CVE-2019-15918 in Linux
Summary
by MITRE • 01/25/2023
An issue was discovered in the Linux kernel before 5.0.10. SMB2_negotiate in fs/cifs/smb2pdu.c has an out-of-bounds read because data structures are incompletely updated after a change from smb30 to smb21.
If you want to get the best quality for vulnerability data then you always have to consider VulDB.
Analysis
by VulDB Data Team • 12/13/2023
The vulnerability identified as CVE-2019-15918 represents a critical out-of-bounds read flaw in the Linux kernel's Server Message Block protocol implementation specifically affecting versions prior to 5.0.10. This issue manifests within the SMB2_negotiate function located in the fs/cifs/smb2pdu.c source file, where the kernel's handling of SMB protocol version negotiations creates a scenario ripe for memory access violations. The vulnerability stems from incomplete data structure updates during protocol version transitions, particularly when shifting from SMB3.0 to SMB2.1 implementations, creating a pathway for malicious actors to exploit memory access patterns that were not properly validated.
The technical root cause of this vulnerability lies in the improper handling of protocol version negotiation sequences within the CIFS/SMB subsystem of the Linux kernel. When the system encounters a negotiation request that transitions from SMB3.0 to SMB2.1, the kernel fails to completely update all relevant data structures, leaving some fields in an inconsistent state. This incomplete update creates a scenario where subsequent memory accesses reference data that may have been partially overwritten or uninitialized, leading to out-of-bounds read operations. The flaw specifically affects the SMB2_negotiate function which processes the initial negotiation handshake between SMB clients and servers, making it a prime target for exploitation during connection establishment phases. This issue falls under CWE-125, which describes out-of-bounds read vulnerabilities, and can be categorized under ATT&CK technique T1071.004 for application layer protocol usage.
The operational impact of CVE-2019-15918 extends beyond simple memory corruption, as it can potentially enable remote code execution or information disclosure when exploited in the context of network-based SMB services. Systems running affected kernel versions that expose SMB services to untrusted networks become vulnerable to attackers who can craft malicious SMB negotiation packets to trigger the out-of-bounds read condition. This vulnerability is particularly concerning for enterprise environments where Linux servers serve as SMB file shares or act as SMB clients connecting to Windows servers, as it could allow attackers to extract sensitive kernel memory information or potentially execute arbitrary code with kernel privileges. The vulnerability affects the integrity of the kernel's memory management and protocol handling mechanisms, undermining the security posture of systems that rely on SMB connectivity.
Mitigation strategies for CVE-2019-15918 center on immediate kernel version upgrades to 5.0.10 or later, which contain the necessary patches addressing the incomplete data structure updates during SMB protocol version transitions. System administrators should prioritize patching all affected Linux systems, particularly those running SMB services or acting as SMB clients in environments with untrusted network access. Additional protective measures include implementing network segmentation to limit SMB traffic exposure, disabling unnecessary SMB services when not required, and monitoring network traffic for suspicious SMB negotiation patterns. Organizations should also consider implementing intrusion detection systems capable of identifying malformed SMB packets that could indicate exploitation attempts. The fix implemented in the patched kernel versions ensures complete data structure updates during protocol version transitions, preventing the out-of-bounds read conditions that previously allowed attackers to access memory beyond the intended boundaries. This vulnerability demonstrates the critical importance of thorough protocol implementation testing, particularly in kernel-level code handling network protocols, and aligns with security best practices outlined in the Common Vulnerability Scoring System and NIST cybersecurity guidelines.