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Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')

CVE-2025-43859

High
9.1/10

Summary

The package h11 is a Python implementation of HTTP/1.1. In versions prior to 0.16.0, leniency in h11's parsing of line terminators in chunked-coding message bodies can lead to request smuggling vulnerabilities under certain conditions. Since exploitation requires the combination of a buggy h11 implementation and a buggy (reverse) proxy, fixing either component is sufficient to mitigate the issue.

  • LOW
  • NETWORK
  • HIGH
  • UNCHANGED
  • NONE
  • NONE
  • HIGH
  • NONE

CWE-444 - HTTP Request Smuggling

Entities such as web servers, web caching proxies, and application firewalls could parse HTTP requests differently. When there are two or more such entities in the path of an HTTP request, an attacker can send a specially crafted HTTP request that is seen as two different sets of requests by the attacked devices, allowing the attacker to smuggle a request into one device without the other device being aware of it. Such a vulnerability can prove devastating, for it enables further attacks on the application, like web cache poisoning, session hijacking, cross-site scripting, security bypassing, and sensitive information exposure.

References

Advisory Timeline

  • Published