CVE-2025-58056: Netty Request Smuggling Vulnerability

CVE-2025-58056 Overview

CVE-2025-58056 is an HTTP Request Smuggling vulnerability affecting Netty, the popular asynchronous event-driven network application framework used for developing high-performance protocol servers and clients. The vulnerability exists in how Netty processes chunked transfer encoding, where it incorrectly accepts standalone line feed (LF) characters as chunk-size line terminators instead of requiring the proper carriage return line feed (CRLF) sequence as mandated by HTTP/1.1 standards defined in RFC 9112.

When deployed behind reverse proxies that interpret LF characters differently—treating them as part of the chunk extension rather than as terminators—attackers can craft malicious HTTP requests that are parsed as a single request by the proxy but processed as two separate requests by Netty. This parsing discrepancy enables HTTP request smuggling attacks, potentially allowing attackers to bypass security controls, access unauthorized resources, or poison web caches.

Critical Impact

Attackers can exploit parsing inconsistencies between reverse proxies and Netty to smuggle malicious HTTP requests, potentially bypassing security controls, hijacking user sessions, or performing cache poisoning attacks.

Affected Products

• Netty version 4.1.124.Final
• Netty versions 4.2.0.Alpha3 through 4.2.4.Final

Discovery Timeline

• September 3, 2025 - CVE-2025-58056 published to NVD
• September 8, 2025 - Last updated in NVD database

Technical Details for CVE-2025-58056

Vulnerability Analysis

This vulnerability is classified as CWE-444 (Inconsistent Interpretation of HTTP Requests), commonly known as HTTP Request Smuggling. The flaw arises from Netty's non-compliant parsing of chunked transfer encoding in HTTP/1.1 requests.

According to RFC 9112, chunk-size lines in chunked transfer encoding must be terminated with CRLF (\r\n). However, vulnerable versions of Netty accept a standalone LF (\n) as a valid terminator, regardless of whether it is preceded by a CR (\r). This lenient parsing behavior creates an exploitable inconsistency when Netty operates behind reverse proxies that strictly follow HTTP standards or interpret these characters differently.

The attack surface exists in network-accessible deployments where Netty-based applications receive HTTP traffic through intermediate proxies, load balancers, or web application firewalls that handle chunked encoding parsing differently.

Root Cause

The root cause is improper input validation in Netty's HTTP codec when parsing chunk-size lines in chunked transfer encoding. The code fails to enforce the HTTP/1.1 specification requirement that chunk-size lines must terminate with the CRLF sequence (\r\n). By accepting a bare LF character as a valid line terminator, Netty deviates from the standard behavior expected by other HTTP implementations, creating the parsing discrepancy that enables request smuggling.

Attack Vector

The attack is network-based and requires the attacker to send specially crafted HTTP requests to a target system where Netty operates behind a reverse proxy with different chunk parsing behavior. The attacker constructs a request where:

1. The reverse proxy interprets the LF as part of the chunk extension, seeing one complete request
2. Netty interprets the LF as a line terminator, parsing the payload as two separate requests
3. The "smuggled" second request is processed by Netty with the security context of the legitimate first request

This parsing differential allows attackers to inject arbitrary HTTP requests that bypass proxy-level security controls. The attack does not require authentication or user interaction, though it does require specific proxy-backend configurations to be exploitable.

Detailed technical analysis of this chunked encoding parsing vulnerability class is available in the W4KE Blog on Funky Chunks and the GitHub Security Advisory.

Detection Methods for CVE-2025-58056

Indicators of Compromise

• HTTP requests containing unusual chunk-size line formats with standalone LF terminators without preceding CR characters
• Anomalous HTTP request patterns where backend servers log different requests than what appeared in proxy logs
• Evidence of cache poisoning or session hijacking attempts
• Discrepancies between proxy access logs and application-level request logs indicating potential request smuggling

Detection Strategies

• Implement deep packet inspection rules to flag HTTP requests with non-standard chunked encoding line terminators
• Deploy web application firewalls configured to detect request smuggling patterns and chunk encoding anomalies
• Monitor for discrepancies between frontend proxy and backend server request counts that may indicate smuggled requests
• Use application-level logging to correlate HTTP requests across proxy and backend layers

Monitoring Recommendations

• Enable detailed access logging on both reverse proxies and Netty-based applications to facilitate correlation analysis
• Implement alerting on unusual HTTP request patterns, particularly those involving chunked transfer encoding
• Monitor for signs of cache poisoning such as unexpected cache entries or user complaints about incorrect content
• Review security logs for evidence of unauthorized access or privilege escalation that could result from successful smuggling attacks

How to Mitigate CVE-2025-58056

Immediate Actions Required

• Upgrade Netty to version 4.1.125.Final or 4.2.5.Final immediately to address the vulnerability
• Audit your infrastructure to identify all applications using affected Netty versions
• Review proxy and load balancer configurations to understand potential exposure
• Implement additional request validation at the application layer as defense in depth

Patch Information

Netty has released patched versions that enforce strict CRLF terminator requirements in chunked transfer encoding parsing:

• Version 4.1.125.Final - For users on the 4.1.x branch
• Version 4.2.5.Final - For users on the 4.2.x branch

The fix is available in commit edb55fd8e0a3bcbd85881e423464f585183d1284, implemented via Pull Request #15611.

Organizations should prioritize this update for any Netty deployments accessible from untrusted networks or operating behind reverse proxies.

Workarounds

• Configure reverse proxies to normalize chunked encoding before forwarding requests to Netty backends
• Implement strict HTTP request validation at the proxy layer to reject requests with non-standard line terminators
• Deploy web application firewall rules to detect and block HTTP request smuggling attempts
• Consider temporarily disabling chunked transfer encoding support if operationally feasible until patches can be applied

# Example: Check Netty version in Maven projects
grep -r "netty" pom.xml | grep -E "version|artifactId"

# Update Netty dependency to patched version in pom.xml
# Change version from 4.1.124.Final to 4.1.125.Final
# Or change version from 4.2.x to 4.2.5.Final