CVE-2026-18341 in IBM
Summary
by MITRE • 09/04/2026
IBM i 7.6, 7.5, 7.4, and 7.3 could allow a remote authenticated attacker to corrupt memory due to an integer underflow.
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Analysis
by VulDB Data Team • 09/04/2026
The vulnerability identified in IBM i operating system versions 7.3 through 7.6 represents a critical security flaw rooted in improper handling of numerical data during internal processing operations. Specifically, the issue stems from an integer underflow condition that occurs when the software fails to adequately validate or bound-check input values before performing arithmetic calculations. In many enterprise environments running IBM i, such as those utilizing DB2 for i and associated system utilities, processes often handle large volumes of transactional data where memory allocation is dynamically determined based on calculated sizes. When an attacker provides a specifically crafted value that results in a negative number due to underflow, the system may interpret this erroneous result as a very large positive integer or fail to allocate sufficient buffer space, leading directly to heap corruption. This type of flaw is particularly dangerous because it allows for memory corruption without requiring physical access, relying instead on network-based interactions with authenticated services that process user-supplied data structures.
From a technical perspective, an integer underflow typically occurs when a subtraction operation results in a value lower than the minimum limit supported by the variable's data type. In C and similar low-level languages commonly used in system software development, signed integers wrap around to negative values while unsigned integers wrap around to extremely large positive values depending on implementation details. If subsequent code assumes that the calculated size is valid for memory allocation or array indexing, it may write beyond the bounds of an allocated buffer. This out-of-bounds write can overwrite adjacent memory structures, including function return addresses, exception handlers, or other critical control data within the process space. The presence of this vulnerability implies a weakness in input validation logic where boundary checks are either missing or incorrectly implemented for specific edge cases involving small negative inputs or values that trigger wrap-around behavior during arithmetic operations.
The operational impact of exploiting this memory corruption flaw is severe and multifaceted. Once an attacker successfully triggers the underflow, they gain the ability to manipulate the state of a privileged process running on the IBM i system. Depending on how the corrupted memory is utilized, this can lead to arbitrary code execution with the privileges of the affected service or application. In many cases, these services run with high-level authority within the operating environment, potentially granting the attacker full control over the underlying host machine. This level of access allows for comprehensive data exfiltration, modification of critical system configurations, installation of persistent backdoors, and lateral movement to other systems on the network. Furthermore, if the vulnerability affects core subsystems like DB2 or security services, it could compromise the integrity of stored financial records, customer data, and operational logs that are central to IBM i deployments in banking, healthcare, and manufacturing sectors.
This flaw aligns with Common Weakness Enumeration (CWE) category CWE-190, which defines integer overflow or wraparound when converting between different numeric types, as well as CWE-787, out-of-bounds write, since the underflow leads to memory corruption that permits writes outside intended boundaries. In terms of tactical classification within the MITRE ATT&CK framework, this vulnerability facilitates initial access and privilege escalation techniques. An attacker would likely leverage this flaw during the execution phase to inject malicious payloads into the process memory or modify existing code segments. The requirement for authentication means that threat actors must first compromise valid credentials through phishing, credential stuffing, or other social engineering tactics before they can exploit this specific technical weakness, making it a secondary but high-impact vector in an attack chain rather than a standalone remote exploitation path.
Mitigation strategies should focus on both immediate remediation and long-term architectural improvements. The primary defense is the application of vendor-provided patches that correct the underlying arithmetic logic by implementing strict input validation and using safe integer libraries that detect overflow or underflow conditions before they occur. Administrators must ensure that all systems running IBM i 7.3, 7.4, 7.5, and 7.6 are updated to the latest authorized service levels provided by IBM. Beyond patching, organizations should enforce least-privilege principles for services that handle external inputs, ensuring that even if memory corruption occurs, the resulting process does not possess excessive permissions. Network segmentation can also limit exposure by restricting access to vulnerable components from untrusted networks. Additionally, deploying intrusion detection systems configured to monitor for anomalous behavior patterns associated with buffer overflow attempts or unusual memory allocation requests can provide an additional layer of defense against exploitation efforts targeting this specific class of vulnerabilities.