CVE-2026-94243: Apache Sling Security Bundle Auth Bypass

CVE-2026-94243 Overview

CVE-2026-94243 is an origin validation weakness in the Apache Sling Security Bundle. The ReferrerFilter component accepts evidence weaker than a full origin when validating incoming requests. This allows network-based attackers to bypass referrer-based protections designed to defend against cross-site request forgery and related attacks. The flaw affects all Apache Sling Security Bundle versions prior to 1.3.2 and is tracked under CWE-346: Origin Validation Error. The Apache Sling project has released version 1.3.2 to remediate the issue.

Critical Impact

Attackers can bypass the ReferrerFilter over the network without authentication or user interaction, undermining a security control intended to block cross-origin request forgery against Sling-based applications.

Affected Products

• Apache Sling Security Bundle versions before 1.3.2
• Applications built on the Apache Sling framework that rely on ReferrerFilter for request origin validation
• Adobe Experience Manager and other downstream products that embed vulnerable versions of the Sling Security Bundle

Discovery Timeline

• 2026-09-23 - CVE-2026-94243 published to the National Vulnerability Database
• 2026-09-23 - Vulnerability disclosed on the Apache Security mailing list and OpenWall OSS-Security list
• 2026-09-23 - Last updated in NVD database

Technical Details for CVE-2026-94243

Vulnerability Analysis

Apache Sling ships a security bundle that includes a ReferrerFilter. The filter inspects the HTTP Referer header on state-changing requests to confirm they originate from an approved host. The filter is a common defense against cross-site request forgery in Sling-based content management deployments.

In versions before 1.3.2, the filter accepts evidence that is weaker than a full origin. In practice this means the filter can be satisfied by request metadata that does not uniquely identify the true request origin. An attacker who can induce a victim's browser or a network client to submit a request can craft that request so the weakened check passes. The result is that requests originating from untrusted sources are treated as if they came from a trusted origin, defeating the filter's purpose.

The impact aligns with the CWE-346 origin validation error category and touches confidentiality, integrity, and availability of the protected application.

Root Cause

The root cause is incomplete validation of the request origin inside ReferrerFilter. The component treats sub-origin evidence, such as partial header values or missing components, as sufficient rather than requiring a full scheme, host, and port match against the allow-list.

Attack Vector

Exploitation is remote and unauthenticated. An attacker crafts a request that manipulates or omits the Referer header in a way that satisfies the weakened check. The request is delivered either directly or through a victim's browser interacting with attacker-controlled content. No user interaction beyond visiting a page is required in browser-driven scenarios. See the Apache Security mailing list thread and the OpenWall OSS-Security discussion for maintainer commentary.

No public exploit or proof of concept is currently available. The vulnerability mechanism is described in prose because no verified exploitation code has been released.

Detection Methods for CVE-2026-94243

Indicators of Compromise

• Unexpected state-changing POST, PUT, or DELETE requests to Sling endpoints with missing, empty, or malformed Referer headers
• Requests to administrative Sling paths such as /system/console, /bin, or /libs from external client IP addresses
• Spikes in HTTP 200 responses on endpoints previously guarded by ReferrerFilter denials
• Correlated activity between suspicious outbound browser traffic and inbound Sling write operations from the same user session

Detection Strategies

• Inventory all deployments to identify Apache Sling Security Bundle versions below 1.3.2
• Enable verbose request logging on the Sling instance and alert on write requests that bypass expected referrer checks
• Add web application firewall rules to log requests to Sling endpoints with absent or non-conforming Referer headers
• Correlate authentication events with content modification events to identify requests that lack a plausible user-initiated origin

Monitoring Recommendations

• Forward Sling and reverse-proxy access logs to a centralized analytics platform for baseline and anomaly analysis
• Monitor for changes to Sling repository content that were not preceded by a normal editor session
• Track the ratio of allowed to denied requests through ReferrerFilter and alert on sudden shifts after deployment changes

How to Mitigate CVE-2026-94243

Immediate Actions Required

• Upgrade the Apache Sling Security Bundle to version 1.3.2 or later on every affected instance
• Audit downstream products such as Adobe Experience Manager for bundled versions of the Sling Security Bundle and apply vendor updates
• Review ReferrerFilter configuration and confirm the allow-list only includes hosts required for legitimate operations
• Restrict network access to Sling administrative endpoints to trusted management networks

Patch Information

Apache Sling has released Sling Security Bundle version 1.3.2, which corrects the origin validation logic in ReferrerFilter. Administrators should upgrade the bundle through their normal OSGi deployment process. Refer to the Apache Security mailing list thread for the maintainer announcement and to the OpenWall OSS-Security discussion for additional context.

Workarounds

• Place a reverse proxy or web application firewall in front of Sling and enforce strict Referer and Origin header validation for state-changing methods
• Block requests to sensitive Sling endpoints when the Referer header is absent or does not match an approved host
• Require authenticated sessions and CSRF tokens for all write operations that were previously protected only by ReferrerFilter
• Segment Sling administrative interfaces onto internal networks that are not reachable from the public internet

# Example NGINX snippet enforcing Referer validation for Sling write methods
location / {
    if ($request_method ~ ^(POST|PUT|DELETE|PATCH)$) {
        set $bad_referer 1;
    }
    if ($http_referer ~* ^https://(www\.)?example\.com/) {
        set $bad_referer 0;
    }
    if ($bad_referer) {
        return 403;
    }
    proxy_pass http://sling_backend;
}