CVE-2026-103877 in LDAP API
Summary
by MITRE • 10/02/2026
Deserialization of Untrusted Data vulnerability in Apache Directory LDAP API.
A rogue/compromised LDAP server (or pre-TLS MITM) can answer a client's loadSchema() subschema search with a schema object that contains a serialized Java class, allowing some potential RCE.
This issue affects Apache Directory LDAP API: from 2.1.0 before 2.1.9.
Users are recommended to upgrade to version 2.1.9, which fixes the issue.
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Analysis
by VulDB Data Team • 10/02/2026
The vulnerability identified in Apache Directory LDAP API versions prior to 2.1.9 represents a critical deserialization of untrusted data flaw that compromises the integrity and confidentiality of applications relying on this library for Lightweight Directory Access Protocol operations. This specific weakness arises from the way the client handles schema information retrieved during the loadSchema() subschema search process. In standard LDAP interactions, clients often retrieve directory schemas to understand available attributes, object classes, and operational rules necessary for constructing valid queries or interpreting responses. However, in vulnerable versions of the Apache Directory LDAP API, the mechanism used to parse these schema objects fails to adequately sanitize input before processing it as a Java serialized stream. This oversight creates an attack vector where malicious actors can inject arbitrary Java bytecode into the data flow, leading directly to remote code execution under specific conditions.
The operational context for this vulnerability involves scenarios where an LDAP client connects to a server that is either rogue or compromised by an attacker. It also applies to situations involving pre-TLS man-in-the-middle attacks, where network traffic between the client and a legitimate directory server can be intercepted and modified without detection due to the absence of encryption. In these environments, the attacker positions themselves to intercept the subschema search response initiated by the victim application. Instead of returning standard LDAP schema definitions encoded in LDIF or ASN.1 formats as expected, the malicious actor crafts a response containing a serialized Java object. When the vulnerable version of the Apache Directory library processes this response, it invokes Java's native deserialization mechanisms on the untrusted input without validating its origin or content structure. This allows the execution of arbitrary code within the context of the victim application process, effectively granting the attacker full control over the affected system.
From a technical perspective, this flaw is classified under CWE-502, which denotes Deserialization of Untrusted Data. The root cause lies in the lack of strict type checking or allow-listing mechanisms during the parsing phase of schema retrieval. Java deserialization has long been recognized as a high-risk operation because it can trigger complex object graphs and side effects that are difficult to predict without deep static analysis. By allowing unvalidated serialized objects derived from network sources, the library violates the principle of least privilege and fails to enforce strict input validation boundaries. This aligns with MITT&CK technique T1059, Command and Scripting Interpreter, as the successful exploitation typically results in the execution of system commands or scripts via the compromised Java Virtual Machine environment. The severity is further amplified by the fact that schema retrieval is often an automatic process performed during initial connection setup, meaning users may not even be aware they are interacting with a malicious server unless they explicitly disable auto-schema loading.
The impact of this vulnerability extends beyond simple remote code execution to include potential data exfiltration, lateral movement within internal networks, and complete system compromise if the application runs with elevated privileges. Since LDAP directories often contain sensitive authentication credentials and organizational structures, an attacker gaining shell access through this vector can pivot to other systems or extract valuable intelligence. The risk is particularly acute in enterprise environments where directory services are central to identity management and single sign-on implementations. An adversary exploiting this flaw could manipulate the application's behavior, bypass security controls, or establish persistent backdoors that evade traditional signature-based detection methods due to the legitimate nature of Java deserialization traffic.
To mitigate this vulnerability, organizations must immediately upgrade Apache Directory LDAP API to version 2.1.9 or later, where the issue has been resolved through improved input validation and safer handling of schema data. For applications unable to update promptly, implementing network-level controls such as TLS encryption for all LDAP connections is essential to prevent pre-TLS man-in-the-middle attacks that facilitate this exploit. Additionally, deploying application firewalls or intrusion detection systems capable of detecting anomalous deserialization patterns can provide an additional layer of defense. Developers should also consider disabling automatic schema loading if it is not strictly required by their business logic, thereby reducing the attack surface exposed to potential malicious servers. Regular security audits and dependency management practices are crucial to ensuring that such libraries remain patched against known exploitation techniques in dynamic threat landscapes.