CVE-2025-59835 is a high-severity vulnerability affecting LangBot, a global instant messaging bot platform designed to interface with large language models (LLMs). The vulnerability exists in versions 4.1.0 up to but not including 4.3.5. It involves a relative path traversal flaw combined with improper file upload handling in the /api/v1/files/documents API endpoint. Authorized attackers with limited privileges can exploit this endpoint to upload arbitrary files to the server. The core issue is that the application does not strictly restrict the storage directory for uploaded files, allowing attackers to specify paths that traverse directories and place files in sensitive system locations. This can lead to overwriting critical files, uploading malicious scripts or binaries, and potentially achieving remote code execution or persistent backdoors. The vulnerability is classified under CWE-23 (Relative Path Traversal) and CWE-434 (Unrestricted Upload of File with Dangerous Type). The CVSS v4.0 score is 8.6 (high), reflecting network attack vector, low attack complexity, no user interaction, and significant impacts on confidentiality, integrity, and availability. The vulnerability was publicly disclosed on October 2, 2025, and fixed in LangBot version 4.3.5. No known exploits are currently reported in the wild, but the potential for severe impact makes this a critical issue for organizations using affected versions of LangBot.

For European organizations, the impact of this vulnerability can be substantial, especially for enterprises and public sector entities relying on LangBot for internal or customer-facing communication automation with LLMs. Exploitation could allow attackers to upload malicious files to critical directories, leading to unauthorized code execution, data breaches, or service disruptions. This could compromise sensitive personal data protected under GDPR, resulting in regulatory penalties and reputational damage. Additionally, disruption of automated communication services could impact business continuity and customer trust. Organizations in sectors such as finance, healthcare, government, and critical infrastructure that use LangBot are particularly at risk. The vulnerability's ability to affect confidentiality, integrity, and availability simultaneously elevates the threat level. Even though exploitation requires authorized access, insider threats or compromised credentials could facilitate attacks. The lack of user interaction requirement further lowers the barrier for exploitation once access is obtained.

European organizations should immediately upgrade LangBot to version 4.3.5 or later, where the vulnerability is patched. Until the upgrade is applied, organizations should restrict access to the /api/v1/files/documents endpoint to only highly trusted users and monitor file upload activities closely for suspicious patterns, such as unusual file paths or types. Implement strict input validation and sanitization on file upload paths to prevent directory traversal attempts. Employ application-layer firewalls or runtime application self-protection (RASP) solutions to detect and block malicious file uploads. Conduct regular audits of server directories to identify unauthorized or suspicious files. Additionally, enforce strong authentication and authorization controls to minimize the risk of credential compromise. Organizations should also prepare incident response plans to quickly address any exploitation attempts. Finally, consider isolating LangBot instances in segmented network zones to limit lateral movement in case of compromise.