CVE-2017-17740 in OpenLDAP
Summary
by MITRE
contrib/slapd-modules/nops/nops.c in OpenLDAP through 2.4.45, when both the nops module and the memberof overlay are enabled, attempts to free a buffer that was allocated on the stack, which allows remote attackers to cause a denial of service (slapd crash) via a member MODDN operation.
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Analysis
by VulDB Data Team • 12/15/2019
The vulnerability identified as CVE-2017-17740 represents a critical memory management flaw within the OpenLDAP software ecosystem, specifically affecting versions through 2.4.45. This issue manifests when two particular components are simultaneously enabled: the nops module and the memberof overlay. The flaw occurs during the processing of member MODDN operations, which are LDAP operations used to modify the distinguished names of members within directory entries. The nops module, designed for testing and debugging purposes, when combined with the memberof overlay that tracks group membership relationships, creates a dangerous condition where stack-allocated memory is improperly freed. This improper memory management creates a classic use-after-free vulnerability that can be exploited remotely by attackers without authentication requirements.
The technical execution of this vulnerability relies on the specific combination of module configurations and the sequence of operations performed during LDAP directory modifications. When an attacker crafts a MODDN request that targets a member within a group structure managed by the memberof overlay, the system processes this request through the nops module's code path. The flaw occurs because the code attempts to free memory that was allocated on the stack frame of the function handling this operation, rather than on the heap where allocated memory should be freed. This mismanagement creates a situation where the stack memory becomes corrupted or the program attempts to deallocate memory that is no longer valid, leading to immediate program termination. The vulnerability demonstrates a fundamental flaw in memory management practices and improper handling of stack-based allocations within the LDAP server's processing pipeline.
The operational impact of CVE-2017-17740 is significant as it allows remote attackers to execute a denial of service attack against OpenLDAP servers without requiring any authentication credentials. This makes the vulnerability particularly dangerous in production environments where LDAP services are exposed to untrusted networks. The attack results in a complete service disruption as the slapd daemon crashes and must be restarted manually or automatically by system management tools. Organizations relying on OpenLDAP for directory services, authentication, or authorization may experience service outages that can affect numerous applications and systems depending on the directory service. The vulnerability affects any environment where both the nops module and memberof overlay are enabled, which may be common in testing environments or configurations where debugging capabilities are enabled in production systems.
Mitigation strategies for this vulnerability should focus on immediate configuration changes to disable the problematic module combinations. Organizations should first disable the nops module if it is not actively required for debugging purposes, as this module should never be enabled in production environments. Additionally, administrators should carefully review their OpenLDAP configurations to ensure that the memberof overlay is not unnecessarily enabled when the nops module is active. The recommended long-term solution involves upgrading to OpenLDAP versions 2.4.46 or later where this vulnerability has been patched through proper memory management implementation. Security teams should also implement monitoring for suspicious MODDN operations and consider network segmentation to limit exposure of LDAP services to untrusted networks. This vulnerability aligns with CWE-415, which describes improper deallocation of memory on the stack, and represents a typical example of how seemingly innocuous debugging features can create security risks when combined with other system components. The ATT&CK framework categorizes this as a denial of service attack vector that leverages memory corruption vulnerabilities in system services, making it particularly relevant for organizations implementing comprehensive cybersecurity frameworks.