Executive Summary

The vulnerability in http-proxy-middleware affects the router feature's host+path substring matching mechanism. The middleware uses unanchored substring matching on the Host header combined with the request path to determine backend routing. An attacker can craft a Host header that is a superstring containing a configured host+path key, causing the request to be routed to an unintended backend.

For example, if the router key is "localhost:3000/api" and the attacker sends a Host header like "evillocalhost:3000", the concatenated string "evillocalhost:3000/api" still contains the router key as a substring. This causes the request to be proxied incorrectly.

This happens because the middleware checks if the concatenated Host header and path string contains the router key anywhere, rather than matching it exactly or anchored, allowing bypass of intended routing rules.

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Impact Analysis

This vulnerability can impact you by allowing unauthenticated external clients to bypass routing boundaries within your application.

Specifically, it can cause backend-selection integrity issues, where requests are routed to unintended backends. This can break tenant separation or segregation between public and sensitive upstream services.

While it does not directly compromise confidentiality or availability, it can lead to unauthorized access to different backend services, potentially exposing sensitive data or functionality.

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Detection Guidance

This vulnerability can be detected by checking if your system is running a vulnerable version of the http-proxy-middleware library (versions >= 0.16.0 < 2.0.10, >= 3.0.0 < 3.0.6, and >= 4.0.0 < 4.1.0) and if the proxy-table router feature is enabled with host+path rules.

To detect potential exploitation attempts on your network, you can monitor HTTP requests for suspicious Host headers that are superstrings of configured host+path keys.

• Use network packet capture tools like tcpdump or Wireshark to filter HTTP requests with unusual Host headers.
• Example tcpdump command to capture HTTP requests with Host headers containing a specific substring (replace example.com with your configured host):

tcpdump -A -s 0 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)' | grep 'Host: evillocalhost'

• Check your application logs for requests with Host headers that do not exactly match configured hosts but contain them as substrings.

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Mitigation Strategies

The immediate mitigation step is to upgrade the http-proxy-middleware library to a fixed version: 2.0.10, 3.0.6, or 4.1.0 or later.

If upgrading immediately is not possible, consider disabling the proxy-table router feature or avoid using host+path rules until the update can be applied.

Additionally, implement strict validation of Host headers in your application or at the proxy level to reject unexpected or malformed Host headers.

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