IBM SECURITY ADVISORY First Issued: Wed Oct 21 07:14:39 CDT 2026 The most recent version of this document is available here: https://aix.software.ibm.com/aix/efixes/security/ntp_tmux_advisory15.asc Security Bulletin: AIX is vulnerable to a denial of service due to NTP and tmux. =============================================================================== SUMMARY: Multiple vulnerabilities in NTP and tmux could allow a remote attacker to cause Integer Overflow (CVE-2026-63384), Out-of-bounds Read (CVE-2026-63383), Memory Corruption (CVE-2026-63381, CVE-2026-63387, CVE-2026-63388), HTTP Request Smuggling / Header Injection (CVE-2026-63379, CVE-2026-63382, CVE-2026-63385). =============================================================================== VULNERABILITY DETAILS: CVEID: CVE-2026-63379 https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2026-63379 DESCRIPTION: Libevent is vulnerable to HTTP request smuggling due to improper handling of chunked HTTP trailers. An unauthenticated remote attacker could inject security-sensitive headers through trailers, potentially leading to authorization bypass, proxy-header spoofing, header smuggling, or cache poisoning. CVSS Base Score: 6.3 https://exchange.xforce.ibmcloud.com/ for the current score CVSS Vector: (CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N) CVEID: CVE-2026-63381 https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2026-63381 DESCRIPTION: Libevent is vulnerable to a use-after-free condition in buffer processing. A local attacker able to trigger a specific buffer state could cause memory corruption or a process crash, resulting in denial of service. CVSS Base Score: 5.8 https://exchange.xforce.ibmcloud.com/ for the current score CVSS Vector: (CVSS:4.0/AV:L/AC:L/AT:P/PR:L/UI:N/VC:L/VI:L/VA:H/SC:N/SI:N/SA:N) CVEID: CVE-2026-63382 https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2026-63382 DESCRIPTION: Libevent is vulnerable to HTTP request smuggling due to improper handling of Transfer-Encoding headers and chunked message framing. A remote attacker could desynchronize request boundaries, bypass access controls, or poison shared caches. CVSS Base Score: 9.2 https://exchange.xforce.ibmcloud.com/ for the current score CVSS Vector: (CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:L/SC:L/SI:L/SA:N) CVEID: CVE-2026-63383 https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2026-63383 DESCRIPTION: Libevent is vulnerable to an out-of-bounds read when processing malformed tagged RPC data. A remote attacker could exploit this vulnerability to crash the application, resulting in denial of service. CVSS Base Score: 8.7 https://exchange.xforce.ibmcloud.com/ for the current score CVSS Vector: (CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N) CVEID: CVE-2026-63384 https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2026-63384 DESCRIPTION: Libevent is vulnerable to an integer overflow or wraparound issue when processing attacker-controlled payload lengths. A remote attacker could trigger excessive memory allocation and cause a denial of service. CVSS Base Score: 8.7 https://exchange.xforce.ibmcloud.com/ for the current score CVSS Vector: (CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N) CVEID: CVE-2026-63385 https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2026-63385 DESCRIPTION: Libevent is vulnerable to HTTP parsing weaknesses that could allow header injection, request smuggling, or access control bypass. A remote attacker could exploit inconsistencies in URI and header processing to evade security checks. CVSS Base Score: 9.2 https://exchange.xforce.ibmcloud.com/ for the current score CVSS Vector: (CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:L/SC:L/SI:L/SA:N) CVEID: CVE-2026-63387 https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2026-63387 DESCRIPTION: Libevent is vulnerable to a stack-based buffer overflow when formatting DNS records. By sending a specially crafted DNS response, a remote attacker could cause memory corruption or crash the affected process. CVSS Base Score: 7.0 https://exchange.xforce.ibmcloud.com/ for the current score CVSS Vector: (CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H) CVEID: CVE-2026-63388 