IBM SECURITY ADVISORY
First Issued: Mon Mar 17 15:30:45 CDT 2025
The most recent version of this document is available here:
https://aix.software.ibm.com/aix/efixes/security/openssh_advisory18.asc
Security Bulletin: AIX is vulnerable to a denial of service (CVE-2025-26466)
and a machine-in-the-middle attack (CVE-2025-26465) due to OpenSSH
===============================================================================
SUMMARY:
Vulnerabilities in AIX's OpenSSH could allow a remote attacker to cause a
denial of service (CVE-2025-26466) or a machine-in-the-middle attack
(CVE-2025-26465). OpenSSH is used by AIX for remote login.
===============================================================================
VULNERABILITY DETAILS:
CVEID: CVE-2025-26466
https://www.cve.org/CVERecord?id=CVE-2025-26466
DESCRIPTION: A flaw was found in the OpenSSH package. For each ping packet
the SSH server receives, a pong packet is allocated in a memory buffer
and stored in a queue of packages. It is only freed when the
server/client key exchange has finished. A malicious client may keep
sending such packages, leading to an uncontrolled increase in memory
consumption on the server side. Consequently, the server may become
unavailable, resulting in a denial of service attack.
CVSS Base Score: 5.9
CVSS Vector: (CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H)
CVEID: CVE-2025-26465
https://www.cve.org/CVERecord?id=CVE-2025-26465
DESCRIPTION: A vulnerability was found in OpenSSH when the
VerifyHostKeyDNS option is enabled. A machine-in-the-middle attack can
be performed by a malicious machine impersonating a legit server. This
issue occurs due to how OpenSSH mishandles error codes in specific
conditions when verifying the host key. For an attack to be considered
successful, the attacker needs to manage to exhaust the client's
memory resource first, turning the attack complexity high.
CVSS Base Score: 6.8
CVSS Vector: (CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N)
AFFECTED PRODUCTS AND VERSIONS:
AIX 7.2, 7.3
VIOS 3.1, 4.1
The following fileset levels are vulnerable:
key_fileset = aix
Fileset Lower Level Upper Level KEY
---------------------------------------------------------------
openssh.base.client 9.2.112.0 9.7.3013.1000 key_w_fs
openssh.base.server 9.2.112.0 9.7.3013.1000 key_w_fs
openssh.base.client 9.9.3015.1000 9.9.3015.1000 key_w_fs
openssh.base.server 9.9.3015.1000 9.9.3015.1000 key_w_fs
Note:
A. OpenSSH versions below 9.7 are out-of-support. Customers are
advised to update to the latest OpenSSH version from the Web
Download site:
https://www.ibm.com/resources/mrs/assets?source=aixbp
To find out whether the affected filesets are installed
on your systems, refer to the lslpp command found in AIX user's guide.
Example: lslpp -L | grep -i openssh.base.client
REMEDIATION:
A. FIXES
IBM strongly recommends addressing the vulnerability now.
AIX and VIOS fixes are available.
The AIX and VIOS fixes can be downloaded via https from:
https://aix.software.ibm.com/aix/efixes/security/openssh_fix18.tar
The link above is to a tar file containing this signed
advisory, fix packages, and OpenSSL signatures for each package.
The fixes below include prerequisite checking. This will
enforce the correct mapping between the fixes and AIX
Technology Levels.
Note that the tar file contains Interim fixes that are based on
OpenSSH version, and AIX OpenSSH fixes are cumulative.
You must be on the 'prereq for installation' level before
applying the interim fix. This may require installing a new
level(prereq version) first from:
https://www.ibm.com/resources/mrs/assets?source=aixbp
AIX Level Interim Fix (*.Z) Fileset Name(prereq for installation) KEY
---------------------------------------------------------------------------------------
7.2, 7.3 973013sa.250306.epkg.Z openssh.base(9.7.3013.1000) key_w_fix
7.2, 7.3 993015sa.250306.epkg.Z openssh.base(9.9.3015.1000) key_w_fix
VIOS Level Interim Fix (*.Z) Fileset Name(prereq for installation) KEY
--------------------------------------------------------------------------------------------
3.1, 4.1 973013sa.250306.epkg.Z openssh.base(9.7.3013.1000) key_w_fix
3.1, 4.1 993015sa.250306.epkg.Z openssh.base(9.9.3015.1000) key_w_fix
To extract the fixes from the tar file:
tar xvf openssh_fix18.tar
cd openssh_fix18
Verify you have retrieved the fixes intact:
The checksums below were generated using the
"openssl dgst -sha256 [filename]" command as the following:
openssl dgst -sha256 filename KEY
-----------------------------------------------------------------------------------------------------
88c77fa830bb7b9745e0e1efc5d680bdf9e727785f18f225c43610a1b13f39eb 973013sa.250306.epkg.Z key_w_csum
a2f0e3b13b3bf4275dbaad82bcc3aeb5f62cc52c67523557e6e57af4e361df9f 993015sa.250306.epkg.Z key_w_csum
These sums should match exactly. The OpenSSL signatures in the tar
file and on this advisory can also be used to verify the
integrity of the fixes. If the sums or signatures cannot be
confirmed, contact IBM Support at
http://ibm.com/support/ and describe the discrepancy.
