CVE-2026-89028 in MikroTik
Summary
by MITRE • 09/16/2026
MikroTik RouterOS before 7.24 contains a heap memory corruption vulnerability in the userspace SMB daemon that allows remote attackers to corrupt adjacent heap memory by supplying a crafted uniPwdLen value in the SMB1 SessionSetupAndX handler. An attacker can send a malformed SMB1 request with a uniPwdLen field that triggers an integer underflow, causing the resulting value to be used as the copy length in a memory copy operation into a smaller heap buffer, corrupting adjacent heap memory.
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Analysis
by VulDB Data Team • 09/16/2026
The vulnerability identified in MikroTik RouterOS versions prior to 7.24 represents a critical security flaw within the userspace Server Message Block version 1 SMB daemon. This specific weakness is classified as a heap-based buffer overflow resulting from an integer underflow, which falls under CWE-190 Integer Overflow or Wrap Around and CWE-122 Heap-Based Buffer Overflow in standard vulnerability taxonomies. The core of the issue lies in how the SMB1 SessionSetupAndX handler processes authentication parameters during session establishment. Specifically, the daemon fails to adequately validate the uniPwdLen field, which indicates the length of the user password provided by the client. When an attacker supplies a crafted value for this field that is smaller than expected or improperly formatted relative to other buffer size indicators, it triggers an integer underflow condition within the application logic.
This mathematical error results in a significantly larger unsigned integer being calculated as the copy length for subsequent memory operations. The SMB daemon then proceeds to execute a memory copy operation using this erroneously large value into a heap-allocated buffer that is substantially smaller than the specified length. Consequently, data written during this operation overflows the boundaries of the intended destination buffer and corrupts adjacent memory regions on the heap. This type of out-of-bounds write can overwrite critical metadata structures managed by the heap allocator or alter function pointers stored in nearby objects. Such corruption destabilizes the application state and creates a pathway for arbitrary code execution, allowing remote attackers to gain control over the affected device with the privileges of the SMB daemon process.
From an operational perspective, this vulnerability poses severe risks to network infrastructure integrity. Since it is remotely exploitable without authentication, any host running vulnerable versions of RouterOS that have the SMB service enabled and accessible from untrusted networks becomes a prime target for exploitation. Successful exploitation can lead to complete system compromise, enabling attackers to install persistent backdoors, exfiltrate sensitive configuration data, or pivot into internal network segments protected by the router. The impact aligns with high-severity entries in the MITRE ATT&CK framework, particularly those related to Initial Access via Exploitation of Remote Services and Execution through Arbitrary Code Injection. Attackers can leverage this flaw as an entry point for broader campaigns targeting IoT devices and small-to-medium enterprise networks that rely on MikroTik hardware for connectivity management.
Mitigation strategies must prioritize immediate software updates to version 7.24 or later, where the vendor has addressed the input validation flaws in the SMB1 implementation. In environments where upgrading is not immediately feasible, administrators should disable the SMB service entirely if it is not strictly required for network operations, thereby removing the attack surface associated with this daemon. Additionally, implementing strict firewall rules to restrict access to port 445 and related SMB ports only from trusted internal subnets can provide a layer of defense-in-depth against external exploitation attempts. Regular vulnerability scanning and monitoring for anomalous traffic patterns indicative of malformed SMB requests are also recommended to detect potential probing or active exploitation efforts in real-time.