RClone: Unauthenticated operations/fsinfo allows attacker-controlled backend instantiation and local command execution

Summary

The RC endpoint operations/fsinfo is exposed without AuthRequired: true and accepts attacker-controlled fs input. Because rc.GetFs(...) supports inline backend definitions, an unauthenticated attacker can instantiate an attacker-controlled backend on demand. For the WebDAV backend, bearer_token_command is executed during backend initialization, making single-request unauthenticated local command execution possible on reachable RC deployments without global HTTP authentication.

Preconditions

Preconditions for this vulnerability are:

• The rclone remote control API must be enabled, either by the --rc flag or by running the rclone rcd server

• The remote control API must be reachable by the attacker - by default rclone only serves the rc to localhost unless the --rc-addr flag is in use

• The rc must have been deployed without global RC HTTP authentication - so not using --rc-user/--rc-pass/--rc-htpasswd/etc

Details

The root cause consists of the following pieces:

1. operations/fsinfo is not protected with AuthRequired: true

2. operations/fsinfo calls rc.GetFs(...) on attacker-controlled input

3. rc.GetFs(...) supports inline backend creation through object-valued fs

4. WebDAV backend initialization executes bearer_token_command

Relevant code paths:

• fs/operations/rc.go

  • operations/fsinfo is registered without AuthRequired: true

  • rcFsInfo() calls rc.GetFs(ctx, in)

• fs/rc/cache.go

  • GetFs() / GetFsNamed() can parse an object-valued fs

  • getConfigMap() converts attacker-controlled JSON into a backend config string

• backend/webdav/webdav.go

  • bearer_token_command is a supported backend option

  • NewFs(...) calls fetchAndSetBearerToken() when bearer_token_command is set

  • fetchBearerToken() invokes exec.Command(...)

This creates a practical single-request unauthenticated command-execution primitive on reachable RC servers without global HTTP authentication.

This was alidated on:

• current master as of 2026-04-14: bf55d5e6d37fd86164a87782191f9e1ffcaafa82

• latest public release tested locally: v1.73.4

This was also validated on a public amd64 Ubuntu host controlled by the tester, using direct host execution (not containerized PoC execution).

PoC

Minimal single-request form PoC

Start a vulnerable RC server:

rclone rcd --rc-addr 127.0.0.1:5572

No --rc-user, no --rc-pass, no --rc-htpasswd.

Then send a single request:

curl -sS -X POST http://127.0.0.1:5572/operations/fsinfo \
  --data-urlencode "fs=:webdav,url='http://127.0.0.1/',vendor=other,bearer_token_command='/usr/bin/touch /tmp/rclone_fsinfo_rce_poc_marker':"

Expected result:

• HTTP 200 JSON response from operations/fsinfo

• /tmp/rclone_fsinfo_rce_poc_marker is created on the host

Impact

This is effectively a single-request unauthenticated command-execution vulnerability on reachable RC deployments without global HTTP authentication.

In practice, command execution in the rclone process context can lead to higher-impact outcomes such as local file read, file write, or shell access, depending on the deployed environment.

Testing performed

This was successfully reproduced:

• on a local test environment

• on a public amd64 Ubuntu host controlled by the tester

On the public host it was confirmed:

• the unauthenticated operations/fsinfo exploit worked

• command execution occurred on the host

• the issue was reproducible through direct host execution