CVE-2026-41853: Spring Framework Multipart Request Smuggling Vulnerability

Technical summary

CVE-2026-41853 is a multipart request smuggling vulnerability (CWE-444) in Spring Framework's handling of HTTP multipart requests. The vulnerability exists in Spring MVC and WebFlux request processing pipelines across multiple version branches: 7.0.0–7.0.7, 6.2.0–6.2.18, 6.1.0–6.1.27, and 5.3.0–5.3.48. Attackers can exploit inconsistencies in how Spring parses multipart boundaries and request content to cause HTTP request desynchronization attacks, potentially bypassing security controls or routing requests to unintended handlers. The CVSS 3.1 score of 5.3 (MEDIUM) reflects network-accessible exploitation with low complexity and integrity impact.

Business impact

Organizations running Spring-based applications face potential integrity compromise through request smuggling attacks. An attacker could manipulate application state, submit unauthorized data, trigger unintended business logic, or bypass input validation controls. While confidentiality is not directly compromised and availability impact is minimal, the integrity risk is significant—particularly for applications handling financial transactions, authentication/authorization decisions, or sensitive state changes. The attack requires no authentication or user interaction, making it exploitable at scale if exposed to untrusted networks.

Affected systems

All Spring Framework instances in the following branches are affected: version 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. Both Spring MVC (servlet-based) and Spring WebFlux (reactive) implementations are vulnerable. Applications using earlier versions (5.2 and below) and patched versions beyond those ranges are not affected. The vulnerability applies to any deployment—on-premises, cloud, or containerized—running vulnerable Spring Framework versions.

Exploitability

This vulnerability has a low attack complexity and requires no privileges or user interaction. An attacker with network access to a Spring application can craft a specially formed multipart HTTP request to trigger the smuggling condition. No pre-authentication or social engineering is needed. However, practical exploitation depends on the application's HTTP routing configuration and whether intermediate proxies or load balancers interpret multipart boundaries differently than Spring does. The lack of KEV (Known Exploited Vulnerability) designation as of the source data date suggests active wild exploitation has not been publicly documented, but the low barrier to exploitation means defensive action should not be delayed.

Remediation

Upgrade Spring Framework to patched versions beyond the affected ranges: use 7.0.8 or later for the 7.0 branch, 6.2.19 or later for 6.2, 6.1.28 or later for 6.1, or 5.3.49 or later for 5.3. Verify the exact patch version against the official VMware Spring Framework security advisory to ensure completeness. Organizations unable to patch immediately should implement request validation to detect and reject malformed multipart payloads, and consider deploying Web Application Firewall (WAF) rules to block smuggling patterns. Network segmentation limiting untrusted access to Spring applications reduces immediate risk.

Patch guidance

Patches are available for all affected version branches. Priority your upgrades by application criticality: prioritize production systems, then non-production environments. For Spring Boot applications, update the spring-framework dependency in your build configuration (Maven pom.xml or Gradle build.gradle) and run your full test suite before deployment. Verify patch application by checking the Spring Framework version string in application logs or through dependency inspection tools. Test multipart file upload functionality specifically post-patch to confirm no regression. Rolling deployments are recommended for high-availability environments to maintain uptime during the upgrade.

Detection guidance

Monitor HTTP request logs for suspicious multipart requests with abnormal or repeated boundary markers, incomplete Content-Length headers, or boundaries appearing in unexpected positions within the request body. Network-based detection can inspect for request smuggling patterns using IDS/IPS signatures targeting multipart desynchronization techniques. Application-level monitoring should flag requests that trigger parsing errors in multipart handling. Check application server logs for 400-series errors related to multipart parsing, which may indicate attack attempts. If Web Application Firewall is deployed, enable multipart request validation rules.

Why prioritize this

Although CVSS 5.3 (MEDIUM) may seem modest, the combination of network accessibility, zero authentication requirements, and integrity-focused impact warrants prompt patching. Multipart request smuggling can be weaponized against applications that rely on request integrity for security decisions. Organizations with publicly exposed Spring endpoints should prioritize this patch in the near term. The fact that no ransomware or active wild exploitation has been documented provides a small operational window; however, exploit simplicity means that delay increases risk unnecessarily.

Risk score, explained

CVSS 3.1/5.3 reflects: Attack Vector = Network (AV:N) because the vulnerability is remotely exploitable; Attack Complexity = Low (AC:L) because no special conditions are required; Privileges Required = None (PR:N) and User Interaction = None (UI:N) because no authentication or user click-through is needed; Confidentiality = None (C:N) and Availability = None (A:N) because the attack does not leak secrets or crash services; Integrity = Low (I:L) because the attacker can influence request routing and data integrity without full control. The MEDIUM severity reflects material but not critical risk.

Frequently asked questions