CVE-2026-15064: IBM WebSphere HTTP Smuggling Flaw

CVE-2026-15064 Overview

CVE-2026-15064 is an HTTP Response Smuggling vulnerability in IBM WebSphere Application Server. The flaw affects WebSphere Application Server versions 9.0 and 8.5, along with WebSphere Application Server Liberty versions 17.0.0.3 through 26.0.0.7. The root cause is improper handling of non-standard HTTP version tokens, tracked under CWE-444 (Inconsistent Interpretation of HTTP Requests). Attackers can exploit the parsing inconsistency across network-facing HTTP infrastructure without authentication or user interaction.

Critical Impact
Successful exploitation enables attackers to smuggle HTTP responses, poison caches, hijack sessions, and bypass front-end security controls, compromising confidentiality and integrity of application traffic.

Affected Products

IBM WebSphere Application Server 9.0
IBM WebSphere Application Server 8.5
IBM WebSphere Application Server Liberty 17.0.0.3 through 26.0.0.7

Discovery Timeline

2026-07-28 - CVE-2026-15064 published to NVD
2026-07-30 - Last updated in NVD database

Technical Details for CVE-2026-15064

Vulnerability Analysis

The vulnerability resides in how IBM WebSphere Application Server parses HTTP version tokens in incoming requests. When the server receives non-standard HTTP version strings, it interprets them inconsistently compared to upstream proxies, load balancers, or reverse proxies. This mismatch enables HTTP Response Smuggling, a variant of request/response desynchronization attacks. Attackers craft requests that intermediary devices parse one way and the WebSphere back end parses another way. The resulting desynchronization allows adversaries to inject data into other users' responses. Impact includes cache poisoning, credential theft, cross-user response manipulation, and bypass of front-end web application firewalls. The attack requires high complexity because the attacker must understand the exact intermediary chain and craft payloads that trigger the parser divergence reliably.

Root Cause

The server fails to strictly validate or normalize HTTP version tokens in request lines. Non-standard tokens such as malformed HTTP/ version strings are accepted and processed, but front-end proxies interpret the same tokens differently. This creates a parser differential aligned with CWE-444, the classic precondition for HTTP smuggling.

Attack Vector

Exploitation occurs remotely over the network against any WebSphere endpoint fronted by an HTTP intermediary. The attacker sends a crafted HTTP request containing a non-standard version token, followed by a smuggled payload. The intermediary forwards the traffic as a single request, while WebSphere treats part of the payload as a separate response boundary. Subsequent users of the connection or shared cache receive attacker-controlled content. Refer to the IBM Support advisory for technical details.

Detection Methods for CVE-2026-15064

Indicators of Compromise

HTTP request logs containing malformed or non-standard HTTP version tokens such as HTTP/1.11, HTTP/0.9x, or embedded whitespace in the version field.
Unexpected Content-Length and Transfer-Encoding header combinations reaching WebSphere back ends.
Cache entries or session responses served to users who never requested the associated resource.
Anomalous response bodies delivered on keep-alive connections after crafted requests.

Detection Strategies

Inspect access logs for HTTP request lines that deviate from RFC 7230 version syntax and alert on any non-HTTP/1.0 or non-HTTP/1.1 tokens.
Deploy WAF or reverse proxy rules that normalize or reject malformed version strings before traffic reaches WebSphere.
Correlate front-end proxy logs with WebSphere access logs to identify request count or byte count discrepancies indicating desynchronization.

Monitoring Recommendations

Enable verbose HTTP channel logging on WebSphere Liberty and analyze for parser warnings tied to version tokens.
Monitor cache infrastructure for unauthorized content substitution and abnormal cache hit patterns.
Baseline connection reuse behavior on load balancers and alert on unexpected mid-connection response boundaries.

How to Mitigate CVE-2026-15064

Immediate Actions Required

Apply the interim fixes and updated versions published by IBM in the IBM Support advisory for node 7281625.
Inventory all WebSphere Application Server 8.5, 9.0, and Liberty 17.0.0.3 through 26.0.0.7 deployments and prioritize internet-facing instances.
Configure front-end proxies to strictly validate HTTP version tokens and drop non-compliant requests.

Patch Information

IBM has published remediation guidance and fixes on the IBM Support Page. Administrators should install the vendor-supplied interim fix or upgrade to a fixed release for both traditional WebSphere Application Server and WebSphere Liberty. Verify patch application by confirming the fix pack level after deployment.

Workarounds

Terminate HTTP connections at a strict-parsing reverse proxy or WAF that rejects malformed HTTP version tokens.
Disable HTTP connection reuse and keep-alive on intermediaries where feasible to reduce smuggling impact.
Restrict WebSphere HTTP endpoints to trusted networks until patching is complete.

bash

# Example: reject non-standard HTTP version tokens at an NGINX front end
# Place in the server block fronting WebSphere
if ($server_protocol !~ "^HTTP/1\.(0|1)$") {
    return 400;
}