openssl dgst -sha256 -verify [pubkey_file] -signature [advisory_file].sig [advisory_file]
openssl dgst -sha256 -verify [pubkey_file] -signature [ifix_file].sig [ifix_file]
Published advisory OpenSSL signature file location:
https://aix.software.ibm.com/aix/efixes/security/openssh_advisory18.asc.sig
B. FIX AND INTERIM FIX INSTALLATION
If possible, it is recommended that a mksysb backup of the system
be created. Verify it is both bootable and readable before
proceeding.
To preview a fix installation:
installp -a -d fix_name -p all # where fix_name is the name of the
# fix package being previewed.
To install a fix package:
installp -a -d fix_name -X all # where fix_name is the name of the
# fix package being installed.
Interim fixes have had limited functional and regression
testing but not the full regression testing that takes place
for Service Packs; however, IBM does fully support them.
Interim fix management documentation can be found at:
https://www.ibm.com/support/pages/managing-interim-fixes-aix
To preview an interim fix installation:
emgr -e ipkg_name -p # where ipkg_name is the name of the
# interim fix package being previewed.
To install an interim fix package:
emgr -e ipkg_name -X # where ipkg_name is the name of the
# interim fix package being installed.
WORKAROUNDS AND MITIGATIONS:
None.
===============================================================================
CONTACT US:
Note: Keywords labeled as KEY in this document are used for parsing
purposes.
If you would like to receive AIX Security Advisories via email,
please visit "My Notifications":
http://www.ibm.com/support/mynotifications
Contact IBM Support for questions related to this announcement:
https://ibm.com/support/
For information on how to securely verify AIX security bulletins and fixes:
https://www.ibm.com/support/pages/node/6985269
To obtain the OpenSSL public key that can be used to verify the
signed advisories and ifixes:
Download the key from our web page:
https://aix.software.ibm.com/aix/efixes/security/systems_p_os_aix_security_pubkey.txt
To verify the AIX/VIOS security bulletin:
Published advisory OpenSSL signature file location:
https://aix.software.ibm.com/aix/efixes/security/openssh_advisory18.asc.sig
openssl dgst -sha256 -verify [pubkey_file] -signature [advisory_file].sig [advisory_file]
Please contact your local IBM AIX support center for any
assistance.
REFERENCES:
Complete CVSS v3 Guide: https://www.first.org/cvss/user-guide
On-line Calculator v3:
https://www.first.org/cvss/calculator/3.0
RELATED INFORMATION:
IBM Secure Engineering Web Portal
https://www.ibm.com/support/pages/ibm-security-vulnerability-management
IBM Product Security Incident Response Blog
https://www.ibm.com/blogs/psirt/
Security Bulletin: AIX is vulnerable to a denial of service (CVE-2025-26466)
and a machine-in-the-middle attack (CVE-2025-26465) due to OpenSSH
https://www.ibm.com/support/pages/node/7186519
ACKNOWLEDGEMENTS:
None.
CHANGE HISTORY:
First Issued: Mon Mar 17 15:30:45 CDT 2025
===============================================================================
*The CVSS Environment Score is customer environment specific and will
ultimately impact the Overall CVSS Score. Customers can evaluate the impact
of this vulnerability in their environments by accessing the links in the
Reference section of this Security Bulletin.
Disclaimer
According to the Forum of Incident Response and Security Teams (FIRST), the
Common Vulnerability Scoring System (CVSS) is an "industry open standard
designed to convey vulnerability severity and help to determine urgency and
priority of response." IBM PROVIDES THE CVSS SCORES "AS IS" WITHOUT WARRANTY
OF ANY KIND, INCLUDING THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
FOR A PARTICULAR PURPOSE. CUSTOMERS ARE RESPONSIBLE FOR ASSESSING THE IMPACT
OF ANY ACTUAL OR POTENTIAL SECURITY VULNERABILITY.