CVE-2019-11767 in phpBB
Summary
by MITRE
Server side request forgery (SSRF) in phpBB before 3.2.6 allows checking for the existence of files and services on the local network of the host through the remote avatar upload function.
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Analysis
by VulDB Data Team • 09/12/2023
The vulnerability CVE-2019-11767 represents a critical server-side request forgery flaw in phpBB versions prior to 3.2.6 that exposes the application to unauthorized network reconnaissance and resource access. This vulnerability specifically affects the remote avatar upload functionality, which allows users to specify external URLs for avatar images. The flaw enables attackers to manipulate the avatar upload process to make requests to internal network resources, effectively bypassing normal network restrictions and creating a pathway for internal network enumeration. The vulnerability stems from insufficient input validation and sanitization of URLs provided during the avatar upload process, allowing malicious actors to craft requests that target internal services or files. This issue falls under the CWE-918 category of Server-Side Request Forgery, which is classified as a critical security weakness that enables attackers to make arbitrary requests from the server. The ATT&CK framework categorizes this as a technique for Initial Access through exploitation of vulnerable web applications, specifically targeting the server-side processing of user inputs. The vulnerability's impact extends beyond simple network reconnaissance as it can potentially expose sensitive internal services, databases, or file systems that are not properly isolated from the web server. Attackers can leverage this flaw to map internal network topology, identify running services, and potentially gain access to restricted resources that should only be accessible within the internal network perimeter. The flaw particularly affects environments where phpBB instances are deployed in corporate or organizational networks where internal resources are not properly firewalled from the web-facing servers.
The technical implementation of this vulnerability occurs when phpBB processes avatar URLs through its remote avatar functionality without proper validation of the requested resources. When a user submits a URL for an avatar, the application attempts to fetch the image from the specified location and process it for display. However, due to inadequate URL parsing and validation, the system accepts and processes URLs that point to internal network resources or local files. This allows attackers to specify URLs such as file://localhost/etc/passwd or http://internal-server:8080/admin, effectively enabling them to probe internal services and potentially access restricted files. The vulnerability exists because the application fails to properly sanitize the input URL, particularly when handling protocols like file://, ftp://, or custom protocols that could be used to access local resources. The flaw is particularly dangerous because it can be exploited by unauthenticated users or those with minimal privileges within the phpBB system, making it an attractive target for reconnaissance activities. The SSRF vulnerability essentially allows attackers to use the phpBB server as a proxy to make requests to internal resources that would normally be inaccessible from the internet. This creates a significant risk for organizations where the web server is not properly isolated from internal networks, as it provides a direct pathway for attackers to enumerate internal services and potentially exploit other vulnerabilities within the internal network.
The operational impact of CVE-2019-11767 is substantial as it provides attackers with a method to perform internal network reconnaissance without requiring direct access to the internal network. This vulnerability can be exploited to identify running services, check for open ports, and potentially access sensitive files or systems that are not properly secured. Organizations using vulnerable versions of phpBB are at risk of having their internal network architecture exposed, which could lead to more sophisticated attacks targeting other systems within the network. The vulnerability also poses a risk for data exfiltration, as attackers can potentially access local files or retrieve information from internal services that are not properly protected. The impact is particularly severe in environments where phpBB is used for community forums or user-generated content platforms, as these systems often have fewer security controls and may be more accessible to potential attackers. Additionally, the vulnerability can be combined with other exploits to create more complex attack scenarios, such as using the information gathered through SSRF to target other systems within the internal network. Organizations may also face compliance and regulatory issues if sensitive internal information is exposed through this vulnerability, particularly in environments governed by standards such as pci dss, hipaa, or soc 2. The vulnerability demonstrates the importance of proper input validation and the principle of least privilege in web application security, as it allows attackers to leverage a legitimate application function to gain unauthorized access to internal resources.
Mitigation strategies for CVE-2019-11767 focus primarily on upgrading to phpBB version 3.2.6 or later, which includes patches that address the SSRF vulnerability in the avatar upload functionality. Organizations should also implement network-level restrictions that prevent the web server from accessing internal network resources, including blocking access to private IP ranges and localhost addresses from the application server. Input validation and sanitization should be enhanced to ensure that URLs provided for avatar uploads are properly validated and restricted to allow only external web addresses. Security configurations should include disabling or restricting the use of protocols such as file://, ftp://, and other potentially dangerous protocols in the avatar upload functionality. Organizations should also implement network segmentation and firewall rules that isolate the web server from internal resources, ensuring that even if the vulnerability is exploited, the attacker's access is limited. Regular security assessments and penetration testing should be conducted to identify similar vulnerabilities in other applications and systems. The implementation of web application firewalls and security monitoring solutions can help detect and prevent exploitation attempts of this vulnerability. Additionally, organizations should establish proper access controls and user permissions to limit the ability of untrusted users to upload avatars or modify system configurations that could be leveraged for SSRF attacks. These mitigations align with security best practices outlined in standards such as iso 27001 and nist cybersecurity framework, which emphasize the importance of input validation, network segmentation, and regular security updates. The vulnerability serves as a reminder of the critical importance of keeping web applications updated and implementing proper security controls to prevent attackers from exploiting legitimate application features for malicious purposes.