fix(lfs): reject unknown SSH LFS sub-verbs to prevent auth bypass (#38008)

An authenticated SSH user could pass a malformed sub-verb (e.g.
`git-lfs-authenticate <repo> badverb`) so getAccessMode falls through to
AccessModeNone (0). The permission check in routers/private/serv.go then
evaluates `userMode < 0` which is always false, granting a valid LFS JWT
for any private repository. The HTTP LFS handler only validates the Op
claim on writes, so the token works for downloads.

Validate the sub-verb in runServ before calling getAccessMode and fail
fast for anything other than upload/download.

cmd/serv_test.go

@@ -0,0 +1,56 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package cmd
+
+import (
+	"testing"
+
+	"gitea.dev/models/perm"
+	"gitea.dev/modules/git"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGetAccessMode(t *testing.T) {
+	cases := []struct {
+		verb, lfsVerb string
+		expected      perm.AccessMode
+	}{
+		{git.CmdVerbUploadPack, "", perm.AccessModeRead},
+		{git.CmdVerbUploadArchive, "", perm.AccessModeRead},
+		{git.CmdVerbReceivePack, "", perm.AccessModeWrite},
+		{git.CmdVerbLfsAuthenticate, git.CmdSubVerbLfsUpload, perm.AccessModeWrite},
+		{git.CmdVerbLfsAuthenticate, git.CmdSubVerbLfsDownload, perm.AccessModeRead},
+		{git.CmdVerbLfsTransfer, git.CmdSubVerbLfsUpload, perm.AccessModeWrite},
+		{git.CmdVerbLfsTransfer, git.CmdSubVerbLfsDownload, perm.AccessModeRead},
+	}
+	for _, tc := range cases {
+		t.Run(tc.verb+"/"+tc.lfsVerb, func(t *testing.T) {
+			mode, ok := getAccessMode(tc.verb, tc.lfsVerb)
+			assert.True(t, ok)
+			assert.Equal(t, tc.expected, mode)
+		})
+	}
+}
+
+// TestGetAccessModeUnknownVerb locks in the invariant that getAccessMode reports
+// ok=false for unrecognised verbs and LFS sub-verbs, so runServ rejects them. An
+// unknown verb has no valid access mode; if it were treated as AccessModeNone (0)
+// it would pass the `userMode < mode` permission check in routers/private/serv.go
+// and hand out valid LFS JWTs for any private repository.
+func TestGetAccessModeUnknownVerb(t *testing.T) {
+	cases := []struct{ verb, lfsVerb string }{
+		{git.CmdVerbLfsAuthenticate, ""},
+		{git.CmdVerbLfsAuthenticate, "badverb"},
+		{git.CmdVerbLfsTransfer, "badverb"},
+		{"git-unknown-verb", ""},
+	}
+	for _, tc := range cases {
+		t.Run(tc.verb+"/"+tc.lfsVerb, func(t *testing.T) {
+			mode, ok := getAccessMode(tc.verb, tc.lfsVerb)
+			assert.False(t, ok)
+			assert.Equal(t, perm.AccessModeNone, mode)
+		})
+	}
+}