HTTP request smuggling
Summary: HTTP request smuggling vulnerabilities arise when websites route HTTP requests through web servers that parse HTTP inconsistently, letting an attacker supply a request that different servers read as having different lengths. The attacker can then poison the back-end TCP/TLS socket and prepend arbitrary data to the next request, which PortSwigger says can bypass front-end security rules, access internal systems, poison web caches, and attack users browsing the site.
Why it matters: The fix PortSwigger gives is to have the front-end server use HTTP/2 exclusively when talking to back-end systems, or to run identical web server software and configuration across the chain; disabling back-end connection reuse reduces impact but does not stop all exploits. Any inconsistency in how your servers parse request length is enough for an attacker to prepend data to the next user's request.
Read the original at PortSwigger Research ↗
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