CVE-2021-47776 in Umbraco
Summary
by MITRE • 01/15/2026
Umbraco CMS v8.14.1 contains a server-side request forgery vulnerability that allows attackers to manipulate baseUrl parameters in multiple dashboard and help controller endpoints. Attackers can craft malicious requests to the GetContextHelpForPage, GetRemoteDashboardContent, and GetRemoteDashboardCss endpoints to trigger unauthorized server-side requests to external hosts.
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Analysis
by VulDB Data Team • 01/15/2026
The CVE-2021-47776 vulnerability represents a critical server-side request forgery flaw within Umbraco CMS version 8.14.1 that fundamentally undermines the application's security boundaries. This vulnerability stems from insufficient input validation and sanitization mechanisms within the CMS's dashboard and help controller components, specifically affecting three key endpoints: GetContextHelpForPage, GetRemoteDashboardContent, and GetRemoteDashboardCss. The flaw enables malicious actors to manipulate the baseUrl parameter through carefully crafted HTTP requests, effectively bypassing the intended security controls that should prevent unauthorized communication between the CMS server and external systems. The vulnerability exists due to the application's failure to properly validate and sanitize user-supplied input before using it in server-side requests, creating a pathway for attackers to leverage the CMS server as an unwitting proxy for malicious network activity.
The technical exploitation of this vulnerability follows a pattern that aligns with common server-side request forgery attack vectors classified under CWE-918, which specifically addresses server-side request forgery in web applications. Attackers can construct malicious requests that target the vulnerable endpoints and manipulate the baseUrl parameter to point to internal network resources or external malicious servers. When the CMS processes these requests, it performs server-side HTTP requests to the attacker-controlled destinations, potentially enabling reconnaissance of internal network infrastructure, data exfiltration, or further exploitation of internal systems. The attack chain typically involves crafting HTTP requests that include malicious baseUrl values, which are then processed by the vulnerable controller methods without proper validation, leading to unauthorized network communications originating from the CMS server itself.
The operational impact of CVE-2021-47776 extends beyond immediate data exposure to encompass broader security implications for organizations relying on Umbraco CMS. This vulnerability can enable attackers to perform internal network reconnaissance by targeting internal IP addresses or services that are normally inaccessible from the internet, effectively using the CMS server as a pivot point for further attacks. The vulnerability also poses risks for data exfiltration, as attackers can configure the vulnerable endpoints to send sensitive information to external servers controlled by the attacker. Additionally, the flaw can facilitate more sophisticated attacks such as internal port scanning, service enumeration, or even the exploitation of other vulnerabilities within the internal network that the CMS server can reach. Organizations may face compliance violations and regulatory penalties if sensitive data is compromised through this vulnerability, particularly in environments subject to standards such as pci dss or hipaa.
Mitigation strategies for CVE-2021-47776 must address both immediate remediation and long-term architectural improvements to prevent similar vulnerabilities from emerging. The most critical immediate action involves upgrading to Umbraco CMS version 8.15.0 or later, where the vulnerability has been patched through enhanced input validation and parameter sanitization mechanisms. Organizations should implement network-level restrictions to prevent the CMS server from accessing external resources unless absolutely necessary, utilizing firewalls and network segmentation to limit outbound communications. Input validation should be strengthened across all user-supplied parameters, with strict whitelisting of acceptable baseUrl values and comprehensive sanitization of all input before processing. Security monitoring should be enhanced to detect unusual outbound network requests from the CMS server, particularly those targeting internal network addresses or unusual external domains. The vulnerability also highlights the importance of implementing proper security controls such as those defined in the mitre ATT&CK framework, specifically focusing on prevention of command and control communications and limiting the attack surface through proper access controls and network segmentation. Organizations should conduct thorough security assessments of their web applications to identify similar input validation weaknesses that could be exploited in similar fashion.