Jinja is a widely used extensible templating engine in Python web applications. The vulnerability CVE-2024-34064 affects the xmlattr filter in Jinja versions prior to 3.1.4. This filter is designed to convert a dictionary of key-value pairs into XML/HTML attributes. However, it improperly accepts keys containing characters such as spaces, '/', '>', or '=', which are not valid in XML/HTML attribute names. When an application accepts user input as keys (rather than just values) for this filter, an attacker can craft keys that inject additional attributes or malicious payloads, leading to cross-site scripting (XSS). This can result in the execution of arbitrary JavaScript in the context of other users viewing the affected page. The vulnerability was partially addressed in CVE-2024-22195 by blocking spaces in keys, but other dangerous characters remained exploitable. The Jinja project now explicitly considers accepting keys as user input an unintended and insecure use case. The vulnerability is fixed in Jinja 3.1.4, which properly restricts invalid characters in keys. Exploitation requires no privileges but does require user interaction (e.g., a victim visiting a maliciously crafted page). No known exploits are currently in the wild. The CVSS 3.1 score is 5.4 (medium), reflecting the moderate impact and ease of exploitation in specific scenarios.

For European organizations, this vulnerability poses a risk primarily to web applications using Jinja templating versions earlier than 3.1.4 that accept user input as keys in the xmlattr filter. Successful exploitation can lead to cross-site scripting attacks, compromising the confidentiality and integrity of user sessions, potentially leading to session hijacking, credential theft, or unauthorized actions performed on behalf of users. This can damage organizational reputation, lead to regulatory non-compliance (e.g., GDPR breaches if personal data is exposed), and cause operational disruptions. The impact is heightened in sectors with sensitive data or critical services, such as finance, healthcare, and government. However, the vulnerability requires specific unsafe coding practices (accepting keys as user input), so the overall risk depends on application design. The absence of known exploits reduces immediate threat but patching is advised to prevent future attacks.

1. Upgrade all Jinja installations to version 3.1.4 or later to ensure the vulnerability is patched. 2. Audit application code to identify any use of the xmlattr filter where user input is accepted as keys; refactor such code to avoid this practice entirely. 3. Implement strict input validation and sanitization on any user-supplied data that might be used in templating, especially keys for attribute generation. 4. Employ Content Security Policy (CSP) headers to reduce the impact of potential XSS by restricting script execution sources. 5. Conduct security code reviews and penetration testing focusing on template injection and XSS vectors. 6. Educate developers about the risks of accepting user input as keys in templating filters and encourage secure coding practices. 7. Monitor web application logs and user reports for signs of XSS exploitation attempts. 8. If immediate upgrade is not possible, consider temporary mitigations such as disabling features that accept user input as keys or applying custom filters to sanitize keys before rendering.