CVE-2026-94439 in nethttp
Summary
by MITRE • 10/09/2026
When an HTTP server handler sends a 2xx response to an HTTP/1 CONNECT request and returns without hijacking the connection, the server improperly continues to read and serve requests from the connection. Since a 2xx response to an HTTP/1 CONNECT converts the connection into a tunnel, the server should not treat the connection as continuing to contain HTTP. The impact of this misbehavior is mostly limited to potential request smuggling, where an intermediate proxy considers the data on the connection to be tunneled and the server considers it to be HTTP.
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Analysis
by VulDB Data Team • 10/09/2026
The vulnerability described constitutes a critical protocol handling flaw within HTTP/1 servers that fail to properly transition connections into tunnels upon receiving a successful CONNECT response. In standard HTTP/1.1 operations, when a client issues a CONNECT request to establish a secure tunnel through an intermediary proxy or server for purposes such as HTTPS communication, the server is expected to respond with a 2xx status code and then cease interpreting subsequent data on that connection as HTTP requests. Instead, it must treat the stream as raw binary data destined for end-to-end encryption between the client and the final destination host. However, in this specific misbehavior, the server acknowledges the tunnel establishment by sending the appropriate success response but erroneously continues to parse incoming bytes as if they were standard HTTP request lines and headers. This deviation from RFC 7231 and related specifications creates a fundamental ambiguity regarding where one protocol ends and another begins on the same TCP connection.
This architectural inconsistency primarily facilitates HTTP Request Smuggling attacks, specifically those relying on conflicting interpretations of message boundaries by different entities in the network path. When an intermediate proxy or load balancer observes the 2xx response to CONNECT, it correctly assumes that all subsequent traffic is tunneled and opaque, forwarding it directly without inspection. Conversely, the vulnerable server continues to interpret this same data as HTTP requests. This discrepancy allows an attacker to craft malicious payloads that are interpreted differently by each party. For instance, an attacker might embed a second request within what appears to be tunnel data from the proxy's perspective but is parsed as a valid new HTTP request by the backend server. This can lead to unauthorized access to internal resources, bypassing authentication controls or security policies enforced at the network edge because the intermediate device never inspected the smuggled payload.
The operational impact of this vulnerability extends beyond simple request smuggling. It undermines the integrity and confidentiality guarantees that HTTPS tunnels are designed to provide. If an attacker can inject requests into a tunnel intended for encrypted traffic, they may be able to manipulate backend services in ways that bypass rate limiting, input validation, or access control lists applied by front-end infrastructure. Furthermore, this flaw can potentially lead to server-side request forgery if the smuggled requests are directed toward internal microservices or APIs that trust connections originating from trusted proxies. The severity is heightened because many modern web architectures rely heavily on reverse proxies and CDNs for security enforcement; exploiting this misconfiguration effectively neutralizes those protective layers by tricking them into ignoring malicious traffic while allowing it to reach vulnerable backend systems.
Mitigation strategies must focus on strict adherence to protocol specifications regarding connection state management. Server implementations should be audited to ensure that upon sending a 2xx response to an HTTP CONNECT request, the handler immediately stops parsing input as HTTP and switches to raw byte streaming mode for tunneling purposes. Developers must verify that their web server software is updated to versions where this logic error has been corrected. Additionally, network security teams should implement strict validation rules at intermediate proxies to detect anomalies in connection handling, such as unexpected protocol transitions or malformed headers within established tunnels. Regular penetration testing focused on HTTP desynchronization and request smuggling techniques can help identify instances of this vulnerability before it is exploited in production environments. Aligning server behavior with industry standards like CWE-436 for interpretation ambiguity and mapping the attack vector to MITRE ATT&CK technique T1071 Application Layer Protocol: Web Protocols ensures a comprehensive defense against such protocol-level exploits.