The vulnerability identified as CVE-2026-24486 affects the python-multipart library, a streaming multipart parser used in Python applications to handle file uploads. The flaw is a path traversal vulnerability (CWE-22) that occurs when the library is configured with the non-default options UPLOAD_DIR and UPLOAD_KEEP_FILENAME=True. Under these conditions, an attacker can craft a malicious filename in a multipart upload request that includes directory traversal sequences (e.g., ../) to escape the designated upload directory. This allows the attacker to write files to arbitrary locations on the filesystem, potentially overwriting critical files or placing malicious executables. The vulnerability does not require any authentication or user interaction, making it remotely exploitable over the network. The CVSS v3.1 base score of 8.6 reflects the ease of exploitation (network vector, low attack complexity), no privileges required, and the significant impact on integrity and some impact on availability and confidentiality. The vulnerability was patched in python-multipart version 0.0.22 by properly sanitizing and restricting file paths to prevent directory traversal. No known exploits are reported in the wild yet, but the high severity and ease of exploitation make it a critical risk for applications using vulnerable versions. Organizations relying on python-multipart for handling file uploads should upgrade promptly or disable the UPLOAD_KEEP_FILENAME option to mitigate the vulnerability.

For European organizations, this vulnerability poses a significant risk especially to web applications and services that accept file uploads using python-multipart. Successful exploitation can lead to arbitrary file writes outside intended directories, enabling attackers to overwrite configuration files, deploy web shells, or place malware, thereby compromising system integrity and potentially leading to full system compromise. Confidential data could be exposed if sensitive files are overwritten or accessed. Availability could be impacted if critical system files are corrupted or replaced. The lack of authentication and user interaction requirements increases the risk of automated exploitation attempts. Sectors such as finance, healthcare, government, and critical infrastructure that rely on Python-based web services are particularly vulnerable. The vulnerability could also be leveraged as a foothold for lateral movement within networks, escalating the overall impact. Given the widespread use of Python and open-source libraries in Europe, the threat surface is considerable.

To mitigate this vulnerability, organizations should immediately upgrade python-multipart to version 0.0.22 or later, where the path traversal issue is fixed. If upgrading is not immediately feasible, disable the UPLOAD_KEEP_FILENAME configuration option to prevent the library from preserving user-supplied filenames, thereby avoiding the path traversal vector. Additionally, implement strict input validation and sanitization on filenames at the application level to reject any path traversal sequences or suspicious characters. Employ filesystem permissions to restrict the write access of the application process to only the intended upload directories, minimizing the impact of any potential exploitation. Use application-layer security controls such as web application firewalls (WAFs) to detect and block malicious multipart requests containing directory traversal payloads. Regularly audit and monitor file upload directories and logs for unusual activity. Finally, incorporate secure coding practices and dependency management to promptly identify and patch vulnerable libraries.