CVE-2026-16821 is a high severity (CVSS 7.0) format string vulnerability in IBM AIX 7.2 and 7.3 and PowerVM VIOS 4.1. A local attacker could gain elevated privileges. IBM fixes it through service packs and VIOS fix packs listed in one bulletin.
What's affected
IBM AIX 7.2 and 7.3, and IBM PowerVM VIOS 4.1. IBM's bulletin lists the affected fileset as bos.mp64 at these levels:
- AIX 7.2.5: 7.2.5.0 through 7.2.5.212
- AIX 7.3.2: 7.3.2.0 through 7.3.2.5
- AIX 7.3.3: 7.3.3.0 through 7.3.3.2
- AIX 7.3.4: 7.3.4.0 through 7.3.4.1
The same bulletin covers a large group of AIX and VIOS issues, and one set of fixes addresses all of them. Per the bulletin, the remediation levels are AIX 7.2 TL05 SP13, AIX 7.3 TL02 SP5, AIX 7.3 TL03 SP3 and AIX 7.3 TL04 SP2, and VIOS fix packs 4.1.0.50, 4.1.1.30 and 4.1.2.20. The service packs and fix packs are cumulative.
How to check if you're exposed
Run these on each AIX LPAR and on each VIOS:
oslevel -s
lslpp -L | grep -i bos.mp64
On a VIOS, log in as padmin and run ioslevel. Compare the bos.mp64 level against the ranges above. If the fileset is at or below the upper level for its release, the system is in scope. You can also check for the APAR that matches your release with instfix -ik IJ59565 (use IJ59566 for 7.2.5, IJ59565 for 7.3.2, IJ59564 for 7.3.3 and IJ59563 for 7.3.4).
What to do now
- List every AIX and VIOS system and record
oslevel -soriosleveloutput. - Compare each against the bos.mp64 ranges in IBM's bulletin 7283858.
- Download the matching service pack or VIOS fix pack from Fix Central. Plan for an LPAR reboot, or use AIX Live Update where it suits you.
- If you patch through NIM with nimsh secure, read the IBM note linked in the bulletin first, because the protocol between master and client changed.
- On VIOS 4.1.0 and 4.1.1, follow the post-update Postgres15 migration steps IBM links in the bulletin.
- Re-run the version check and record the new level.
Why it matters
CWE-134 is use of an externally controlled format string. A function that formats output takes a string from input as its template, so special sequences in that input can read or write memory. The record says a local attacker could use this to gain elevated privileges. Attack complexity is rated high, which keeps the score at 7.0.
Local flaws matter on multi-user systems and on VIOS, where an unprivileged shell account or a compromised application user may be the starting point. The fix is a routine service pack, so there is little reason to delay it beyond change windows.
The weakness class here is CWE-134 (Use of Externally-Controlled Format String). The NVD record scores it 7.0, which is high severity. IBM's bulletin gives the same score. The CVSS vector reads attack vector with local access, high attack complexity, low privileges required, no user interaction. Scope is unchanged. Impact on confidentiality, integrity and availability is rated high, high and high. The record's KEV flag is false, so this CVE is not in CISA's Known Exploited Vulnerabilities catalog, and the advisory I fetched does not report exploitation.
Safeguard's Gold directory tracks this CVE, and reachability analysis helps you decide which affected components matter most for your own software.
Quick check
Is this reachable from the network?
No. The vector is local, so an attacker needs a way to run code or commands on the system first.
Sources
- IBM security bulletin for AIX and VIOS
- Safeguard Gold entry for CVE-2026-16821, which summarizes the NVD record
Last reviewed: 18 October 2026.
