CVE-2021-48011 in servor
Summary
by MITRE • 10/13/2026
Servor 1.0.0 through 4.0.2 contains a path traversal vulnerability that allows unauthenticated remote attackers to escape the served root by sending URL-encoded dot segments. Attackers can send %2e%2e/ sequences, which decodeURI() decodes before path.join() in serveStaticFile and serveDirectoryListing, to read arbitrary files and list directories outside the root.
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Analysis
by VulDB Data Team • 10/13/2026
Servor versions 1.0.0 through 4.0.2 are susceptible to a critical path traversal vulnerability that stems from an improper sequence of input sanitization operations within its static file serving logic. The core technical flaw lies in how the application processes URL-encoded characters before validating directory boundaries. Specifically, when handling requests for files or directories, the software utilizes JavaScript's decodeURI function to interpret percent-encoded sequences such as %2e%2e/ into their literal dot-dot-slash representation. This decoding step occurs prior to the execution of path.join(), which is intended to resolve the file system path relative to a designated root directory. Because the traversal sequence is resolved before boundary checks are applied, an attacker can effectively bypass security controls that rely on verifying whether the resulting path remains within the allowed serving root.
This architectural oversight allows unauthenticated remote attackers to escape the served root directory and access arbitrary files or list directories outside of the intended scope. By injecting sequences like %2e%2e/ into URL parameters, an attacker can manipulate the file system navigation logic to traverse upward through the directory hierarchy. This capability enables the reading of sensitive configuration files, source code, environment variables, or other data that should remain inaccessible via the web server interface. The vulnerability is particularly dangerous because it does not require any form of authentication, making it immediately exploitable by any remote actor with network access to the service.
The operational impact of this vulnerability extends beyond simple information disclosure. Depending on the specific files accessible through path traversal, attackers may gain insights into internal system architecture, retrieve credentials stored in configuration files, or identify other vulnerabilities within the application stack. In severe cases where sensitive data such as private keys or database connection strings are exposed, the compromise can lead to further lateral movement and deeper infiltration of the target environment. The ability to list directories outside the root also facilitates reconnaissance efforts, allowing attackers to map out the file system structure for subsequent exploitation attempts.
From a classification perspective, this vulnerability aligns with CWE-22: Improper Limitation of a Pathname to a Restricted Directory, which describes flaws where software does not properly neutralize special elements within a pathname that can cause the path to resolve to a location outside of the restricted directory. Additionally, in terms of attack tactics, this behavior corresponds to ATT&CK technique T1083: File and Directory Discovery, as it enables an attacker to enumerate files and directories on the target system without authorization. The lack of authentication requirement further exacerbates the risk profile, placing this vulnerability in a high-severity category due to its ease of exploitation and broad impact potential.
To mitigate this vulnerability, immediate updates to Servor versions greater than 4.0.2 are strongly recommended if available from the official vendor channels. For environments where upgrading is not immediately feasible, defensive coding practices must be implemented within any custom wrappers or middleware interacting with the static file serving components. Developers should ensure that path resolution and validation occur in a strict order: first resolve the full absolute path using safe methods such as path.resolve(), then verify that the resulting path starts with the intended root directory before proceeding to serve the content. It is crucial to avoid relying solely on decodeURI for security checks, as decoding must be followed by rigorous boundary verification. Additionally, implementing input validation to reject or sanitize percent-encoded traversal sequences at the entry point can provide an additional layer of defense against such attacks.