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CVE-2026-9520 blitz-js CVE debrief

A cross-site scripting (XSS) vulnerability exists in blitz-js blitz versions up to 3.0.2, specifically within the LoginForm.tsx component's handling of the 'Next' argument during sign-in operations. The vulnerability allows remote attackers to inject malicious scripts through manipulation of this parameter. The issue was disclosed to the vendor without response prior to public disclosure. The vulnerability has been assigned a LOW severity CVSS score of 2.1, reflecting the required user interaction for exploitation. Public exploit availability increases the practical risk despite the low severity rating. The vulnerability affects an unknown function in the file packages/generator/templates/app/src/app/auth/components/LoginForm.tsx and is classified under CWE-79 (Improper Neutralization of Input During Web Page Generation) and CWE-94 (Improper Control of Generation of Code).

Vendor
blitz-js
Product
blitz
CVSS
LOW 2.1
CISA KEV
Not listed in stored evidence
Original CVE published
2026-05-26
Original CVE updated
2026-05-26
Advisory published
2026-05-26
Advisory updated
2026-05-26

Who should care

Organizations using blitz-js blitz framework versions up to 3.0.2 for authentication flows; development teams maintaining applications generated from the affected template; security teams monitoring for XSS in authentication components; vendors integrating blitz-js authentication patterns.

Technical summary

The vulnerability resides in the blitz-js blitz framework's generator template for authentication components. Specifically, the LoginForm.tsx file fails to properly sanitize the 'Next' URL parameter used in sign-in flows, enabling reflected or stored XSS depending on implementation. The CVSS 4.0 vector (AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:L/VA:N) indicates network accessibility, low attack complexity, no privileges required, but mandatory user interaction. The 'E:P' metric confirms public exploit availability. The vulnerability is deferred in NVD status, suggesting ongoing analysis or vendor coordination challenges.

Defensive priority

medium

Recommended defensive actions

  • Review and sanitize all user-controlled input parameters, particularly the 'Next' argument in authentication flows, using context-appropriate encoding (e.g., HTML entity encoding for output contexts)
  • Implement Content Security Policy (CSP) headers to mitigate impact of XSS vulnerabilities in authentication components
  • Audit customizations to the LoginForm.tsx template in packages/generator/templates/app/src/app/auth/components/ for unsafe parameter handling
  • Consider migrating to blitz-js blitz version 3.0.3 or later if available, or apply vendor patches when released
  • Monitor vendor communications and security advisories for official remediation guidance
  • Implement additional logging and monitoring for authentication component access patterns that may indicate exploitation attempts

Evidence notes

Vulnerability disclosed via Vuldb with public exploit availability confirmed. Vendor non-response documented. CVSS 4.0 vector indicates network attack vector with low attack complexity, no privileges required, but user interaction required. Impact limited to integrity (low) with no confidentiality or availability impact.

Official resources

2026-05-26