacontenturlescape, which can be used to disable escaping URLs in actions in the meta content attribute which follow "url=" by setting htmlmetacontenturlescape=0.