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cmd/go, x/mod/sumdb/tlog: fix transparency log tile verification bypass #80744
Description
A malicious GOPROXY was previously capable of forging
up to two sumdb tiles that allow for a requested module
to bypass the GOSUMDB check and persist attacker-controlled
module content to a local Go module cache.
This attack allows for a malicious GOPROXY to serve
malicious module content that cannot be detected
by evaluating the transparency log.
All tiles are now correctly verified against their parents.
In order to determine if you have been affected:
rm -r go.sum go.work.sum vendor/ && go mod tidy
Thanks to Filippo Valsorda (Geomys) for reporting this issue.
This is CVE-2026-56865 and Go issue https://go.dev/issue/80744.
This was a PRIVATE track issue, tracked in http://b/527431546.
Activity
thatnealpatel commentedon Aug 6, 2026
Hi @gopherbot, please open backport issues for this security issue.
gopherbot commentedon Aug 6, 2026
Backport issue(s) opened: #80764 (for 1.25), #80765 (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.
[release-branch.go1.26] cmd/vendor: fix CVE-2026-56865 in x/mod
gopherbot commentedon Aug 13, 2026
Change https://go.dev/cl/814802 mentions this issue:
[release-branch.go1.26] cmd/vendor: fix CVE-2026-56865 in x/modgopherbot commentedon Aug 13, 2026
Change https://go.dev/cl/814822 mentions this issue:
[release-branch.go1.25] cmd/vendor: fix CVE-2026-56865 in x/modgopherbot commentedon Aug 13, 2026
Change https://go.dev/cl/814862 mentions this issue:
[release-branch.go1.27] cmd/vendor: fix CVE-2026-56865 in x/mod[release-branch.go1.25] cmd/vendor: fix CVE-2026-56865 in x/mod
[release-branch.go1.27] cmd/vendor: fix CVE-2026-56865 in x/mod
gopherbot commentedon Aug 13, 2026
Change https://go.dev/cl/815020 mentions this issue:
all: update vendored x/modgopherbot commentedon Aug 13, 2026
Change https://go.dev/cl/815080 mentions this issue:
[internal-branch.go1.27-vendor] sumdb/tlog: fix TileHashReader authentication bypass[-]security: fix CVE-2026-56865[/-][+]x/mod/sumdb/tlog: fix transparency log tile verification bypass[/+]gopherbot commentedon Aug 13, 2026
Change https://go.dev/cl/815081 mentions this issue:
[internal-branch.go1.26-vendor] sumdb/tlog: fix TileHashReader authentication bypassgopherbot commentedon Aug 13, 2026
Change https://go.dev/cl/815083 mentions this issue:
[internal-branch.go1.25-vendor] sumdb/tlog: fix TileHashReader authentication bypass[internal-branch.go1.26-vendor] sumdb/tlog: fix TileHashReader authen…
[internal-branch.go1.25-vendor] sumdb/tlog: fix TileHashReader authen…
[internal-branch.go1.27-vendor] sumdb/tlog: fix TileHashReader authen…
gopherbot commentedon Aug 13, 2026
Change https://go.dev/cl/815240 mentions this issue:
[release-branch.go1.27] cmd/internal/moddeps: restore testsall: update vendored x/mod
[release-branch.go1.27] cmd/internal/moddeps: restore tests
[-]x/mod/sumdb/tlog: fix transparency log tile verification bypass[/-][+]cmd/go, x/mod/sumdb/tlog: fix transparency log tile verification bypass[/+]