CVE-2026-75922 in Reverse::Proxy
Summary
by MITRE • 08/23/2026
Reverse::Proxy versions before 0.04 for Perl allow HTTP request smuggling via a percent-decoded PATH_INFO written unencoded to the upstream request line.
PSGI hands PATH_INFO to an application percent-decoded, so a %XX sequence in the client URL has become a raw byte by the time the proxy sees it. The proxy appends that byte string to the upstream base URL, and for an Upgrade tunnel writes it into a request line it serializes itself, re-encoding nothing in either path. The HTTP client that sends the resulting URL does not validate the target either. A path containing %0d%0a therefore arrives at the upstream as a CRLF that ends the request line, and a decoded space, '?' or '#' truncates it the same way.
Everything the client writes after the CRLF is read by the upstream as a second request. On the buffered path it arrives on a keep-alive connection the proxy pools and reuses for other clients. Its method, path and headers are all chosen by the client, and the upstream attributes it to the proxy, so it reaches upstream paths that the proxy's own routing does not expose.
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Analysis
by VulDB Data Team • 08/23/2026
The vulnerability identified in Reverse::Proxy versions prior to 0.04 represents a critical HTTP request smuggling flaw rooted in improper handling of percent-encoded characters within the PATH_INFO variable during reverse proxy operations. This issue arises from the interaction between the PSGI interface, which automatically decodes URL components such as PATH_INFO into raw bytes before passing them to application code, and the proxy's subsequent serialization logic. When a client submits an HTTP request containing encoded special characters like percent-encoded carriage return and line feed sequences (%0d%0a), these are decoded by the PSGI layer into actual control characters. The Reverse::Proxy module then appends this raw byte string directly to the upstream base URL without performing any additional encoding or validation, effectively injecting malicious CRLF sequences into the HTTP request line destined for the backend server.
This technical flaw allows an attacker to perform HTTP request smuggling by manipulating the boundaries between requests on a connection that supports persistent connections such as keep-alive. By inserting %0d%0a within the URL path sent to the proxy, the resulting upstream request contains artificial line breaks that terminate the initial request prematurely. The data following these injected characters is interpreted by the upstream server as a distinct and separate HTTP request. Because this second request arrives on a pooled connection that the proxy may reuse for subsequent legitimate clients from different origins, the smuggled request inherits the context of the proxy rather than the original client. This misattribution enables attackers to bypass access controls or routing logic implemented at the reverse proxy layer, as the upstream server processes commands based on its own configuration and trust in the proxy identity.
The operational impact of this vulnerability is severe, facilitating unauthorized access to internal resources that are not exposed through standard proxy routes. An attacker can craft requests with methods such as POST, PUT, or DELETE, along with custom headers, which are then executed against backend services under the assumption they originate from the trusted reverse proxy. This capability undermines security boundaries established by firewalls and web application firewalls that rely on the proxy to filter traffic. Furthermore, because the smuggled request is processed within an existing connection pool, it can lead to response splitting or cache poisoning if the upstream server's responses are cached based on incomplete or manipulated request identifiers. The vulnerability affects both buffered and non-buffered paths, indicating a systemic failure in input sanitization across different operational modes of the proxy software.
From a classification perspective, this flaw aligns with CWE-113, Improper Neutralization of CRLF Sequences in HTTP Headers, which describes vulnerabilities where applications fail to neutralize special characters that can alter message structure. It also maps closely to ATT&CK technique T1071, Application Layer Protocol: Web Protocols, specifically within the context of request smuggling variants such as CL.TE or TE.CL attacks depending on how the upstream server parses content length and transfer encoding headers in conjunction with the smuggled data. The core issue lies in the lack of strict validation for decoded URL components before they are embedded into protocol-level structures like HTTP request lines, violating fundamental principles of secure web gateway design where all inputs must be treated as untrusted until explicitly validated against a whitelist of allowed characters and formats.
Mitigation strategies require immediate upgrading to Reverse::Proxy version 0.04 or later, which addresses this encoding oversight by ensuring that special characters are properly escaped or rejected before being appended to upstream requests. In the interim, administrators should implement strict input validation at the network perimeter using web application firewalls configured to detect and block CRLF injection attempts within URL paths. Additionally, disabling keep-alive connections for traffic destined to vulnerable backend servers can reduce the impact by preventing connection reuse that facilitates request smuggling. Developers integrating PSGI applications with reverse proxies must also ensure that any manual manipulation of PATH_INFO or other decoded variables includes rigorous sanitization routines to prevent control character injection into serialized HTTP messages.