Security Bulletin
Summary
A vulnerability in zlib could allow a remote attacker to cause a denial of service (CVE-2026-27171). AIX uses zlib as part of its data compression functions.
Vulnerability Details
CVEID: CVE-2026-27171
DESCRIPTION: zlib before 1.3.2 allows CPU consumption via crc32_combine64 and crc32_combine_gen64 because x2nmodp can do right shifts within a loop that has no termination condition.
CWE: CWE-1284: Improper Validation of Specified Quantity in Input
CVSS Source: NVD
CVSS Base score: 5.5
CVSS Vector: (CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H)
Affected Products and Versions
AFFECTED PRODUCTS AND VERSIONS:
| Affected Product(s) | Version(s) |
| AIX | 7.3 |
| VIOS | 4.1 |
The following fileset levels are vulnerable:
key_fileset = aix
| FIleset | Lower Level | Upper Level | KEY | PRODUCTS |
| rpm.rte | 4.9.1.3 | 4.15.1.1019 | key_w_fs | zlib |
| rpm.rte | 4.15.1.2000 | 4.15.1.2015 | key_w_fs | zlib |
| rpm.rte | 4.18.1.2000 | 4.18.1.2008 | key_w_fs | zlib |
| rpm.rte | 4.20.1.2000 | 4.20.1.2001 | key_w_fs | zlib |
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 rpm.rte
Remediation/Fixes
REMEDIATION:
A. FIXES
IBM strongly recommends addressing the vulnerability now.
AIX and VIOS fixes are available. The fixes are cumulative and
address previously issued AIX/VIOS zlib security bulletins with
respect to SP and TL.
The AIX and VIOS fixes can be downloaded via https from:
https://aix.software.ibm.com/aix/efixes/security/zlib_fix3.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.
AIX Level Fix KEY PRODUCT(S)
---------------------------------------------------------------
7.2.5 rpm.rte.4.15.1.1020 key_w_fix zlib
7.3.1 rpm.rte.4.15.1.2016 key_w_fix zlib
7.3.2 rpm.rte.4.18.1.2009 key_w_fix zlib
7.3.3 rpm.rte.4.18.1.2009 key_w_fix zlib
7.3.4 rpm.rte.4.20.1.2002 key_w_fix zlib
VIOS Level Fix KEY PRODUCT(S)
---------------------------------------------------------------
4.1.0 rpm.rte.4.15.1.2016 key_w_fix zlib
4.1.1 rpm.rte.4.18.1.2009 key_w_fix zlib
4.1.2 rpm.rte.4.20.1.2002 key_w_fix zlib
Please reference the Affected Products and Version section above
for help with checking installed fileset levels.
To extract the fixes from the tar file:
tar xvf zlib_fix3.tar
cd zlib_fix3
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
-----------------------------------------------------------------------------------------------------
5e86c65d156c56a674ef24c02584901e630ff2d3114ff4a3aa398392fdd1c758 rpm.rte.4.15.1.1020 key_w_csum
5e107252975c9472e31d1e7174f3fd8c7f67a1dceab77e5e2453403c80381680 rpm.rte.4.15.1.2016 key_w_csum
c16080e725e0ad91a12fbb83f46114d26579da2d9c7d277d46f4b27906d61d59 rpm.rte.4.18.1.2009 key_w_csum
261cae99c2bd24cd2cba8dcba51f09bedc037b4769636e5a3ef0bb5ea11452e0 rpm.rte.4.20.1.2002 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/zlib_advisory3.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:
http://www14.software.ibm.com/webapp/set2/sas/f/aix.efixmgmt/home.html
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
Get Notified about Future Security Bulletins
References
Acknowledgement
Change History
22 Jul 2026: Initial Publication
*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. In addition to other efforts to address potential vulnerabilities, IBM periodically updates the record of components contained in our product offerings. As part of that effort, if IBM identifies previously unidentified packages in a product/service inventory, we address relevant vulnerabilities regardless of CVE date. Inclusion of an older CVEID does not demonstrate that the referenced product has been used by IBM since that date, nor that IBM was aware of a vulnerability as of that date. We are making clients aware of relevant vulnerabilities as we become aware of them. "Affected Products and Versions" referenced in IBM Security Bulletins are intended to be only products and versions that are supported by IBM and have not passed their end-of-support or warranty date. Thus, failure to reference unsupported or extended-support products and versions in this Security Bulletin does not constitute a determination by IBM that they are unaffected by the vulnerability. Reference to one or more unsupported versions in this Security Bulletin shall not create an obligation for IBM to provide fixes for any unsupported or extended-support products or versions.
Document Location
Worldwide
Was this topic helpful?
Document Information
Modified date:
23 July 2026
Initial Publish date:
22 July 2026
UID
ibm17280739