CVE-2026-73259 in Mongoose
Summary
by MITRE • 08/20/2026
Mongoose is an embedded web server and network library. Prior to 7.22, a remote attacker can send a crafted percent-encoded request path to a deployment using MG_ENABLE_DIRLIST and persuade a user to visit it. The mg_http_serve_dir() and listdir() path in src/http.c places the decoded request URI into the title and h1 elements without HTML entity encoding. The resulting reflected cross-site scripting executes in the Mongoose origin and can expose session data or perform 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 adopted embedded web server and network library, frequently utilized in Internet of Things devices, microcontrollers, and lightweight backend services due to its compact footprint and ease of integration. The vulnerability identified prior to version 7.22 resides within the directory listing functionality enabled by the MG_ENABLE_DIRLIST configuration flag. This feature allows the server to automatically generate an HTML index page when a client requests a directory path that does not correspond to a specific file, thereby exposing the contents of that directory to remote users. The core technical flaw lies in how the library handles user-supplied input within this auto-generated content. Specifically, when processing HTTP GET requests for directories, the function mg_http_serve_dir and its helper listdir retrieve the requested URI path from the client request headers or URL structure.
The critical failure occurs during the rendering of the directory listing page. The implementation takes the decoded version of the percent-encoded request path and injects it directly into HTML title and h1 elements without applying any form of output encoding, such as HTML entity encoding. This behavior creates a classic reflected cross-site scripting vulnerability. Because the input is not sanitized or escaped before being embedded in the DOM structure, an attacker can craft a malicious URL containing script payloads encoded using percent-encoding to bypass basic filters that might check for literal angle brackets or script tags. When a victim user visits this crafted link while authenticated with the web server hosting Mongoose, their browser will parse and execute the injected JavaScript within the context of the vulnerable application's origin.
The operational impact of this vulnerability is significant due to its potential for session hijacking and unauthorized actions on behalf of the victim. Since the script executes in the same security context as the legitimate website, it has full access to cookies, local storage, and other sensitive data associated with that domain. An attacker can leverage this capability to steal authentication tokens or session identifiers, effectively taking over user accounts without needing a password. Furthermore, the malicious script can perform state-changing actions on behalf of the victim, such as modifying account settings, initiating transactions, or exfiltrating internal network information if combined with other vulnerabilities like cross-site request forgery defenses being absent. This reflects a fundamental failure in input validation and output encoding principles, categorized under CWE-79: Improper Neutralization of Input During Web Page Generation commonly known as Cross-site Scripting.
From an offensive security perspective, this vulnerability aligns with the MITRE ATT&CK technique T1059.007, which covers JavaScript execution within web browsers. The attack vector relies on social engineering to persuade a victim to visit a maliciously crafted URL, making it relevant to initial access and credential harvesting tactics in broader cyber kill chains. To mitigate this risk, organizations using versions of Mongoose prior to 7.22 must upgrade immediately to version 7.22 or later where the issue has been resolved by implementing proper HTML entity encoding for user-supplied data before rendering it into web pages. For environments that cannot update promptly due to legacy constraints, a temporary mitigation involves disabling the MG_ENABLE_DIRLIST feature if directory listing is not strictly required, thereby removing the attack surface entirely. Additionally, deploying Web Application Firewalls with robust cross-site scripting detection rules can provide an additional layer of defense by blocking requests containing known malicious payload patterns in URL paths before they reach the application logic.