CVE-2026-103552 in Directory LDAP API
Summary
by MITRE • 10/02/2026
Stack Overflow vulnerability in Apache Directory LDAP API.
Before binding, a client can send a deeply nested search filter that overflows the stack in the server's decoder.
This issue affects Apache Directory LDAP API: from 1.2.0 before 1.2.9.
Users are recommended to upgrade to version 1.2.9, which fixes the issue.
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Analysis
by VulDB Data Team • 10/02/2026
The vulnerability identified as a stack overflow in the Apache Directory LDAP API represents a critical memory management flaw within the server-side processing of Lightweight Directory Access Protocol requests. This specific weakness arises from an improper restriction on input data depth during the parsing phase of search filter operations. When a client initiates a binding request, it may include complex search filters that are recursively structured to an excessive degree. The LDAP API decoder, responsible for interpreting these filters before authentication is completed, fails to enforce adequate limits on recursion or nesting levels. Consequently, as the parser traverses deeply nested logical expressions, each recursive call consumes additional space on the system stack until the allocated memory limit is exceeded, triggering a buffer overflow condition that corrupts the execution context and leads to application instability.
From a technical perspective, this flaw aligns with CWE-121, which describes a stack-based buffer overflow where data exceeds the fixed size of the stack allocation without proper bounds checking. In the context of LDAP protocols, search filters are often represented as Abstract Syntax Notation One structures that require recursive parsing to resolve logical operators such as AND, OR, and NOT. The vulnerability exploits this inherent recursion by providing inputs with nesting depths far beyond normal operational parameters. Because the overflow occurs in the decoder thread before authentication is verified, it allows unauthenticated attackers to trigger the condition remotely. This bypasses standard access controls that would otherwise restrict exploitation to authorized users only, significantly lowering the barrier for entry and increasing the attack surface available to malicious actors targeting directory services infrastructure.
The operational impact of this vulnerability centers on service availability rather than immediate code execution or data exfiltration in its basic form. A successful stack overflow typically results in a segmentation fault or an abrupt termination of the affected thread within the Apache Directory Server process. In distributed environments, such as those utilizing load balancers or high-availability clusters, repeated exploitation can lead to cascading failures if multiple nodes are targeted simultaneously. This effectively constitutes a Denial of Service attack against critical identity and access management systems. Organizations relying on LDAP for user authentication, authorization checks, and directory queries may experience significant downtime, preventing legitimate users from accessing applications that depend on these services. While stack overflows can sometimes be leveraged to execute arbitrary code through return address manipulation, the primary immediate risk here is the disruption of service continuity and potential data corruption within memory structures associated with active sessions.
Mitigation strategies must prioritize both immediate remediation and long-term architectural resilience. The most effective solution is to upgrade the Apache Directory LDAP API to version 1.2.9 or later, where developers have implemented proper input validation mechanisms to limit filter nesting depth during parsing. For environments where upgrading is not immediately feasible, network-level controls such as Web Application Firewalls can be configured to inspect incoming LDAP traffic and drop packets containing unusually deep search filters based on heuristic analysis of packet size and structure. Additionally, implementing rate limiting on authentication endpoints can mitigate the impact by slowing down rapid-fire exploitation attempts. From a defensive development standpoint, this incident underscores the importance of adhering to secure coding practices that enforce strict input validation limits on recursive algorithms, ensuring that all external inputs are sanitized before being processed by core system components.