html/template: backticks not treated as string delimiters (CVE-2023-24538)

[rolandshoemaker]

Templates did not properly consider backticks (`) as Javascript string delimiters, and as such did
not escape them as expected. Backticks are used, since ES6, for JS template literals. If a template
contained a Go template action within a Javascript template literal, the contents of the action could
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, we've decided
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.
Template.Parse will now return an Error when it encounters templates like this, with a currently unexported
ErrorCode with a value of 12. This ErrorCode will be exported in the next major release.

Users who rely on this 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.

Thanks to Sohom Datta, Manipal Institute of Technology, for reporting this issue.

This is CVE-2023-24538 and Go issue https://go.dev/issue/59234.

/cc @golang/security and @golang/release

[julieqiu]

@gopherbot please open backport issues.

[gopherbot]

Backport issue(s) opened: #59271 (for 1.19), #59272 (for 1.20).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.

[gopherbot]

Change https://go.dev/cl/481981 mentions this issue: [release-branch.go1.19] html/template: disallow actions in JS template literals

[gopherbot]

Change https://go.dev/cl/481987 mentions this issue: [release-branch.go1.19] html/template: disallow actions in JS template literals

[gopherbot]

Change https://go.dev/cl/481993 mentions this issue: [release-branch.go1.20] html/template: disallow actions in JS template literals

[gopherbot]

Change https://go.dev/cl/482079 mentions this issue: html/template: disallow actions in JS template literals

[hundt]

I do not think the solution merged handles the case where a template literal contains a string interpolation with another template literal, as in:

package main

import (
	"bytes"
	"fmt"
	"html/template"
)

func main() {
	t := template.Must(template.New("test").Parse("<script>var v = `${function(){return `{{.V}}+1`}()}`;</script>"))
	buf := new(bytes.Buffer)
	err := t.Execute(buf, map[string]string{"V": "${alert(1)}"})
	if err != nil {
		panic(err)
	}
	fmt.Printf("%s", buf)
}

Go's simple JS state machine incorrectly thinks that "{{.V}}" appears outside of a JS template literal and escapes it as if it were the javascript context (by just adding quotes around it). As a result the output of this program is

<script>var v = `${function(){return `"${alert(1)}"+1`}()}`;</script>

which, if inserted as HTML in a web page, will run the alert function.

#9200 has a good discussion of the complexities of this issue.

[gopherbot]

Change https://go.dev/cl/482238 mentions this issue: html/template,mime/multipart: document new GODEBUG settings

[gopherbot]

Change https://go.dev/cl/482535 mentions this issue: [release-branch.go1.19] html/template,mime/multipart: document new GODEBUG settings

[gopherbot]

Change https://go.dev/cl/482555 mentions this issue: [release-branch.go1.20] html/template,mime/multipart: document new GODEBUG settings

[karelbilek]

As @hundt mentioned, I don't think this should be closed, the vulnerability is still present

[atc0005]

@julieqiu just making sure you saw the remarks by @hundt and @karelbilek.

See also:

• html/template: add support for template strings (backticks) #9200

[julieqiu]

Thank you for letting us know. We will investigate and post updates to #9200.

[gopherbot]

Change https://go.dev/cl/484075 mentions this issue: html/template: treat nested template literals properly