CVE-2020-24327 in Discourse
Summary
by MITRE • 09/24/2021
Server Side Request Forgery (SSRF) vulnerability exists in Discourse 2.3.2 and 2.6 via the email function. When writing an email in an editor, you can upload pictures of remote websites.
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Analysis
by VulDB Data Team • 10/02/2021
The vulnerability CVE-2020-24327 represents a critical server side request forgery flaw within the Discourse platform version 2.3.2 and 2.6, specifically manifesting through the email function's image upload capability. This vulnerability stems from insufficient input validation and sanitization mechanisms that govern how remote resources are handled during email composition. The flaw allows malicious actors to exploit the system's trust in remote resources by uploading images from external websites, creating a pathway for unauthorized server communication that bypasses normal security controls. The vulnerability operates under the CWE-918 category, which specifically addresses server side request forgery attacks where applications fail to properly validate and sanitize remote resource requests, making it a direct implementation of the server side request forgery weakness.
The technical execution of this vulnerability occurs when users compose emails within the Discourse editor and attempt to upload images from remote web addresses. The system accepts these remote image URLs without adequate validation, allowing attackers to craft malicious requests that can target internal network resources or sensitive server endpoints. This flaw essentially enables attackers to perform unauthorized requests from the server's perspective, potentially accessing internal services, bypassing firewall restrictions, or conducting reconnaissance against backend systems that would normally be protected from external access. The vulnerability directly relates to the ATT&CK technique T1071.004, which involves application layer protocol usage for command and control communications, as it allows attackers to leverage the legitimate email functionality to establish unauthorized communication channels.
The operational impact of CVE-2020-24327 extends beyond simple data exfiltration, as it can enable attackers to conduct extensive reconnaissance of internal network infrastructure, access sensitive server configurations, or even facilitate further exploitation through the discovery of additional vulnerabilities. The vulnerability's potential for abuse increases significantly when considering that Discourse is commonly used for community forums, support systems, and collaborative platforms where users may have elevated privileges or access to sensitive information. Attackers can leverage this vulnerability to map internal network topology, identify running services, or even attempt to exploit other vulnerabilities present on internal systems that are normally protected by network segmentation. The flaw essentially transforms a legitimate email attachment feature into a potential attack vector for lateral movement and privilege escalation within the network environment.
Mitigation strategies for CVE-2020-24327 should focus on implementing robust input validation and sanitization for all remote resource requests, including the restriction of image upload sources to trusted domains only. Organizations should deploy network-level restrictions to prevent outbound requests to internal network ranges from email processing systems, while also implementing proper URL validation and sanitization mechanisms. The recommended approach involves configuring the Discourse platform to reject or sanitize any remote URLs that do not conform to strict whitelisting policies, ensuring that only known good domains can be used for image uploads. Additionally, implementing proper network segmentation and access controls can help limit the potential impact of successful exploitation attempts, while monitoring and logging mechanisms should be established to detect anomalous outbound requests that may indicate exploitation of this vulnerability. The fix should align with security best practices outlined in the OWASP Top Ten and NIST cybersecurity frameworks, specifically addressing the prevention of server side request forgery attacks through proper input validation and resource access controls.