CVE-2021-22969 in Concrete
Summary
by MITRE • 11/19/2021
Concrete CMS (formerly concrete5) versions below 8.5.7 has a SSRF mitigation bypass using DNS Rebind attack giving an attacker the ability to fetch cloud IAAS (ex AWS) IAM keys.To fix this Concrete CMS no longer allows downloads from the local network and specifies the validated IP when downloading rather than relying on DNS.Discoverer: Adrian Tiron from FORTBRIDGE ( https://www.fortbridge.co.uk/ )The Concrete CMS team gave this a CVSS 3.1 score of 3.5 AV:N/AC:H/PR:L/UI:N/S:C/C:L/I:N/A:N . Please note that Cloud IAAS provider mis-configurations are not Concrete CMS vulnerabilities. A mitigation for this vulnerability is to make sure that the IMDS configurations are according to a cloud provider's best practices.This fix is also in Concrete version 9.0.0
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Analysis
by VulDB Data Team • 11/25/2021
CVE-2021-22969 represents a critical server-side request forgery vulnerability in Concrete CMS that was addressed in versions 8.5.7 and 9.0.0. This vulnerability stems from an insufficient mitigation mechanism that could be bypassed through DNS rebind attacks, allowing attackers to access internal network resources that would otherwise be restricted. The flaw specifically affected versions prior to 8.5.7, making them susceptible to exploitation where an attacker could leverage the application's download functionality to retrieve sensitive cloud infrastructure credentials. The vulnerability operates through a sophisticated attack vector that combines DNS rebinding techniques with improper IP validation, effectively circumventing the intended network boundary protections.
The technical implementation of this vulnerability involves the application's handling of external resource downloads where it previously relied on DNS resolution to determine target IP addresses rather than enforcing explicit IP validation. This approach created a window of opportunity for attackers to manipulate DNS responses and redirect requests to internal network addresses, particularly targeting cloud infrastructure metadata services such as AWS EC2 Instance Metadata Service. The attack methodology typically involves setting up malicious DNS records that initially resolve to external addresses but then redirect to internal network addresses during the request lifecycle, exploiting the time window between DNS resolution and actual connection establishment. This technique aligns with the ATT&CK framework's reconnaissance and credential access phases, specifically targeting the T1580 (Cloud Infrastructure Misconfiguration) and T1566 (Phishing) tactics.
The operational impact of this vulnerability extends beyond simple information disclosure, as it could potentially provide attackers with access to cloud infrastructure credentials that grant elevated privileges within the affected cloud environment. The CVSS score of 3.5 reflects the relatively low complexity of exploitation but the significant potential for credential compromise, particularly in environments where cloud services are heavily integrated with local applications. The vulnerability's classification under CWE-918 (Server-Side Request Forgery) indicates the fundamental flaw in input validation and resource access control mechanisms within the application's network handling code. Organizations using affected versions of Concrete CMS faced a substantial risk of unauthorized access to cloud metadata services, which often contain sensitive information including IAM keys, instance identifiers, and other privileged credentials.
The remediation implemented by the Concrete CMS team involved strengthening the download functionality by eliminating local network access restrictions and enforcing explicit IP validation rather than relying on DNS resolution for determining target addresses. This approach directly addresses the root cause by ensuring that any download operations are explicitly validated against known IP addresses rather than allowing DNS resolution to determine target locations. The fix also demonstrates adherence to the principle of least privilege by removing the ability for applications to make arbitrary network requests to internal addresses without explicit validation. Organizations should implement additional mitigations including network segmentation, firewall rules to restrict access to internal metadata services, and ensuring that cloud infrastructure configurations follow best practices for metadata service access control. The vulnerability's resolution in version 9.0.0 also reflects the importance of maintaining current software versions and implementing proper security controls around cloud infrastructure access. This vulnerability underscores the critical need for robust input validation and network boundary enforcement mechanisms, particularly when applications need to interact with external resources while maintaining security boundaries.