CVE-2026-103866 in Sling Engine
Summary
by MITRE • 11/05/2026
Inconsistent interpretation of HTTP requests ('HTTP Request/Response smuggling') vulnerability in Apache Sling Engine. The engine's query/form parser accepts non-standard percent-escapes, wrong charsets, and continues request processing on such errors.
This issue affects Apache Sling Engine: before 3.0.4.
Users are recommended to upgrade to version 3.0.4, which fixes the issue.
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Analysis
by VulDB Data Team • 11/05/2026
The vulnerability identified in Apache Sling Engine prior to version 3.0.4 represents a critical flaw rooted in the inconsistent interpretation of HTTP requests, specifically manifesting as an HTTP Request Smuggling attack vector. This class of vulnerabilities arises when there is a discrepancy between how different components or layers within a system parse and interpret incoming network traffic. In this specific instance, the core issue lies within the engine's query string and form data parser, which exhibits overly permissive behavior regarding character encoding standards. The parser accepts non-standard percent-escapes and incorrectly handles wrong charsets without raising appropriate errors that would halt processing. Instead of rejecting malformed input as a security violation or syntax error, the system continues to process the request, leading to ambiguous states where the front-end proxy and the backend application server may disagree on the boundaries of individual HTTP requests.
From a technical perspective, this inconsistency allows an attacker to craft maliciously formatted HTTP requests that exploit the parsing divergence between network intermediaries and the Apache Sling Engine itself. By injecting non-standard percent-escapes or utilizing incorrect charset declarations within form data or query parameters, an adversary can manipulate how the request is segmented. If a reverse proxy interprets one part of the payload as the end of the current request while the backend server continues to read it as part of that same request, subsequent legitimate requests from other users may be misinterpreted by the backend. This desynchronization enables the attacker to inject arbitrary HTTP commands into the stream, effectively allowing them to impersonate other users or access restricted resources without proper authentication. The flaw is particularly dangerous because it relies on subtle encoding anomalies rather than complex logic errors in application business rules, making it difficult to detect through standard input validation checks that assume strict adherence to RFC standards for URL and form data encoding.
The operational impact of this vulnerability extends beyond simple unauthorized access. Successful exploitation can lead to session hijacking, where an attacker steals the authentication cookies or tokens of legitimate users by smuggling their requests into a victim's active session context. It also facilitates cross-site request forgery attacks with elevated privileges if the application performs sensitive actions based on smuggled headers that were intended for administrative functions but are instead processed as part of a regular user transaction. Furthermore, this vulnerability can be used to bypass access control mechanisms or firewalls that rely on strict parsing rules, effectively rendering security perimeter defenses ineffective against encoded smuggling payloads. The ability to continue processing requests despite encoding errors means that the system lacks robust input sanitization at the transport layer, exposing it to persistent and potentially automated attacks targeting any instance of Apache Sling Engine running versions older than 3.0.4.
This vulnerability aligns with CWE-697, which describes incorrect comparison by software, specifically in the context of how different entities interpret message boundaries or data formats. It also maps directly to MITRE ATT&CK technique T1557, Adversary-in-the-Middle, as it enables an attacker to position themselves between a client and server to intercept or manipulate communications. Additionally, aspects of this flaw relate to CWE-20, Improper Input Validation, due to the failure to reject non-compliant percent-encoded sequences that deviate from standard URI syntax definitions. The persistence of such issues in web frameworks highlights the importance of strict adherence to HTTP specifications during request parsing phases.
To mitigate this risk, organizations must immediately upgrade Apache Sling Engine to version 3.0.4 or later, where these parsing inconsistencies have been addressed and corrected. In environments where immediate patching is not feasible, deploying a Web Application Firewall with specific rulesets designed to detect and block HTTP Request Smuggling patterns can provide temporary protection. These security controls should be configured to reject requests containing non-standard percent-escapes in query strings or form data that do not conform to RFC 3986 standards for URI syntax. Additionally, implementing strict Content-Length header validation and ensuring consistent parsing logic across all layers of the infrastructure from load balancers to application servers is essential to prevent desynchronization attacks. Regular security audits focusing on input handling and encoding normalization should be conducted to identify similar vulnerabilities in other components of the web stack.