CVE-2026-63718 Overview
CVE-2026-63718 is an HTTP response smuggling vulnerability in Apache HTTP Server affecting the mod_proxy_uwsgi module. The flaw stems from inconsistent interpretation of HTTP responses when a crafted uwsgi response contains a Transfer-Encoding header. Attackers can exploit this to desynchronize HTTP response parsing between the proxy and downstream clients. The vulnerability affects Apache HTTP Server versions 2.4.30 through 2.4.68. It is classified under [CWE-444] (Inconsistent Interpretation of HTTP Requests).
Critical Impact
Attackers can smuggle crafted responses through mod_proxy_uwsgi, enabling cache poisoning, response splitting, and injection of malicious content into legitimate client sessions.
Affected Products
- Apache HTTP Server 2.4.30 through 2.4.68
- Deployments using the mod_proxy_uwsgi module
- Reverse proxy configurations fronting uwsgi application servers
Discovery Timeline
- 2026-10-01 - CVE-2026-63718 published to NVD
- 2026-10-06 - Last updated in NVD database
Technical Details for CVE-2026-63718
Vulnerability Analysis
The vulnerability resides in mod_proxy_uwsgi, the Apache module responsible for proxying requests to uwsgi application servers. When the backend uwsgi application returns a response containing a Transfer-Encoding header, Apache and downstream HTTP clients interpret the response boundary differently. This parsing inconsistency allows an attacker who controls or influences the backend response to append arbitrary HTTP response data that the client treats as a separate, legitimate response.
Response smuggling attacks of this class enable cache poisoning against shared caches, hijacking of subsequent responses on persistent connections, and injection of attacker-controlled content into other users' sessions. The impact concentrates on integrity, consistent with the CVSS vector indicating high integrity impact without confidentiality or availability consequences.
Root Cause
The root cause is improper handling of the Transfer-Encoding header in uwsgi responses. HTTP/1.1 specifications require strict precedence between Transfer-Encoding and Content-Length, but mod_proxy_uwsgi does not reconcile these headers consistently with downstream parsers. The resulting ambiguity creates the smuggling primitive.
Attack Vector
Exploitation requires an attacker to influence the uwsgi backend response, typically by compromising an upstream application or exploiting a backend input-handling flaw that reflects attacker data into response headers. Once a crafted response with a malicious Transfer-Encoding header traverses mod_proxy_uwsgi, the proxy forwards content that downstream clients or intermediate caches parse as multiple distinct responses. The attack is network-reachable, requires no authentication, and no user interaction. See the Apache HTTPD Vulnerabilities advisory and the OpenWall OSS-Security discussion for technical references.
Detection Methods for CVE-2026-63718
Indicators of Compromise
- Backend uwsgi responses containing unexpected Transfer-Encoding: chunked headers alongside Content-Length
- Cached responses in front-end proxies or CDNs with content that does not match the originally requested URL
- Anomalous HTTP response boundaries or duplicate response bodies observed on persistent connections
- Client reports of receiving responses intended for unrelated requests
Detection Strategies
- Inspect Apache access and error logs for uwsgi backend responses with conflicting framing headers
- Deploy a web application firewall rule to flag or strip Transfer-Encoding headers originating from uwsgi backends
- Compare response hashes between origin and cache tiers to identify poisoned entries
- Correlate HTTP parsing anomalies across proxy, cache, and client telemetry
Monitoring Recommendations
- Monitor the version of httpd and mod_proxy_uwsgi across the fleet and alert on versions in the 2.4.30 to 2.4.68 range
- Instrument reverse proxies to log full response headers from uwsgi backends during triage
- Track CDN and shared cache purge events for unexplained content mismatches
How to Mitigate CVE-2026-63718
Immediate Actions Required
- Upgrade Apache HTTP Server to a release later than 2.4.68 that patches mod_proxy_uwsgi
- Audit all reverse proxy configurations that use ProxyPass or SetHandler with the uwsgi:// scheme
- Review the trust boundary with uwsgi backends and restrict which applications can set Transfer-Encoding headers
- Purge shared caches that may contain poisoned responses generated prior to patching
Patch Information
Refer to the Apache HTTPD 2.4 Vulnerabilities page for the fixed version and release notes. Apply vendor-supplied packages from your Linux distribution as soon as they are available, and restart httpd to load the updated module.
Workarounds
- Temporarily disable mod_proxy_uwsgi and use an alternative proxy module such as mod_proxy_http where feasible
- Deploy a reverse proxy rule that strips Transfer-Encoding headers from uwsgi backend responses before they reach clients
- Harden backend uwsgi applications to prevent reflection of untrusted input into response headers
- Terminate HTTP/1.1 persistent connections between the proxy and downstream clients where response integrity cannot be guaranteed
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.