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.go

@@ -113,23 +113,25 @@ func handleCliResponseExtra(extra private.ResponseExtra) error {
 	return nil
 }
 
-func getAccessMode(verb, lfsVerb string) perm.AccessMode {
+// getAccessMode maps an SSH git/LFS verb to the access mode it requires, with
+// ok=false for an unrecognised verb. Callers MUST reject the request when ok is
+// false: AccessModeNone would otherwise pass the `userMode < mode` permission
+// check in routers/private/serv.go and grant access.
+func getAccessMode(verb, lfsVerb string) (mode perm.AccessMode, ok bool) {
 	switch verb {
 	case git.CmdVerbUploadPack, git.CmdVerbUploadArchive:
-		return perm.AccessModeRead
+		return perm.AccessModeRead, true
 	case git.CmdVerbReceivePack:
-		return perm.AccessModeWrite
+		return perm.AccessModeWrite, true
 	case git.CmdVerbLfsAuthenticate, git.CmdVerbLfsTransfer:
 		switch lfsVerb {
 		case git.CmdSubVerbLfsUpload:
-			return perm.AccessModeWrite
+			return perm.AccessModeWrite, true
 		case git.CmdSubVerbLfsDownload:
-			return perm.AccessModeRead
+			return perm.AccessModeRead, true
 		}
 	}
-	// should be unreachable
-	setting.PanicInDevOrTesting("unknown verb: %s %s", verb, lfsVerb)
-	return perm.AccessModeNone
+	return perm.AccessModeNone, false
 }
 
 func runServ(ctx context.Context, c *cli.Command) error {
@@ -247,7 +249,10 @@ func runServ(ctx context.Context, c *cli.Command) error {
 		}
 	}
 
-	requestedMode := getAccessMode(verb, lfsVerb)
+	requestedMode, ok := getAccessMode(verb, lfsVerb)
+	if !ok {
+		return fail(ctx, "Unknown git command", "Unknown git command %s %s", verb, lfsVerb)
+	}
 
 	results, extra := private.ServCommand(ctx, keyID, username, reponame, requestedMode, verb, lfsVerb)
 	if extra.HasError() {

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)
+		})
+	}
+}