html/template: improper handling of empty HTML attributes

[rolandshoemaker]

Templates containing actions in unquoted HTML attributes (e.g. "attr={{.}}") executed with empty input could result in output that would have unexpected results when parsed due to HTML normalization rules. This may allow injection of arbitrary attributes into tags.

Thanks to Juho Nurminen of Mattermost for reporting this issue.

This is CVE-2023-29400 and Go issue https://go.dev/issue/59722.

/cc @golang/security and @golang/release

[neild]

@gopherbot please open backport issues for this security fix

[gopherbot]

Backport issue(s) opened: #59815 (for 1.19), #59816 (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/491357 mentions this issue: [release-branch.go1.19] html/template: emit filterFailsafe for empty unquoted attr value

[gopherbot]

Change https://go.dev/cl/491358 mentions this issue: [release-branch.go1.20] html/template: emit filterFailsafe for empty unquoted attr value

[gopherbot]

Change https://go.dev/cl/491617 mentions this issue: html/template: emit filterFailsafe for empty unquoted attr value