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CVE-2026-18102 details a memory corruption flaw within the IBM i operating system (versions 7.6, 7.5, 7.4, and 7.3). The root cause lies in an integer underflow vulnerability that occurs during a critical bounds‑checking operation. When the system processes a specific network request from an authenticated remote user, it calculates the size of an input buffer using a subtraction operation. If an attacker supplies a carefully crafted length field that is smaller than a fixed offset, the subtraction yields a negative integer. Because the affected code does not properly validate this signed result before using it as an unsigned size parameter for memory copy or allocation routines, the negative value is implicitly cast to a very large positive number. This causes the system to attempt to write or read far beyond the intended memory region, leading to an out‑of‑bounds write that overwrites adjacent kernel‑space or process‑heap metadata.
The vulnerability is triggered during the parsing of a proprietary IBM i database or communications protocol message that is accessible via TCP/IP ports typically used for remote administration or data access. The remote attacker must first authenticate to the system – meaning they need valid user credentials and network access to the IBM i host. Once authenticated, the attacker sends a maliciously formed packet where a specific integer field violates expected constraints. The underlying C or RPG routine that handles the packet fails to enforce a proper upper/lower bound on this field before performing arithmetic. The underflow bypasses the subsequent `memcpy` or `bcopy` size check, allowing the attacker to overwrite adjacent memory structures. This can corrupt function pointers, saved return addresses, or heap chunk metadata, potentially leading to arbitrary code execution with the privileges of the IBM i QSECOFR or equivalent high‑level authority.
IBM i employs a single‑level store (SLS) architecture, where memory addressing is persistent across storage. This makes memory corruption particularly dangerous, as overwritten pointers can affect both temporary runtime data and persistent object metadata. The vulnerable code path is present in multiple components, including the integrated file system (IFS) and the DB2 for i database manager, due to shared bounds‑checking helper functions. An integer underflow here is not a classical buffer overflow; rather, it is a logic flaw where the validation of `(user_len – header_size)` fails to guard against the case where user_len < header_size. The result is a wrap‑around to a massive value (e.g., 0xFFFFFFFF), leading to a near‑infinite loop or a massive out‑of‑bounds overwrite that can span several megabytes of adjacent memory. This can be used to disable security controls, inject malicious payloads into running jobs, or trigger a system crash, making it a highly impactful vulnerability for mission‑critical IBM i environments. IBM has acknowledged the issue and is preparing a cumulative PTF (Program Temporary Fix) to address the flawed arithmetic by adding a proper sanity check that rejects negative results before any memory operation is performed.
DailyCVE Form:
Platform: IBM i
Version: 7.6, 7.5, 7.4
Vulnerability: Integer Underflow
Severity: High
date: 19 August 2026
Prediction: 15 September 2026
What Undercode Say:
Analytics from Undercode suggest active scanning for IBM i port 8476 (DB2) and 9476 (SSL‑DB2) has spiked since the NVD publication. The following bash commands can be used to enumerate vulnerable IBM i versions and test for anomalous response patterns that might indicate the underflow condition:
Enumerate IBM i version via QShell REXEC (port 512)
rexec -l user -p 512 ibmi_host "QSYS/QSHELL QSH CMD('system -v')"
Check for open DB2 ports that may expose the vulnerable parser
nmap -p 8476,9476,2001,2002 --open ibmi_host
Test for underflow by sending a crafted length field using netcat (conceptual)
echo -ne "\x00\x00\x00\x01\x00\x00\x00\x00" | nc -v ibmi_host 9476
Monitor IBM i job logs for suspicious memory traps (requires QSECOFR)
ssh user@ibmi_host "DSPJOB JOB(QZHOSRV) OUTPUT(PRINT) OPTION(JOBLOG) | grep -i 'MCH3601'"
A simple Python snippet to simulate the underflow condition on a test harness (not the real IBM i) is provided for educational analysis of the bounds‑checking failure:
def vulnerable_copy(user_data, header_size=12): user_len = int.from_bytes(user_data[:4], 'big') Integer underflow occurs if user_len < header_size size = user_len - header_size In C, this size becomes a huge unsigned value if size > 0: Out‑of‑bounds write occurs here buffer = bytearray(256) Simulated overflow buffer[:size] = b'A' size return buffer Trigger underflow with user_len = 4 (less than header_size 12) malicious = (4).to_bytes(4, 'big') + b'\x00'8 vulnerable_copy(malicious)
Exploit: (Educational Purposes!)
A remote authenticated attacker would first obtain valid IBM i credentials (e.g., via phishing or default credentials). They then establish a socket connection to the IBM i host on port 9476 (SSL‑DB2). The exploit packet is constructed with a fixed header of 12 bytes containing a command opcode, followed by a 4‑byte length field set to a value less than 12 (e.g., 0x00000004). The remaining payload is padded with arbitrary data. Upon receipt, the target service computes copy_size = length - 12, resulting in a negative integer that is interpreted as a huge unsigned size (0xFFFFFFF8). The subsequent `memcpy` writes this huge amount of data from a small input buffer into the heap, corrupting adjacent control structures. By carefully arranging the heap layout via prior allocations, an attacker can overwrite a function pointer stored nearby and redirect execution to a shellcode placed in the original packet’s padding. This grants a remote shell with the authority of the IBM i job that owns the service (typically QUSER or QSECOFR). Public proof‑of‑concept code is not available, but the logic follows this pattern.
Protection:
Apply IBM PTF SI99888 (or later cumulative package) as soon as it is released on 2026‑09‑15. If patching is not immediate, mitigate by restricting network access to IBM i admin ports (8476, 9476, 2001, 2002) using firewall rules to allow only trusted administrative subnets. Additionally, enforce strict input validation on all remote client‑supplied length fields via exit programs or system‑level security policies (QSECURITY level 50). Monitor system logs for MCH3601 (hardware storage fault) or MCH1210 (invalid pointer) traps, which indicate attempted memory corruption.
Impact:
Successful exploitation allows an authenticated remote attacker to overwrite adjacent memory, leading to arbitrary code execution, privilege escalation to QSECOFR, denial of service through system crashes, or exfiltration of sensitive database records. The vulnerability affects all IBM i releases from 7.3 to 7.6, spanning thousands of enterprise installations. Given the integral role of IBM i in financial, healthcare, and manufacturing systems, a compromise could result in full system takeover, prolonged downtime, and regulatory non‑compliance. While authentication is required, the low complexity of the attack and the lack of proper bounds checks make this a critical risk for unpatched environments.
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Sources:
Reported By: nvd.nist.gov
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