CVE-2026-16958 is a medium severity (CVSS 6.5) out-of-bounds write in IBM AIX 7.2 and 7.3 and PowerVM VIOS 4.1. A remote attacker could cause a denial of service. The CVSS vector marks the attack as adjacent network. IBM fixes it in cumulative packs.
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-787 is an out-of-bounds write. In this case the stated impact is denial of service. The record calls the attacker remote, while the CVSS vector in both the record and IBM's bulletin uses the adjacent network attack vector. Both are quoted here. Adjacent means the attacker needs to sit on the same local network segment or similar, not reach the host from the internet.
That narrows exposure for most estates, but flat management networks and shared VLANs make adjacency easy. Availability impact is rated high, so a crash on a VIOS can affect many client partitions. Plan the fix pack with that in mind.
The weakness class here is CWE-787 (Out-of-bounds Write). The NVD record scores it 6.5, which is medium severity. IBM's bulletin gives the same score. The CVSS vector reads attack vector from an adjacent network, low attack complexity, no privileges required, no user interaction. Scope is unchanged. Impact on confidentiality, integrity and availability is rated none, none 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 a remote internet attack?
The vector says adjacent network, so the attacker must be close in network terms. The record's text says remote. Check the network position of your hosts either way.
Sources
- IBM security bulletin for AIX and VIOS
- Safeguard Gold entry for CVE-2026-16958, which summarizes the NVD record
Last reviewed: 18 October 2026.
