CVE-2026-41853 Overview
CVE-2026-41853 is an HTTP request smuggling vulnerability affecting Spring MVC and Spring WebFlux applications. The flaw resides in the multipart request handling logic of the Spring Framework. Attackers can craft malicious multipart requests that bypass intermediary security controls and reach backend application logic with altered content.
The vulnerability maps to [CWE-444: Inconsistent Interpretation of HTTP Requests (HTTP Request/Response Smuggling)]. It is exploitable over the network without authentication or user interaction. The affected versions span Spring Framework 7.0.0 through 7.0.7, 6.2.0 through 6.2.18, 6.1.0 through 6.1.27, and 5.3.0 through 5.3.48.
Critical Impact
Remote attackers can smuggle multipart requests past front-end proxies, security filters, and web application firewalls to manipulate integrity-sensitive backend processing in Spring MVC and WebFlux applications.
Affected Products
- Spring Framework 7.0.0 through 7.0.7
- Spring Framework 6.2.0 through 6.2.18 and 6.1.0 through 6.1.27
- Spring Framework 5.3.0 through 5.3.48
Discovery Timeline
- 2026-06-09 - CVE-2026-41853 published to the National Vulnerability Database (NVD)
- 2026-06-09 - Last updated in NVD database
Technical Details for CVE-2026-41853
Vulnerability Analysis
The vulnerability is a multipart request smuggling flaw in the Spring Framework's HTTP handling layer. Spring MVC and Spring WebFlux parse multipart/form-data requests to extract individual form parts and uploaded files. When the framework interprets multipart boundaries, headers, or part delimiters differently from upstream HTTP intermediaries, a smuggling primitive becomes available.
An attacker constructs a single multipart request that a proxy, load balancer, or web application firewall parses as one logical request while Spring parses it as multiple parts or as a different structure. This desynchronization lets attackers slip part content past inspection, poison subsequent processing, or alter the parameters that downstream controllers observe.
The issue affects the integrity of request processing. Confidentiality and availability impacts are not the primary concern in this advisory, but the integrity impact enables bypassing access controls implemented at the proxy layer, parameter pollution, and tampering with file upload validation.
Root Cause
The root cause is inconsistent interpretation of multipart HTTP boundaries between Spring's parsing implementation and upstream HTTP intermediaries. Differences in handling of malformed boundary delimiters, ambiguous Content-Type headers, or non-standard line terminators inside multipart payloads create the parser desynchronization that defines request smuggling.
Attack Vector
The attack vector is network-based. An unauthenticated remote attacker sends a crafted multipart/form-data request to any Spring MVC or Spring WebFlux endpoint that accepts multipart input. No user interaction is required. The request passes through any intermediary (reverse proxy, WAF, or API gateway) on the path to the Spring application.
Where the intermediary enforces authentication, authorization, content inspection, or file-type policies, the smuggled multipart parts can carry content the intermediary did not validate. The Spring application then processes those parts as if they were legitimate, leading to policy bypass or parameter manipulation. See the Spring Security CVE-2026-41853 advisory for technical details and patched releases.
Detection Methods for CVE-2026-41853
Indicators of Compromise
- Multipart requests containing duplicated, malformed, or ambiguous boundary delimiters in the Content-Type header
- Discrepancies between proxy-logged request bodies and application-logged form parts for the same request ID
- Unexpected form parameters or uploaded files appearing in controllers that should have been blocked by upstream filters
- Multipart payloads containing unusual line terminators or nested Content-Type: multipart/* declarations
Detection Strategies
- Inventory all Spring-based services exposing multipart endpoints and confirm the runtime Spring Framework version against the affected ranges
- Enable verbose request logging at both the proxy and Spring layers, then diff multipart structure between the two views
- Apply signatures that flag malformed multipart boundaries, conflicting Content-Length and Transfer-Encoding headers, and multipart-in-multipart constructs
Monitoring Recommendations
- Centralize web server, proxy, and application logs and correlate by request identifier to surface parser disagreements
- Alert on spikes in 400/415 responses from Spring controllers combined with 200 responses at the proxy for the same client
- Monitor file upload endpoints for unexpected MIME types, oversized parts, or parts whose names do not match the controller's declared @RequestPart bindings
How to Mitigate CVE-2026-41853
Immediate Actions Required
- Upgrade Spring Framework to a fixed release outside the affected ranges as listed in the Spring Security CVE-2026-41853 advisory
- Audit upstream proxies and WAFs to ensure they reject malformed multipart requests rather than forwarding them to backend Spring services
- Disable multipart support on endpoints that do not require file or form-part uploads
Patch Information
VMware/Spring has published fixed versions through the official Spring Security advisory at spring.io/security/cve-2026-41853. Apply the maintenance release that corresponds to the deployed branch: Spring Framework 7.0.x, 6.2.x, 6.1.x, or 5.3.x. Recompile and redeploy applications using the patched framework artifacts and verify the dependency tree resolves the corrected version.
Workarounds
- Place a strict multipart parser in front of Spring that normalizes or rejects requests with ambiguous boundaries before they reach the application
- Restrict the Content-Type values accepted by public endpoints to a documented allowlist and reject requests with conflicting transfer-coding headers
- Limit multipart request size, part count, and header size at both the proxy and Spring layers to reduce the surface for smuggling payloads
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
