CVE-2026-71313

Summary

The local backend relies on its configurable filename encoder to prevent remote filename data from becoming operating-system path syntax. If a local destination uses an encoding that omits Dot, such as Slash, None, or Raw, a remote object's standard-encoded ．． component is decoded into an actual .. component. backend/local.localPath then passes the decoded name to filepath.Join, which resolves the component and produces a path outside the configured local root.

An attacker who can create object names in a remote source that a victim copies or synchronizes to such a local destination can create or overwrite files outside the selected destination directory, with the permissions of the rclone process.

The default local encoding includes Dot and is not affected by that exact path. This finding requires a non-default local encoding that preserves filesystem path syntax. On Windows, a second confirmed form uses a preserved backslash to turn a remote filename into a native ..\file path even when the destination encoding still includes Dot.

This is not merely an odd filename-conversion result. The local remote's configured root is the destination selected by the user, and ordinary backend operations are expected to remain within it. Rclone documents custom and Raw encodings as filename-conversion controls; it does not document them as an opt-out from destination confinement. The defect is that confinement depends on an encoding mask instead of an independent post-conversion path check.

Affected Assets & Attack Surface

Confirmed affected versions

• v1.51.0 through v1.74.4
• Local development commit tested: a0c09f1381ae93e2a9a33c529d170186c61ad058
• Public master inspected through commit c99b2d11edb0986cd2b1190e9fa25a58a3f12661 (2026-07-23)

v1.51.0 introduced the configurable encoding option for the local backend. Encodings such as None or Slash could omit Dot from that version onward. The explicit Raw encoding was introduced later, in v1.68.0.

Required destination configuration

The destination is a local backend whose effective encoding does not safely encode . and .. components. Examples include:

--local-encoding Slash
--local-encoding None
--local-encoding Raw

The first PoC below uses Slash. Default configurations are not affected because the platform-specific encoder.OS masks include Dot.

On Windows, custom encodings that omit BackSlash can introduce an additional traversal form: an object-key component such as ..\marker.txt can become a native path separator plus .., even if the encoding still contains Dot. The fix therefore should enforce containment after conversion to the native path format rather than only require the Dot flag.

Attacker-controlled input

The relevant input is an object name returned by a source backend. The confirmed source case is S3:

• backend/s3/s3.go:2554 converts raw object keys to rclone's standard path representation with f.opt.Enc.ToStandardPath.
• A raw .. component becomes the standard component ．．.
• When the destination local encoder omits Dot, FromStandardPath decodes ．． back to ...

Amazon S3 permits relative path components when their left-to-right cumulative count does not exceed the preceding non-relative components. Consequently, an object named:

tenant/../marker.txt

is valid. When the victim's source remote is rooted at bucket/tenant/, the relative object name becomes ../marker.txt before standard encoding. A malicious S3-compatible endpoint can return equivalent keys without relying on Amazon S3.

Reachable operations

The unsafe path resolver is used throughout the local backend, including:

• backend/local/local.go:798 — localPath
• backend/local/local.go:803 — Put
• backend/local/local.go:979 — Move
• backend/local/local.go:1534 — Object.Update
• backend/local/local.go:1747 — Object.Remove
• Local directory creation and object lookup operations that call localPath

Normal copy and synchronization propagate the source name to the destination:

• fs/sync/sync.go:518 passes src.Remote() to operations.Copy.
• fs/operations/copy.go:390 uses that remote name for destination Put or Update.

Commands that copy attacker-controlled source objects to a local destination are therefore in scope, including copy, sync, and move.

Technical Root Cause Analysis

Rclone represents backend filenames using its standard encoding. lib/encoder/standard.go defines encoder.Standard with EncodeDot, causing raw names equal to . or .. to be represented by fullwidth characters:

.   -> ．
..  -> ．．

When a standard path is converted for a destination backend, lib/encoder/encoder.go:1214-1240 performs the following transformation for every path component:

func FromStandardName(e Encoder, s string) string {
	if e == Standard {
		return s
	}
	return e.Encode(Standard.Decode(s))
}

For a destination encoding that omits Dot:

1. Standard.Decode("．．") returns "..".
2. The destination encoder leaves ".." unchanged.
3. FromStandardPath returns a path containing an actual parent-directory component.

The local backend then constructs the native path without validating containment:

func (f *Fs) localPath(name string) string {
	return filepath.Join(f.root, filepath.FromSlash(f.opt.Enc.FromStandardPath(name)))
}

filepath.Join cleans the resulting path. For example:

root:    /tmp/destination
name:    ../marker.txt
result:  /tmp/marker.txt

Put creates an object from src.Remote(), and Object.Update eventually opens that resolved path using:

os.O_WRONLY | os.O_CREATE | os.O_TRUNC

There is no subsequent filepath.Rel check, anchored filesystem operation, or rejection of an absolute, volume-qualified, . or .. result.

The default encoder masks the defect because it re-encodes .. as a literal fullwidth directory name. That is not a sufficient security boundary: the encoding is explicitly configurable, including an officially documented Raw value that disables conversion.

The local backend contains an existing os.Root mechanism used while translating symlinks, but ordinary local writes do not use it. In the default non---links mode, mkdirAll, openFile, rename, and remove operations use ordinary filesystem paths.

Proof of Concept & Evidence

Deterministic regression test

Add the following test to the backend/local package. It requires no external storage service. It uses S3's actual default encoding mask to construct the same standard Remote() value that an S3 key with a relative .. component produces.

package local

import (
	"bytes"
	"context"
	"os"
	"path/filepath"
	"strings"
	"testing"
	"time"

	"github.com/rclone/rclone/fs/config/configmap"
	"github.com/rclone/rclone/fs/object"
	"github.com/rclone/rclone/lib/encoder"
	"github.com/stretchr/testify/require"
)

func TestLocalEncodingWithoutDotEscapesRoot(t *testing.T) {
	ctx := context.Background()
	outer := t.TempDir()

	// S3's default encoder converts a raw ".." object-key component
	// into rclone's standard fullwidth representation.
	s3Encoding := encoder.EncodeInvalidUtf8 | encoder.EncodeSlash | encoder.EncodeDot
	remote := s3Encoding.ToStandardPath("../marker.txt")
	require.NotEqual(t, "../marker.txt", remote)

	// The default local encoding includes Dot and keeps the path confined.
	safeRaw, err := NewFs(ctx, "safe", filepath.Join(outer, "safe"),
		configmap.Simple{"encoding": encoder.OS.String()})
	require.NoError(t, err)
	safe := safeRaw.(*Fs)
	rel, err := filepath.Rel(safe.root, safe.localPath(remote))
	require.NoError(t, err)
	require.False(t,
		rel == ".." ||
			strings.HasPrefix(rel, ".."+string(filepath.Separator)))

	// Removing Dot converts the same component to a real "..".
	unsafeRaw, err := NewFs(ctx, "unsafe", filepath.Join(outer, "destination"),
		configmap.Simp