CVE-2019-19906 in macOS
Summary
by MITRE
cyrus-sasl (aka Cyrus SASL) 2.1.27 has an out-of-bounds write leading to unauthenticated remote denial-of-service in OpenLDAP via a malformed LDAP packet. The OpenLDAP crash is ultimately caused by an off-by-one error in _sasl_add_string in common.c in cyrus-sasl.
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Analysis
by VulDB Data Team • 11/03/2020
The vulnerability identified as CVE-2019-19906 represents a critical out-of-bounds write condition within the Cyrus SASL implementation that ultimately leads to remote denial-of-service in OpenLDAP deployments. This flaw exists in cyrus-sasl version 2.1.27 and specifically manifests through malformed LDAP packets that are processed by the OpenLDAP server. The root cause of this vulnerability lies in an off-by-one error within the _sasl_add_string function located in the common.c file of the cyrus-sasl library. This type of memory corruption vulnerability falls under the CWE-121 category of stack-based buffer overflow, though the specific manifestation involves an out-of-bounds write rather than a traditional stack overflow. The flaw demonstrates how security vulnerabilities in authentication libraries can cascade into critical service availability issues within directory services.
The operational impact of this vulnerability is significant as it allows unauthenticated remote attackers to trigger a denial-of-service condition against OpenLDAP servers that utilize cyrus-sasl for authentication. When a malformed LDAP packet is received and processed, the off-by-one error in _sasl_add_string causes the application to write data beyond the allocated buffer boundaries. This memory corruption typically results in an immediate crash of the OpenLDAP daemon, effectively rendering the directory service unavailable to legitimate users and applications that depend on it for authentication and directory lookups. The vulnerability is particularly dangerous because it requires no authentication credentials to exploit, making it an attractive target for attackers seeking to disrupt directory services in enterprise environments where OpenLDAP is commonly deployed.
The technical exploitation of this vulnerability demonstrates how seemingly minor programming errors in authentication libraries can have far-reaching consequences for entire infrastructure services. The _sasl_add_string function in common.c contains a boundary check that fails to properly validate input lengths before performing string operations, creating an opportunity for attackers to manipulate the buffer size calculation. This flaw aligns with ATT&CK technique T1499.004 which involves network denial of service attacks, and more specifically with T1071.004 which covers application layer protocol manipulation. The vulnerability's classification as a remote denial-of-service issue means that attackers can exploit it from any network location without requiring physical access or prior authentication, making it particularly concerning for publicly accessible directory services.
Organizations should prioritize immediate patching of this vulnerability by upgrading to cyrus-sasl versions that contain the fix for the off-by-one error in _sasl_add_string. The mitigation strategy should also include network segmentation and access controls to limit exposure of OpenLDAP services to untrusted networks. Monitoring for unusual LDAP traffic patterns and implementing intrusion detection systems can help identify potential exploitation attempts. Additionally, organizations should consider implementing redundant directory services or backup authentication mechanisms to maintain service availability during remediation efforts. The vulnerability serves as a reminder of the critical importance of thorough input validation in authentication libraries, as even minor buffer boundary issues can result in complete service disruption. This case also highlights the need for regular security audits of core infrastructure components and the importance of maintaining up-to-date security patches across all authentication and directory services within enterprise environments.