CVE-2026-110103 in bbs-go
Summary
by MITRE • 10/13/2026
bbs-go through 4.4.6 contains a stored cross-site scripting vulnerability that allows authenticated users to upload HTML or SVG files by spoofing the multipart Content-Type in POST /api/upload. Attackers can upload script-bearing documents served inline from /res and send their URLs to administrators, executing script on the forum's origin.
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Analysis
by VulDB Data Team • 10/13/2026
The security flaw identified in bbs-go versions prior to 4.4.6 represents a significant stored cross-site scripting vulnerability rooted in inadequate validation of file uploads within the application’s API layer. The core technical deficiency lies in how the server processes multipart form data submitted via POST requests to the /api/upload endpoint. Specifically, the system fails to strictly enforce or correctly interpret the Content-Type header associated with uploaded files. By manipulating this header and spoofing it to mimic a safe media type such as image/svg+xml or text/html, an authenticated attacker can bypass standard file extension checks that might otherwise restrict uploads to non-executable formats like images or documents. This allows the injection of malicious HTML or SVG content directly into the server’s storage infrastructure, which is subsequently served inline from the /res directory without proper sanitization or encoding of script tags and event handlers embedded within the markup.
From an operational perspective, this vulnerability enables a persistent attack vector where the malicious payload remains stored on the server until manually removed by an administrator. The impact is particularly severe because it targets privileged users who frequently access the forum interface to manage content or review posts. When an attacker uploads these spoofed files and distributes their URLs through legitimate channels within the application, such as in user profiles, comments, or private messages, any administrator viewing this content will have their browser execute the injected JavaScript code. Because the malicious scripts are served from the same origin as the forum itself, they inherit full access to session cookies, local storage, and other sensitive data associated with that domain. This effectively allows an authenticated but potentially lower-privilege user to escalate their influence by compromising high-privilege accounts through social engineering tactics disguised as legitimate file sharing or resource linking.
This vulnerability aligns closely with CWE-79, which classifies improper neutralization of input during web page generation, commonly known as cross-site scripting. The specific mechanism here also touches upon CWE-436 regarding interpretation ambiguity, where the server misinterprets the intended type of the uploaded file due to header spoofing. In terms of adversary tactics, this scenario maps directly to ATT&CK technique T1059.007, which covers JavaScript execution within a browser environment for lateral movement or credential theft. The stored nature of the payload means that each view constitutes a potential compromise event, increasing the blast radius significantly compared to reflected XSS variants where user interaction is limited to clicking specific links.
To mitigate this risk, immediate action should be taken by upgrading bbs-go to version 4.4.6 or later, which addresses these input validation gaps. In environments where an upgrade is not immediately feasible, defensive measures must focus on strict content-type verification at the server level rather than relying solely on file extensions. Implementing a whitelist of allowed MIME types and ensuring that uploaded files are either served with appropriate Content-Disposition headers as attachments rather than inline or processed through a robust sanitization library can prevent script execution. Additionally, enforcing SameSite cookie attributes and implementing Content Security Policy directives can help limit the impact if an injection does occur by restricting where scripts can be loaded from and preventing unauthorized data exfiltration to external domains.