A sophisticated HTTP request smuggling attack that exploits inconsistent parsing behaviors between front-end proxy servers and back-end application servers.
This newly discovered technique leverages malformed chunked transfer encoding extensions to bypass established security controls and inject unauthorized secondary requests into web applications.
Key Takeaways
1. Exploits malformed HTTP chunked encoding to create front-end/back-end parsing discrepancies.
2. Bypasses security controls by injecting hidden secondary requests.
3. Apply patches and migrate to the HTTP/2 protocol.
The attack targets a fundamental vulnerability in HTTP/1.1 protocol implementation, where different servers interpret ambiguous request formatting inconsistently.
Attackers can exploit these parsing discrepancies to circumvent Web Application Firewalls (WAFs), Content Delivery Networks (CDNs), and load balancers, potentially gaining unauthorized access to sensitive backend resources.
HTTP Smuggling Vulnerability
Imperva reports that the attack mechanism centers on HTTP/1.1’s chunked transfer encoding feature, which allows message bodies to be transmitted in segments using the Transfer-Encoding: chunked header.
According to RFC 9112 specifications, each chunk includes a header containing the size in hexadecimal format, followed by optional chunk extensions prefixed with semicolons.
Researchers discovered that attackers can manipulate chunk extension parsing by sending malformed headers containing bare semicolons without proper extension names.
This creates a critical parsing discrepancy where front-end systems interpret the malformed syntax differently than backend servers.
The attack sequence follows this pattern: the attacker sends a chunk size line ending with a semicolon but no extension name, causing the front-end parser to treat the entire sequence as a single request while the back-end parser interprets the newline after the semicolon as marking the end of the chunk header.
Smuggled request
This allows attackers to embed secondary HTTP requests after zero-length chunks, which backend systems process as legitimate separate requests, effectively bypassing front-end security validation.
The vulnerability stems from HTTP/1.1’s inherent design weaknesses, particularly its reliance on text-based parsing and multiple methods for expressing message boundaries through Content-Length headers, Transfer-Encoding specifications, or delimiters.
Many server implementations prioritize compatibility over strict RFC compliance, leading to lenient parsing of malformed requests that create exploitable inconsistencies.
Security experts emphasize that comprehensive patches have been deployed across affected systems, with organizations maintaining current software versions receiving full protection against this attack vector.
However, the most effective long-term mitigation involves migrating to HTTP/2, which employs binary framing mechanisms that eliminate the ambiguous parsing scenarios, enabling request smuggling attacks.
This reinforces the critical importance of protocol-level security considerations and highlights HTTP/1.1’s fundamental vulnerabilities that continue enabling sophisticated bypass techniques despite existing protective measures.
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