CVE-2025-12799 in Jastow
Summary
by MITRE • 07/07/2026
A flaw was found in Jastow. Jastow is vulnerable to Cross-Site Scripting (XSS) attack. If using a set of combined configuration to allow unescaped characters in URL with embedded Undertow and Jastow, a server might be vulnerable to improper input handling.
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Analysis
by VulDB Data Team • 07/07/2026
The vulnerability identified in Jastow represents a critical cross-site scripting flaw that emerges from improper input validation within the web application framework. This issue specifically manifests when Jastow is configured in conjunction with Undertow web server, creating a scenario where unescaped characters in URLs can be processed without adequate sanitization. The vulnerability stems from the application's failure to properly escape or validate user-supplied input before incorporating it into web responses, allowing malicious actors to inject executable scripts that can compromise user sessions and data integrity.
The technical implementation of this flaw occurs at the input handling layer where Jastow fails to perform proper encoding or validation of URL parameters that contain unescaped characters. When Undertow processes these requests and passes them to Jastow for further handling, the framework does not adequately sanitize the input before rendering it in web pages. This creates an environment where attackers can craft malicious URLs containing script tags or other executable code that gets executed in the context of legitimate user sessions. The vulnerability is particularly concerning because it leverages the combined configuration of two components rather than being inherent to either system individually, making detection and remediation more complex.
The operational impact of this vulnerability extends beyond simple script execution as it can enable attackers to perform session hijacking, steal sensitive information, deface web applications, or redirect users to malicious sites. Users interacting with vulnerable applications may unknowingly execute malicious scripts that can capture cookies, credentials, or other sensitive data. The attack vector is particularly dangerous in environments where users have elevated privileges or access to sensitive systems, as the compromised sessions could lead to broader system compromise. This vulnerability directly maps to CWE-79 - Improper Neutralization of Input During Web Page Generation, which classifies improper input handling as a fundamental weakness in web application security.
Organizations affected by this vulnerability should implement immediate mitigations including proper input validation and output encoding for all user-supplied data, particularly URL parameters. The recommended approach involves configuring Jastow to enforce strict character sanitization rules and ensuring that Undertow is properly configured to handle URL encoding before passing requests to Jastow. Security measures should include implementing Content Security Policies to prevent script execution, deploying web application firewalls that can detect and block malicious input patterns, and conducting thorough code reviews to identify all potential entry points for user input processing. Additionally, organizations should consider implementing automated vulnerability scanning tools that can detect improper input handling patterns as part of their continuous security monitoring processes.
The ATT&CK framework categorizes this vulnerability under T1203 - Exploitation for Client Execution, where attackers leverage web application flaws to execute malicious code in user browsers. This technique specifically targets the client-side execution environment and represents a common attack pattern that aligns with the broader category of web-based attacks that compromise user sessions. The vulnerability also intersects with T1566 - Phishing, as attackers can craft deceptive URLs that appear legitimate but contain malicious payloads designed to exploit this XSS weakness.
Mitigation strategies should focus on comprehensive input validation at multiple layers of the application architecture, with particular emphasis on ensuring that all URL parameters are properly escaped before being rendered in web responses. Organizations should establish secure coding practices that mandate proper encoding of all user-supplied data and implement automated testing procedures that can identify similar vulnerabilities in other components of their web infrastructure. Regular security training for developers on secure input handling and the specific risks associated with cross-site scripting attacks will help reduce the likelihood of similar issues arising in future implementations.
The vulnerability demonstrates how complex software ecosystems can create unexpected security weaknesses when components interact in unforeseen ways. The combination of Jastow's input handling behavior with Undertow's URL processing creates a scenario that requires careful configuration management and thorough testing to prevent exploitation. Security professionals should consider this as a prime example of why comprehensive security testing across integrated systems is essential rather than relying on individual component security measures alone.