CVE-2026-41838: Spring Framework Auth Bypass Vulnerability

CVE-2026-41838 Overview

CVE-2026-41838 affects the spring-websocket module in the Spring Framework. The module generates WebSocket session identifiers that are not cryptographically unpredictable. An attacker who can guess or enumerate session IDs may hijack a WebSocket session when application-level authorization rules are inadequate. The weakness is categorized as [CWE-330] Use of Insufficiently Random Values. Exploitation requires user interaction and elevated attack complexity, limiting opportunistic abuse but leaving targeted scenarios viable. The advisory from Spring identifies four affected release lines spanning Spring Framework 5.3.x through 7.0.x.

Critical Impact
Predictable WebSocket session IDs can enable session hijacking against applications that rely on session ID secrecy for authorization, exposing confidential message streams.

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-2026-41838 published to NVD
2026-06-09 - Last updated in NVD database

Technical Details for CVE-2026-41838

Vulnerability Analysis

The spring-websocket module assigns identifiers to WebSocket sessions to track active connections. According to the Spring Security advisory, these IDs are generated through a mechanism that does not produce cryptographically unpredictable values. When an application uses the session ID as part of its authorization decision, or relies on the ID remaining secret, an attacker who predicts a valid ID can interact with another user's session. The flaw is classified under [CWE-330] Use of Insufficiently Random Values.

The issue is exploitable only when authorization rules are inadequate. Properly designed applications that verify the authenticated principal on every WebSocket message are not directly affected. However, applications that use the session ID as a bearer token, embed it in URLs, or skip per-message authorization checks may permit hijacking. The EPSS score reflects a low observed exploitation probability, but the impact on confidentiality is rated High in the CVSS vector.

Root Cause

The root cause is the use of a non-cryptographic source for generating WebSocket session identifiers within spring-websocket. Cryptographically unpredictable values require a secure pseudorandom number generator such as java.security.SecureRandom. When weaker generators are used, output may be guessable from prior observations or from internal state leakage.

Attack Vector

An attacker observes or predicts WebSocket session IDs issued by the target application. The attacker then connects with a guessed ID and attempts to interact with the session. Success depends on the application failing to re-validate the user identity on subsequent frames. The vector is network-based and requires user interaction, consistent with the published CVSS metrics.

No public proof-of-concept is available at this time. Refer to the Spring Security advisory for vendor-supplied technical detail.

Detection Methods for CVE-2026-41838

Indicators of Compromise

Unexpected WebSocket connections originating from IP addresses that did not perform a prior HTTP authentication handshake for the same session.
WebSocket frames carrying actions for user accounts that do not match the source IP, geolocation, or User-Agent of the original connection.
Bursts of WebSocket connection attempts with session IDs that follow a sequential or guessable pattern.

Detection Strategies

Inspect access logs for WebSocket upgrade requests where the session ID does not correlate with a recently issued authenticated session.
Correlate WebSocket session establishment events with the originating HTTP session to identify mismatches.
Enable verbose logging in org.springframework.web.socket to capture session ID lifecycles for offline analysis.

Monitoring Recommendations

Forward Spring application logs and reverse-proxy WebSocket logs to a centralized analytics platform for correlation.
Alert on a high ratio of failed WebSocket message authorization decisions per source IP.
Track baseline rates of new WebSocket sessions per user and flag anomalies that may indicate session enumeration.

How to Mitigate CVE-2026-41838

Immediate Actions Required

Upgrade to a fixed Spring Framework release as listed in the Spring Security advisory.
Audit WebSocket message handlers to confirm that authorization is enforced on every inbound frame, not only at connection time.
Remove any application logic that exposes WebSocket session IDs in URLs, logs, or client-side storage accessible to other origins.

Patch Information

Pivotal/Spring has published fixes across the affected release lines. Consult the Spring Security advisory for CVE-2026-41838 for the exact patched versions on the 5.3.x, 6.1.x, 6.2.x, and 7.0.x branches and upgrade accordingly.

Workarounds

Enforce per-message authorization in @MessageMapping handlers using Spring Security's MessageMatcherDelegatingAuthorizationManager so that knowledge of a session ID alone is insufficient to act on the session.
Bind the WebSocket session to the authenticated principal at handshake time and reject frames whose principal does not match.
Apply short server-side idle timeouts to WebSocket sessions to reduce the window available for enumeration.

bash

# Configuration example: enforce authenticated access to all WebSocket destinations
# Spring Security Java configuration snippet
# messages.simpDestMatchers("/**").authenticated();
# messages.anyMessage().denyAll();