CVE-2023-24538: Backticks not treated as string delimiters in html/template

Vulnerability summary

CVE-2023-24538 is a critical (CVSS 9.8) Improper Control of Generation of Code vulnerability in go standard library html/template. EPSS estimates a 2.28% chance of exploitation in the next 30 days, higher than 83.1% of scored vulnerabilities.

Timeline

1. 06-04-2023
   PublishedVulnerability first disclosed.
2. 13-02-2025
   Last modifiedVulnerability information updated.

Description

Templates do not properly consider backticks (`) as Javascript string delimiters, and do not escape them as expected. Backticks are used, since ES6, for JS template literals. If a template contains a Go template action within a Javascript template literal, the contents of the action can be used to terminate the literal, injecting arbitrary Javascript code into the Go template. As ES6 template literals are rather complex, and themselves can do string interpolation, the decision was made to simply disallow Go template actions from being used inside of them (e.g. "var a = {{.}}"), since there is no obviously safe way to allow this behavior. This takes the same approach as github.com/google/safehtml. With fix, Template.Parse returns an Error when it encounters templates like this, with an ErrorCode of value 12. This ErrorCode is currently unexported, but will be exported in the release of Go 1.21. Users who rely on the previous behavior can re-enable it using the GODEBUG flag jstmpllitinterp=1, with the caveat that backticks will now be escaped. This should be used with caution.

• CWE-94Improper Control of Generation of CodeThe product constructs all or part of a code segment using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the syntax or behavior of the intended code segment.