CVE-2026-79781 in Rclone
Summary
by MITRE • 08/25/2026
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.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 08/25/2026
The rclone software, a popular command-line program for synchronizing files and directories across various cloud storage providers, contained a critical path traversal vulnerability in its S3 serving functionality prior to version 1.74.4. This flaw specifically affects the s3 serve subcommand, which allows users to expose an Amazon S3-compatible interface over HTTP or HTTPS. The vulnerability arises from insufficient validation of object keys provided by clients during read and write operations. When a client requests access to an object using a key that includes dot-dot segments such as ../, the underlying file system logic fails to properly canonicalize these paths before resolving them against the designated serve root directory. This lack of strict path normalization allows attackers to escape the intended bucket namespace boundary and traverse up into parent directories on the host machine where rclone is executing.
From a technical perspective, this issue represents a classic Path Traversal vulnerability, often categorized under CWE-22 in the Common Weakness Enumeration standards. The core failure lies in the application's handling of user-supplied input without adequate sanitization or restriction to the allowed directory tree. In an S3-compatible interface, object keys are typically treated as virtual paths within a bucket. However, when rclone maps these virtual keys to actual file system locations on disk, it must ensure that any relative path components do not allow navigation outside the predefined root. The absence of this check means that requests containing sequences like ../root-secret.txt can be interpreted by the operating system's file I/O routines as instructions to access files located in directories above the serve root. This effectively bypasses the isolation mechanisms intended to keep bucket contents separate from the host environment, turning a cloud storage abstraction into a direct vector for local file system exploitation.
The operational impact of this vulnerability is severe, particularly in environments where rclone s3 serves are exposed to untrusted networks or public internet endpoints. An authenticated attacker who can interact with the S3 interface could read sensitive configuration files, private keys, credentials stored on disk, or other critical data residing outside the bucket structure. Furthermore, if write permissions are enabled for the served directory, an attacker could overwrite existing system files or inject malicious scripts into locations that might be executed by other processes. This capability extends beyond simple information disclosure to potential remote code execution or service disruption depending on which root-level files can be modified and how they are utilized by the host operating system or surrounding infrastructure. The severity is amplified in containerized environments where rclone runs with elevated privileges, as escaping the container's intended file scope could compromise the underlying host node.
Mitigation strategies primarily involve upgrading to version 1.74.4 or later of rclone, which includes patches that enforce strict path validation and prevent traversal outside the serve root directory. For deployments that cannot immediately upgrade, administrators should ensure that the S3 interface is not exposed directly to untrusted networks by placing it behind a reverse proxy with additional request filtering capabilities. Implementing Web Application Firewall rules that detect and block requests containing dot-dot sequences in URL paths or object keys can provide an effective layer of defense-in-depth. Additionally, running rclone with minimal file system permissions limits the potential damage even if traversal is attempted; ensuring the process runs under a dedicated user account with restricted access to only necessary directories reduces the attack surface significantly. Regular auditing of served directory structures and monitoring for anomalous path patterns in access logs further aids in early detection and response to exploitation attempts aligned with techniques described in MITRE ATT&CK such as T1083 File and Directory Discovery or T1564 Hidden Files and Directories when used for persistence or evasion.