cmd/go: trust layer bypass when using cgo and SWIG

[neild]

A well-crafted SWIG source file could take advantage
of a file-naming convention used inside the trust
boundary of the cgo compiler. Doing so could result
in arbitrary code execution during build time.
 
SWIG files are disallowed from using this convention.
 
Thank you to Juho Forsén of Mattermost for reporting this issue.
 
 
This is CVE-2026-27140 and Go issue https://go.dev/issue/78335.

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This is a PRIVATE issue for CVE-2026-27140, tracked in http://b/479232126 and fixed by https://go-internal-review.git.corp.google.com/c/go/+/3520.

/cc @golang/security and @golang/release

[neild]

@gopherbot please open backport issues for this security fix

[gopherbot]

Backport issue(s) opened: #78421 (for 1.25), #78422 (for 1.26).

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/763549 mentions this issue: [release-branch.go1.26] cmd/go: disallow cgo trust boundary bypass

[gopherbot]

Change https://go.dev/cl/763556 mentions this issue: [release-branch.go1.25] cmd/go: disallow cgo trust boundary bypass

[gopherbot]

Change https://go.dev/cl/763768 mentions this issue: cmd/go: disallow cgo trust boundary bypass