https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2026-63388 DESCRIPTION: Libevent is vulnerable to a heap-based out-of-bounds write when handling AF_UNIX peer addresses. A local attacker could exploit this flaw to cause memory corruption, resulting in loss of confidentiality, integrity, or availability. CVSS Base Score: 8.4 https://exchange.xforce.ibmcloud.com/ for the current score CVSS Vector: (CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H) AFFECTED PRODUCTS AND VERSIONS: AIX 7.2, 7.3 VIOS 4.1 The vulnerabilities in the following filesets are being addressed: key_fileset = aix For NTP: Fileset Lower Level Upper Level KEY PRODUCT(S) ------------------------------------------------------------------------ ntp.rte 7.4.2.8100 7.4.2.8172 key_w_fs NTP ntp.rte 73.4.2.8151 73.4.2.8172 key_w_fs NTP For tmux: Fileset Lower Level Upper Level KEY PRODUCT(S) ------------------------------------------------------------------------ tmux.rte 3.5.1.0 3.5.1.0 key_w_fs tmux 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 ntp.rte Note1: tmux affects only AIX 7.3 TL4. Note2: Only CVEs CVE-2026-63388, CVE-2026-63381 are impacting tmux.  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 NTP security bulletins with respect to fileset level. The AIX and VIOS fixes can be downloaded via https from: https://aix.software.ibm.com/aix/efixes/security/ntp_tmux_fix15.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. For NTP: AIX Level Fix KEY PRODUCT(S) -------------------------------------------------------- 7.2 ntp.rte.7.2 key_w_fix NTP 7.3 ntp.rte.7.3 key_w_fix NTP VIOS Level Fix KEY PRODUCT(S) -------------------------------------------------------- 4.1 ntp.rte.7.3 key_w_fix NTP For tmux: AIX Level Fix KEY PRODUCT(S) --------------------------------------------------------------- 7.3.4 tmux.rte.3.5.1.1 key_w_fix tmux To extract the fixes from the tar file: tar xvf ntp_tmux_fix15.tar cd ntp_tmux_fix15 Verify you have retrieved the fixes intact: The checksums below were generated using the following command: "openssl dgst -sha256 [filename]" openssl dgst -sha256 filename KEY ----------------------------------------------------------------------------------------------- 580ed275e9dbf6c0e984cbdcc587fb6d9b61d025a0acc8de273ca2cfdb27e807 ntp.rte.7.2 key_w_csum 241b57eb5eacdd7cd5e77a2771f9b262b287176e547287bf67c60aa3fe4fcf4c ntp.rte.7.3 key_w_csum fc474ca1efad9e229e4c84560e54ef3571b37ab17fa8620bb3bfc77e4a147ff0 tmux.rte.3.5.1.1 key_w_csum The checksums below were generated using the following command: "openssl dgst -sha512 [filename]" openssl dgst -sha512 filename KEY --------------------------------------------------------------------------------------------------------------------------------------------------------------- d732a5295c145a9be5578c38b0ecf300ef25111de6b6962eb5359f9009acd05ad40721459c5aa99bcb679f7ddd1c9bfe3a2491d642093eab7d7c77c5cd11c7f6 ntp.rte.7.2 key_4K_w_csum 939ef7a4fce25524a8bf3a6bbee9d562b739d16a8069ff11a771434e835d7aebc04ba5c24bfa08bffeba125ac7a6ba3a1f4c29ed3138f67834f8fc847d5e73da ntp.rte.7.3 key_4K_w_csum 1e3835239df01674e22f88396d46e3ad8fefe78aaf2f986d2fbe2d22117d161ad47f3277ed5e72dee1f2e6bfbafef8199d079e21a1fa4a971e5448f39ba3557e tmux.rte.3.5.1.1 key_4K_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/ntp_tmux_advisory15.asc.sig C. 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": https://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: ftp://ftp.software.ibm.com/systems/power/AIX/systems_p_os_aix_security_pubkey.txt 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/ntp_tmux_advisory15.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: http://www.first.org/cvss/user-guide On-line Calculator v3: http://www.first.org/cvss/calculator/3.0 RELATED INFORMATION: IBM Secure Engineering Web Portal http://www.ibm.com/security/secure-engineering/bulletins.html IBM Product Security Incident Response Blog https://www.ibm.com/blogs/psirt/ Security Bulletin: AIX is vulnerable to a denial of service due to NTP and tmux. https://www.ibm.com/support/pages/node/7293282 ACKNOWLEDGEMENTS: None. CHANGE HISTORY: First Issued: Wed Oct 21 07:14:39 CDT 2026 =============================================================================== *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.