CVE-2026-91852 Overview
CVE-2026-91852 is a cross-site scripting (XSS) vulnerability in the Apache Sling XSS module. The flaw stems from improper neutralization of user-controllable input during web page generation [CWE-79]. Versions of Apache Sling XSS prior to 2.4.12 are affected. An attacker who can influence input reflected by the module can inject script content that executes in a victim's browser context. Successful exploitation requires user interaction, such as clicking a crafted link, and results in a scope change with limited confidentiality and integrity impact.
Critical Impact
Successful exploitation allows attackers to execute arbitrary script in a victim's browser, enabling session data theft, defacement, and delivery of secondary payloads.
Affected Products
- Apache Sling XSS versions before 2.4.12
Discovery Timeline
- 2026-09-23 - CVE-2026-91852 published to the National Vulnerability Database (NVD)
- 2026-09-23 - Last updated in NVD database
Technical Details for CVE-2026-91852
Vulnerability Analysis
Apache Sling XSS provides encoding and validation utilities intended to prevent cross-site scripting in applications built on the Apache Sling framework. CVE-2026-91852 indicates that the module itself fails to fully neutralize certain input before it is rendered in generated web pages. Attackers can craft input that bypasses the sanitization logic and reaches the response body as executable script.
The attack is network-reachable and requires no prior authentication. It does require user interaction, and it causes a scope change, meaning injected script can affect resources beyond the vulnerable component's security boundary. Impact on confidentiality and integrity is limited, and there is no direct availability impact.
Root Cause
The root cause is improper neutralization of input during web page generation, classified as [CWE-79]. The sanitization routines in Apache Sling XSS versions before 2.4.12 do not adequately encode or reject specific input patterns, allowing attacker-controlled content to be interpreted as HTML or JavaScript by the browser.
Attack Vector
An attacker delivers a crafted URL or input payload to a victim, typically through phishing, a malicious link, or an attacker-controlled page. When the victim's browser renders content processed by the vulnerable Apache Sling XSS module, the injected script executes under the origin of the hosting application. This enables session token theft, forced actions on behalf of the user, and phishing content injection.
No public proof-of-concept exploit is currently listed, and the vulnerability is not on the CISA Known Exploited Vulnerabilities catalog. Technical detail is available in the Apache Mailing List Discussion and the Openwall OSS-Security Update.
Detection Methods for CVE-2026-91852
Indicators of Compromise
- Web server or application logs containing requests with encoded script tags, event handlers such as onerror= or onload=, or javascript: URIs against Sling-backed endpoints
- Unexpected outbound requests from user browsers to attacker-controlled domains after visiting application pages
- Reports from users describing unexpected pop-ups, redirects, or forced actions within the application
Detection Strategies
- Inventory deployed Apache Sling XSS module versions across build manifests and running containers to identify instances below 2.4.12
- Deploy web application firewall (WAF) rules to identify reflected XSS payloads targeting Sling request parameters and paths
- Review application response bodies for reflected input that has not been HTML-encoded, particularly in components that rely on the Sling XSS API
Monitoring Recommendations
- Enable Content Security Policy (CSP) violation reporting to surface injected script execution attempts
- Correlate authentication anomalies with prior page views for signs of session hijacking following XSS delivery
- Monitor referrer patterns for inbound traffic from unfamiliar external domains linking directly into Sling-rendered pages
How to Mitigate CVE-2026-91852
Immediate Actions Required
- Upgrade Apache Sling XSS to version 2.4.12 or later across all deployments
- Audit dependent Sling modules and applications that transitively import the vulnerable version
- Rotate session tokens and API keys if exploitation is suspected in application access logs
Patch Information
The Apache Sling project resolved the vulnerability in Apache Sling XSS version 2.4.12. Users are recommended to upgrade to this version. Release information is published on the Apache Sling News Update page.
Workarounds
- Apply strict Content Security Policy headers to restrict inline script execution and limit script sources
- Deploy WAF signatures to block common XSS payload patterns until the module can be upgraded
- Restrict user input that is reflected through Sling-rendered components using application-level allowlists
# Example Content Security Policy header to reduce XSS exploitability
Content-Security-Policy: default-src 'self'; script-src 'self'; object-src 'none'; base-uri 'self'; frame-ancestors 'self'
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
