Path Traversal in Python-Multipart

Mitigation Strategies

To mitigate this vulnerability, upgrade the python-multipart library to version 0.0.30 or later.

The fixed version ignores extended parameters in Content-Disposition headers, ensuring that plain parameters remain authoritative and preventing attackers from smuggling different field names or filenames past upstream inspectors.

Useful 0 Not Useful 0

Impact Analysis

This vulnerability can allow attackers to bypass upstream security inspections by smuggling different field names or filenames to the backend server.

• Bypassing access controls that rely on inspecting multipart/form-data fields.
• Potential path traversal attacks through decoded sequences like ..%2F in filenames.
• Confusion or unexpected behavior caused by decoded control bytes such as %00 in filenames.

Overall, this can lead to unauthorized access or manipulation of backend systems.

Useful 0 Not Useful 0

Executive Summary

CVE-2026-53537 is a vulnerability in the python-multipart library versions before 0.0.30. The issue arises because the library parses Content-Disposition headers using RFC 2231/5987 extended parameter syntax, which is not allowed by RFC 7578 §4.2 for multipart/form-data.

This parsing causes the extended parameters (like filename*=charset'lang'value) to override the plain parameters, allowing an attacker to smuggle different field names or filenames past upstream inspectors such as web application firewalls, proxies, or gateways. This means the backend may receive a different field name or filename than what the upstream components see.

This discrepancy can be exploited to bypass security controls or cause confusion in handling uploaded files.

Useful 0 Not Useful 0

Compliance Impact

The vulnerability in python-multipart involves the improper handling of extended parameters in multipart/form-data headers, which can be exploited to smuggle different field names or filenames past upstream inspectors to the backend.

This manipulation could potentially lead to bypassing access controls or path traversal attacks, which may result in unauthorized access or exposure of sensitive data.

Such unauthorized access or data exposure could negatively impact compliance with standards and regulations like GDPR or HIPAA, which require strict controls over personal and sensitive data to prevent breaches.

Therefore, this vulnerability poses a risk to maintaining compliance with these regulations by enabling attackers to circumvent security mechanisms designed to protect sensitive information.

Useful 0 Not Useful 0

Detection Guidance

This vulnerability involves the parsing of Content-Disposition headers with extended parameters in multipart/form-data requests. Detection involves inspecting HTTP traffic for multipart/form-data requests that include extended parameter syntax such as filename*=charset'lang'value or filename*0/filename*1 continuation forms.

You can detect potential exploitation attempts by searching for these extended parameters in HTTP requests passing through your network or system.

• Use network traffic analysis tools like tcpdump or Wireshark to capture HTTP traffic and filter for multipart/form-data content.
• Example tcpdump command to capture HTTP traffic on port 80: sudo tcpdump -A -s 0 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
• Use grep or similar tools to search captured HTTP requests for the presence of 'filename*=' or 'name*=' strings, which indicate extended parameter usage.
• Example grep command on a captured HTTP request log: grep -iE 'filename\*=|name\*=' captured_http_requests.log

Additionally, review your python-multipart library version in your environment. If it is prior to 0.0.30, it is vulnerable and should be upgraded.

Useful 0 Not Useful 0