CVE-2021-39178 in Next.js
Summary
by MITRE • 08/31/2021
Next.js is a React framework. Versions of Next.js between 10.0.0 and 11.0.0 contain a cross-site scripting vulnerability. In order for an instance to be affected by the vulnerability, the `next.config.js` file must have `images.domains` array assigned and the image host assigned in `images.domains` must allow user-provided SVG. If the `next.config.js` file has `images.loader` assigned to something other than default or the instance is deployed on Vercel, the instance is not affected by the vulnerability. The vulnerability is patched in Next.js version 11.1.1.
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Analysis
by VulDB Data Team • 09/03/2021
The vulnerability described in CVE-2021-39178 represents a cross-site scripting flaw within the Next.js React framework that affects specific version ranges and deployment configurations. This issue specifically targets applications using Next.js versions between 10.0.0 and 11.0.0 where the configuration file contains certain image handling parameters. The vulnerability operates through a sophisticated attack vector that exploits the framework's image processing capabilities when combined with specific configuration settings. Security researchers identified that this flaw could enable attackers to inject malicious scripts into web applications through improperly sanitized SVG content, creating a persistent threat that could compromise user sessions and data integrity. The vulnerability's impact is particularly concerning because it leverages the framework's legitimate image optimization features to execute malicious code in the context of the victim's browser session.
The technical mechanism behind this vulnerability stems from how Next.js processes images when the `images.domains` array is configured in the `next.config.js` file. When an application's configuration explicitly allows specific domains for image processing and those domains permit user-provided SVG content, the framework fails to properly sanitize SVG data before rendering it in web pages. This creates a classic cross-site scripting scenario where malicious SVG files containing embedded JavaScript can execute when loaded through the application's image handling pipeline. The vulnerability specifically requires the `images.domains` array to be populated with hosts that accept user-generated SVG content, and the configuration must not use a custom loader implementation. The flaw operates at the application layer, exploiting the trust relationship between the framework and the configured image sources to bypass normal security boundaries. According to CWE standards, this vulnerability maps to CWE-79 which describes cross-site scripting flaws in web applications. The attack surface is further defined by the ATT&CK framework's web application attack patterns, particularly focusing on code injection techniques that leverage legitimate application features.
The operational impact of CVE-2021-39178 extends beyond simple script execution to potentially enable comprehensive session hijacking, data exfiltration, and user impersonation attacks. An attacker exploiting this vulnerability could gain access to user cookies, session tokens, and potentially sensitive application data through the compromised image rendering pipeline. The vulnerability's exploitation requires specific conditions to be met, including proper configuration of the `next.config.js` file, which means not all Next.js applications are affected. However, when the conditions are met, the impact is severe as the attack can occur silently in the background without user awareness, making detection particularly challenging. Organizations using vulnerable versions of Next.js without proper security measures could face significant reputational damage, regulatory penalties, and financial losses due to compromised user data and system integrity. The vulnerability's patch in Next.js version 11.1.1 demonstrates the framework maintainers' recognition of the severity, as the fix addresses the core sanitization issue in the image processing pipeline.
Organizations affected by this vulnerability should implement immediate mitigation strategies to protect their applications. The primary recommended action is to upgrade to Next.js version 11.1.1 or later, which contains the necessary security patches. For environments where immediate upgrades are not feasible, administrators should review their `next.config.js` configurations to ensure that the `images.domains` array does not include hosts that accept user-provided SVG content. Alternative mitigation approaches include implementing proper input validation and sanitization for all user-provided content, particularly SVG files, and configuring the framework to use the default image loader instead of custom implementations. Security teams should also monitor their applications for suspicious image loading patterns and implement web application firewalls to detect and block malicious SVG content. The vulnerability's specific conditions mean that deployments on Vercel or applications using custom image loaders are inherently protected, but organizations should still verify their configurations to ensure no other attack vectors exist. Organizations should also conduct thorough security reviews of their image handling processes and consider implementing additional security controls such as content security policies and regular security scanning to prevent similar vulnerabilities from emerging in their applications.