CVE-2026-47848 in Reactor Netty
Summary
by MITRE • 08/26/2026
In specific scenarios involving WebSocket handshake redirects to a different origin, the Reactor Netty WebSocket client may leak credentials. In order for this to happen, the HTTP client must have been explicitly configured to follow redirects. Reactor Netty 1.3.0 - 1.3.6 Reactor Netty 1.1.0 - 1.2.18 Reactor Netty 1.0.52 and earlier
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Analysis
by VulDB Data Team • 08/26/2026
The vulnerability identified in Reactor Netty versions ranging from 1.0.52 through 1.3.6, specifically affecting the WebSocket client implementation within the 1.1.0 to 1.2.18 branches as well, represents a critical security flaw related to credential leakage during HTTP redirect handling. This issue manifests exclusively in scenarios where the underlying HTTP client has been explicitly configured to automatically follow redirects and encounters a handshake request that is redirected from one origin to a different origin. The core technical failure lies in how Reactor Netty manages authentication headers when transitioning between origins during the WebSocket upgrade process. When an initial connection attempt triggers a redirect response, such as an HTTP 301 or 302 status code pointing to a new URL with a distinct scheme, host, or port, the library incorrectly persists and forwards sensitive authorization credentials intended for the original target server to the newly redirected destination. This behavior violates fundamental security principles regarding origin isolation and credential scoping, effectively bypassing same-origin policies that are designed to prevent unauthorized data access across different domains.
From an operational perspective, this flaw poses a severe risk of information disclosure and potential account compromise. If an attacker can manipulate or control the redirect target, they may intercept sensitive authentication tokens, API keys, session cookies, or other proprietary headers sent by the client application. This is particularly dangerous in microservices architectures or web applications that rely heavily on WebSocket connections for real-time data exchange, as these services often transmit high-value credentials during initial handshake phases to establish authenticated sessions. The impact extends beyond simple credential theft; it can facilitate unauthorized access to backend systems, leading to data breaches, privilege escalation, and further exploitation of connected infrastructure. Since the vulnerability requires explicit configuration to follow redirects, it targets applications that prioritize seamless connectivity over strict security boundaries, making it a significant concern for developers who have not implemented custom redirect handling logic or disabled automatic redirection for WebSocket endpoints.
The technical classification of this flaw aligns with CWE-200, which covers Exposure of Sensitive Information to an Unauthorized Actor, and more specifically CWE-524 regarding the use of cacheable credentials in a way that allows them to be exposed via other channels. In terms of offensive security frameworks, this behavior maps directly to MITRE ATT&CK technique T1539, Steal Web Session Cookie, as well as aspects of T1078, Valid Accounts, if the leaked credentials allow an attacker to impersonate a legitimate user or service identity. The flaw is not inherent to all WebSocket connections but is conditional upon the interaction between redirect following mechanisms and origin validation logic within the Reactor Netty library. This specificity means that while many applications may be unaffected, those utilizing automatic redirection for load balancing, failover, or URL normalization are at direct risk without additional mitigation layers.
To mitigate this vulnerability, organizations must immediately upgrade their dependencies to patched versions of Reactor Netty where these redirect handling behaviors have been corrected to respect origin boundaries and strip sensitive headers during cross-origin redirects. For environments unable to patch immediately due to legacy constraints, developers should implement custom HTTP client configurations that disable automatic redirect following for WebSocket handshake requests or explicitly validate the target origin before forwarding any authentication data. Additionally, applying strict Content Security Policy (CSP) directives and ensuring that WebSocket endpoints do not rely on implicit trust during redirection chains can reduce the attack surface. It is also advisable to audit existing codebases for instances where Reactor Netty clients are configured with followRedirect enabled in conjunction with WebSocket upgrades, as these represent high-risk configurations requiring immediate remediation or architectural adjustment to prevent credential leakage.