CVE-2026-79781: rclone serve s3 Path Traversal Vulnerability

CVE-2026-79781 Overview

CVE-2026-79781 is a path traversal vulnerability in rclone serve s3 versions before 1.74.4. The flaw allows unauthenticated network attackers to read and overwrite files outside the intended bucket namespace. Attackers craft S3 object keys containing dot-dot segments such as ../root-secret.txt to escape the bucket directory and reach files in the serve root. The vulnerability is classified under CWE-22 (Improper Limitation of a Pathname to a Restricted Directory).

Critical Impact
Remote attackers can read and overwrite files outside the intended bucket path when rclone serve s3 is exposed, potentially exposing credentials and configuration data on the host.

Affected Products

rclone serve s3 command versions prior to 1.74.4
Systems exposing rclone's S3-compatible server over a network
Deployments using rclone as an S3 gateway to local filesystems

Discovery Timeline

2026-08-25 - CVE-2026-79781 published to NVD
2026-08-25 - Last updated in NVD database

Technical Details for CVE-2026-79781

Vulnerability Analysis

The vulnerability exists in rclone's S3-compatible server implementation invoked through rclone serve s3. The server accepts client-supplied object keys but fails to sanitize path traversal sequences before resolving them against the serve root. Attackers include ../ segments in the object key portion of S3 API requests. The server joins these keys with the configured serve directory and follows the resulting path outside the bucket boundary.

This allows both GET operations that read arbitrary files and PUT operations that overwrite them. Sensitive targets include configuration files, SSH keys, and credentials stored in the rclone host's filesystem.

Root Cause

The root cause is missing input validation on S3 object key names. The code path constructs filesystem paths by concatenating the serve root with the untrusted object key. Without normalization or rejection of .. components, the resulting path resolves outside the intended bucket namespace.

Attack Vector

Exploitation requires network access to the rclone S3 endpoint. No authentication or user interaction is needed when the service is exposed without credentials. An attacker issues standard S3 GetObject or PutObject requests with keys containing dot-dot sequences, for example GET /bucket/../root-secret.txt. The server resolves the traversal and returns or writes the target file. Refer to the GitHub Security Advisory GHSA-8v25-v8p6-qf7v and the VulnCheck advisory for technical specifics.

Detection Methods for CVE-2026-79781

Indicators of Compromise

HTTP requests to the rclone S3 endpoint containing .. or URL-encoded %2e%2e sequences in object key paths
Unexpected read or write access to files outside the configured bucket directory on the rclone host
Anomalous GET or PUT requests targeting filenames such as passwd, rclone.conf, or SSH key files

Detection Strategies

Inspect reverse proxy and web server access logs for S3 request URIs containing ../, ..%2f, or double-encoded traversal patterns
Monitor file integrity on the rclone serve root directory and its parents for unexpected modifications
Correlate rclone process file access telemetry against the declared bucket path to identify out-of-bounds reads

Monitoring Recommendations

Enable verbose logging on rclone serve s3 and forward logs to a central analytics platform for query and retention
Alert on S3 object key patterns containing traversal sequences or references to sensitive filenames
Track the running rclone version across hosts and flag any instance older than 1.74.4

How to Mitigate CVE-2026-79781

Immediate Actions Required

Upgrade rclone to version 1.74.4 or later on all systems running rclone serve s3
Audit exposure of the rclone S3 endpoint and restrict it to trusted networks or authenticated clients
Review filesystem contents in and above the serve root directory for signs of unauthorized read or write activity

Patch Information

The rclone maintainers fixed the vulnerability in release 1.74.4. The patch normalizes object keys and rejects paths that escape the bucket root. See the GitHub Security Advisory GHSA-8v25-v8p6-qf7v for release details and commit references.

Workarounds

Run rclone serve s3 from a dedicated, isolated directory that contains no sensitive files above or beside the bucket path
Place the rclone service behind an authenticating reverse proxy that strips or rejects requests containing .. in the path
Restrict network access to the S3 endpoint using host firewalls or network policies until the patched version is deployed

bash

# Configuration example: verify installed rclone version and upgrade
rclone version
# If output shows a version below 1.74.4, upgrade:
#   Linux/macOS:
curl https://rclone.org/install.sh | sudo bash
#   Then confirm:
rclone version | head -n1