rclone archive extract allows S3 destination prefix escape via crafted archive paths

Summary

rclone archive extract can write extracted files outside the user-selected destination prefix when extracting a crafted archive. A malicious archive entry containing parent path components such as ../ can escape the requested extraction prefix and create or overwrite sibling objects in the same bucket/path scope.

Details

The affected code path is in cmd/archive/extract/extract.go.

In ArchiveExtract(), the archive entry path is taken from f.NameInArchive. The code strips only a leading ./ prefix and then joins the archive entry path with the destination directory:

remote := f.NameInArchive
remote = strings.TrimPrefix(remote, "./")
if dstDir != "" {
    remote = path.Join(dstDir, remote)
}
_, err = operations.Rcat(ctx, dst, remote, fin, f.ModTime(), nil)

Parent path components such as ../ are not rejected before path.Join() is used.

When the destination is an S3-style remote such as:

:s3:bucket/safe/prefix

rclone creates the destination filesystem rooted at bucket/safe and treats prefix as the destination directory. If the archive contains an entry named:

../escaped-from-prefix.txt

then path.Join("prefix", "../escaped-from-prefix.txt") resolves to:

escaped-from-prefix.txt

As a result, the S3 backend uploads the object to:

bucket/safe/escaped-from-prefix.txt

instead of the expected destination:

bucket/safe/prefix/escaped-from-prefix.txt

This allows an attacker-controlled archive to escape the selected extraction prefix on object-storage remotes.

PoC

Test environment:

• Windows 11
• rclone v1.74.3 official Windows amd64 binary
• Local fake S3 HTTP endpoint
• Crafted ZIP archive containing ../escaped-from-prefix.txt

Steps to reproduce:https://drive.google.com/file/d/1P_cLKFgiWSVSATB8500yP28jdzwt9FAt/view?usp=sharing

1. Extract the attached PoC ZIP.

2. Run the PoC script:

powershell -ExecutionPolicy Bypass -File .\run-poc.ps1 -RcloneExe "C:\path\to\rclone.exe"

3. The PoC creates a ZIP archive containing this entry:

../escaped-from-prefix.txt

4. The PoC starts a local fake S3 endpoint and runs rclone with an S3-style destination prefix:

rclone archive extract malicious.zip :s3:bucket/safe/prefix

5. Observe the fake S3 request log.

Expected safe behavior:

PUT /bucket/safe/prefix/escaped-from-prefix.txt

Observed behavior:

PUT /bucket/safe/escaped-from-prefix.txt?x-id=PutObject

This shows that the archive entry escaped the requested safe/prefix destination and was written under safe/ instead.

The PoC package includes:

• run-poc.ps1
• fake-s3-server.py
• README.md
• report-draft.md
• captured proof logs

Impact

An attacker who supplies an archive that a victim extracts with rclone archive extract can cause extracted files to be written outside the destination prefix selected by the victim when the destination is an S3-style object storage remote.

Depending on the victim's configured remote credentials and bucket permissions, this may allow creation or overwrite of sibling objects outside the intended extraction directory/prefix.

This does not require compromising the S3 service itself. The attack relies on the victim extracting an attacker-controlled archive with rclone into an object-storage prefix.