CVE-2026-73254 in Mongoose
Summary
by MITRE • 08/20/2026
Mongoose is an embedded web server and network library. Prior to 7.22, an attacker who can create a file with an HTML payload in its name can trigger stored cross-site scripting when a user browses a directory served with MG_ENABLE_DIRLIST. The printdirentry() path called by listdir() in src/http.c URL-encodes the href but inserts the raw filesystem filename into the HTML link text. The browser executes the injected markup in the Mongoose origin, which can expose session data or permit actions as the victim. This issue is fixed in version 7.22.
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Analysis
by VulDB Data Team • 08/20/2026
Mongoose serves as a widely utilized embedded web server and network library designed for resource-constrained environments such as IoT devices and microcontrollers. A critical security vulnerability exists within versions prior to 7.22, specifically affecting the directory listing functionality enabled by the MG_ENABLE_DIRLIST configuration flag. This flaw represents a stored cross-site scripting (XSS) vector that arises from improper handling of filenames provided by the underlying operating system when generating HTML content for web browsers. The vulnerability is not triggered by direct user input to an application form but rather through the manipulation of the file system itself, allowing an attacker who has write access to the served directory to inject malicious payloads into the server's output.
The technical root cause lies in the implementation of the printdirentry function within src/http.c, which is invoked during directory listing operations via listdir. When constructing HTML links for files and directories, Mongoose correctly URL-encodes the href attribute value derived from the filename to prevent path traversal or encoding-based bypasses. However, it fails to apply equivalent sanitization or escaping when inserting the raw filesystem filename into the visible link text of the HTML document. This inconsistency creates a discrepancy between how the browser interprets the resource location and how it renders the textual representation of that resource. Because the raw filename is inserted directly into the DOM without proper context-aware encoding, any special characters within the filename are interpreted as part of the HTML structure rather than literal data.
An attacker capable of creating files in a directory served by Mongoose can exploit this flaw by naming a file with an embedded HTML or JavaScript payload. For instance, if an attacker creates a file named script>alert(document.cookie)</script>, and MG_ENABLE_DIRLIST is active for that directory, the server will generate an HTML page containing this string as visible link text. When a victim user navigates to the directory listing via their web browser, the browser parses the response and executes the injected JavaScript within the security context of the Mongoose origin. This execution environment grants the malicious script access to cookies, local storage, session tokens, or other sensitive data associated with that domain. Furthermore, if the application relies on cookie-based authentication without proper HttpOnly flags, the attacker can steal these credentials to hijack user sessions and perform actions as the authenticated victim.
This vulnerability aligns with CWE-79, which classifies improper neutralization of input during web page generation known as cross-site scripting. The attack vector is categorized under ATT&CK technique T1059, specifically Command Scripting via JavaScript, although in this context it manifests through stored injection rather than direct command execution on the server side. The impact extends beyond simple data exfiltration; if the served content includes administrative interfaces or sensitive operational controls, an attacker could manipulate these interfaces to change configurations, delete files, or escalate privileges within the application logic that trusts user-supplied input derived from file names.
The resolution for this vulnerability is implemented in Mongoose version 7.22 and later releases. The fix ensures that all dynamic content inserted into HTML contexts undergoes rigorous encoding appropriate for its specific context, preventing the browser from interpreting filename characters as executable markup. Organizations relying on embedded web servers must prioritize updating to patched versions of Mongoose to mitigate this risk. Additionally, developers should enforce strict file naming conventions within their applications and avoid serving directories with user-writable contents unless absolutely necessary. Implementing Content Security Policy headers can also provide an additional layer of defense by restricting the sources from which scripts are allowed to execute, thereby mitigating the impact even if a similar vulnerability were present in other components of the system.