CVE-2026-11710: IBM WebSphere HTTP Request Smuggling Flaw

CVE-2026-11710 Overview

CVE-2026-11710 is an HTTP request smuggling vulnerability in IBM WebSphere Application Server 8.5. The flaw stems from improper handling of Content-Length headers during HTTP request parsing, classified under [CWE-444] Inconsistent Interpretation of HTTP Requests. Attackers on the network can craft ambiguous requests that front-end and back-end servers parse differently, allowing malicious payloads to bypass security controls or poison cached responses. The vulnerability carries a CVSS 3.1 score of 6.5 with high attack complexity and no authentication requirement.

Critical Impact
Successful exploitation enables attackers to smuggle unauthorized HTTP requests through proxy layers, potentially bypassing authentication filters, accessing restricted resources, or hijacking user sessions on affected WebSphere deployments.

Affected Products

IBM WebSphere Application Server 8.5
Deployments fronted by HTTP proxies or load balancers relying on Content-Length parsing
Applications hosted on WebSphere 8.5 that process untrusted HTTP traffic

Discovery Timeline

2026-09-18 - CVE-2026-11710 published to NVD
2026-09-22 - Last updated in NVD database

Technical Details for CVE-2026-11710

Vulnerability Analysis

HTTP request smuggling occurs when intermediary systems and origin servers disagree on where one HTTP request ends and the next begins. IBM WebSphere Application Server 8.5 improperly processes the Content-Length header, creating parsing desynchronization between upstream proxies and the WebSphere back end. An attacker sends a single crafted request containing conflicting length indicators. The proxy forwards what it interprets as one request, while WebSphere processes it as two, leaving smuggled bytes prepended to the next legitimate request in the connection pipeline.

This desynchronization enables attackers to prepend arbitrary content to another user's request, poison shared caches, or bypass front-end security controls such as web application firewalls and authentication proxies. The attack complexity is elevated because exploitation depends on specific proxy and WebSphere version combinations, timing conditions, and HTTP connection reuse behavior.

Root Cause

The root cause lies in inconsistent handling of Content-Length headers by the WebSphere HTTP parser. When a request contains duplicate Content-Length values, conflicting Content-Length and Transfer-Encoding headers, or malformed length declarations, WebSphere fails to reject the request or normalize it to match upstream proxy interpretation. See the IBM Support Page for vendor technical details.

Attack Vector

The attack vector is network-based and requires no privileges or user interaction. An attacker sends specially constructed HTTP requests directly to a proxy that forwards traffic to a vulnerable WebSphere 8.5 back end. Verified proof-of-concept exploit code is not publicly available at this time, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. Refer to the IBM Support Page for exploitation prerequisites specific to affected configurations.

Detection Methods for CVE-2026-11710

Indicators of Compromise

HTTP requests containing duplicate Content-Length headers or both Content-Length and Transfer-Encoding: chunked headers reaching WebSphere
Unexpected 400-series responses immediately following legitimate requests on the same connection
Cache entries serving content that does not match the originally requested URL
Anomalous session cross-contamination reports from application users

Detection Strategies

Inspect HTTP access logs on proxy and WebSphere tiers for length header inconsistencies and mismatched request boundaries
Deploy web application firewall rules that reject requests with ambiguous framing headers before they reach WebSphere
Correlate front-end and back-end request counts on shared connections to identify desynchronization

Monitoring Recommendations

Enable verbose HTTP request logging on WebSphere and upstream proxies to capture raw headers for forensic review
Alert on repeated requests from single source IPs that combine Transfer-Encoding and Content-Length headers
Monitor cache hit patterns for anomalies suggesting cache poisoning attempts

How to Mitigate CVE-2026-11710

Immediate Actions Required

Apply the IBM security update referenced on the IBM Support Page as the primary remediation
Inventory all WebSphere Application Server 8.5 instances and prioritize internet-facing deployments
Configure upstream proxies to reject HTTP requests containing conflicting Content-Length or Transfer-Encoding headers

Patch Information

IBM has published remediation guidance for CVE-2026-11710 through its official support channel. Administrators should review the IBM Support Page for interim fix identifiers, applicable fix packs, and any prerequisite service levels required before applying the patch to WebSphere Application Server 8.5.

Workarounds

Terminate HTTP keep-alive connections between the proxy and WebSphere back end to prevent request queueing on shared sockets
Enforce strict HTTP/1.1 compliance at the proxy layer and drop malformed length headers before forwarding
Restrict direct network access to WebSphere listeners so only trusted, HTTP-normalizing proxies can reach the server

bash

# Example proxy hardening: reject conflicting framing headers before WebSphere
# (adapt to your reverse proxy configuration language)
if ($http_transfer_encoding ~* chunked) {
    if ($http_content_length) {
        return 400;
    }
}