CVE-2021-28040 in OSSEC
Summary
by MITRE • 03/06/2021
An issue was discovered in OSSEC 3.6.0. An uncontrolled recursion vulnerability in os_xml.c occurs when a large number of opening and closing XML tags is used. Because recursion is used in _ReadElem without restriction, an attacker can trigger a segmentation fault once unmapped memory is reached.
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Analysis
by VulDB Data Team • 03/28/2021
The vulnerability identified as CVE-2021-28040 represents a critical uncontrolled recursion flaw within the OSSEC security solution version 3.6.0. This issue manifests in the os_xml.c component where the _ReadElem function processes XML data without proper recursion depth limitations. The flaw enables attackers to craft malicious XML payloads containing an excessive number of nested opening and closing tags that trigger recursive parsing operations. When the parser encounters these malformed XML structures, it enters an unbounded recursive loop that consumes system resources and eventually leads to segmentation faults. The vulnerability stems from inadequate input validation and lacks proper safeguards against excessive nesting levels that could overwhelm the system's stack memory allocation.
The technical implementation of this vulnerability aligns with CWE-674, which describes "Uncontrolled Recursion" as a weakness where a recursive function lacks proper termination conditions or depth limits. In the context of OSSEC, the _ReadElem function processes XML elements recursively without imposing any reasonable limits on the nesting depth, creating an exploitable condition where attackers can cause the application to consume excessive memory and eventually crash. The segmentation fault occurs when the recursive calls exceed the allocated stack space, leading to unmapped memory access violations that terminate the process. This type of vulnerability falls under the ATT&CK technique T1203, "Exploitation for Client Execution," as it can be leveraged to disrupt services and potentially gain unauthorized access to systems.
The operational impact of CVE-2021-28040 extends beyond simple service disruption, as it represents a potential denial-of-service vector that could be exploited in various attack scenarios. When exploited, the vulnerability allows attackers to crash the OSSEC agent or server processes, potentially leading to loss of security monitoring capabilities and creating windows of opportunity for additional attacks. The vulnerability is particularly concerning in environments where OSSEC is deployed as a critical security monitoring solution, as its exploitation could compromise the integrity of security operations and allow malicious actors to evade detection. The recursive nature of the flaw means that even relatively small malicious XML payloads can cause significant system instability, making it an attractive target for attackers seeking to disrupt security infrastructure.
Mitigation strategies for CVE-2021-28040 should focus on both immediate defensive measures and long-term architectural improvements. Organizations should immediately upgrade to OSSEC versions that contain patches addressing this recursion vulnerability, as the manufacturer has likely released corrected versions with proper recursion depth limits. Implementing input validation measures at the network level can help filter out malformed XML traffic before it reaches the OSSEC components. Additionally, monitoring systems should be configured to detect unusual patterns of process crashes or memory consumption that might indicate exploitation attempts. Security teams should also consider implementing resource limits and process isolation for OSSEC components to prevent complete system compromise. The vulnerability demonstrates the importance of proper input sanitization and recursion control in security applications, as highlighted by ATT&CK's emphasis on preventing exploitation of such fundamental weaknesses in security tooling.