CVE-2026-40986 is a cross-site scripting vulnerability in Spring Web Flow's JavaScript RemotingHandler. The handler renders error response bodies as HTML regardless of the response's content type, allowing attacker-controlled input reflected in error details to execute scripts in the user's browser. This improper neutralization of input during web page generation is classified under CWE-79. The affected versions explicitly include 4.0.0, 3.0.0 through 3.0.1, and 2.5.0 through 2.5.1. No official patch or remediation guidance is currently documented.

Successful exploitation can lead to execution of arbitrary scripts in the context of the user's browser, potentially allowing an attacker to perform actions such as session hijacking or other malicious activities that rely on script execution. The vulnerability does not impact confidentiality or availability directly but has a high impact on integrity due to script injection. The CVSS vector indicates the attack requires network access, high attack complexity, low privileges, and user interaction.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. Until an official fix is available, users should consider applying workarounds such as validating and sanitizing inputs that may be reflected in error responses or disabling JavaScript RemotingHandler error rendering if possible. Monitor vendor communications for updates on patches or official mitigations.