CVE-2026-4942: IBM i TLS Downgrade Vulnerability

CVE-2026-4942 Overview

CVE-2026-4942 is a Transport Layer Security (TLS) protocol downgrade vulnerability affecting IBM i versions 7.6, 7.5, 7.4, and 7.3. A remote attacker can send a specifically crafted message that causes the server to negotiate a TLS protocol version explicitly disabled in the server configuration. The flaw is classified under [CWE-757] (Selection of Less-Secure Algorithm During Negotiation). Successful exploitation weakens the cryptographic protections intended by the administrator and exposes session data to further attack. The issue is remotely reachable without authentication or user interaction, though attack complexity is high.

Critical Impact
A network-adjacent attacker can force IBM i servers to negotiate TLS versions that administrators explicitly disabled, undermining transport security controls and enabling downstream cryptographic attacks on protected sessions.

Affected Products

IBM i 7.6
IBM i 7.5
IBM i 7.4
IBM i 7.3

Discovery Timeline

2026-07-17 - CVE-2026-4942 published to NVD
2026-07-20 - Last updated in NVD database

Technical Details for CVE-2026-4942

Vulnerability Analysis

The vulnerability resides in the TLS negotiation logic on IBM i. When a client initiates a handshake, the server should reject any protocol version that administrators have disabled in the configuration. IBM i fails to enforce that policy under certain crafted handshake messages. As a result, the server accepts and negotiates a disabled, weaker TLS version. This defeats hardening controls that administrators rely on to disable deprecated protocols such as TLS 1.0 or TLS 1.1.

The attack requires network access to the TLS-enabled service but does not require credentials or user interaction. Attack complexity is high because the attacker must craft handshake traffic that triggers the flawed negotiation path. Impact is limited to confidentiality — integrity and availability are not directly affected, though the weakened session can serve as a foundation for further cryptographic attacks.

Root Cause

The root cause is improper enforcement of the configured protocol allow-list during TLS version negotiation [CWE-757]. The negotiation code path does not consistently honor the disabled-protocol list when parsing specific handshake messages, allowing selection of a less-secure algorithm.

Attack Vector

The attack vector is a network-based TLS handshake against any IBM i service exposing TLS. An attacker sends a specifically crafted ClientHello or negotiation message that induces the server to accept a disabled TLS version. Once the downgraded session is established, the attacker or a subsequent adversary can attempt cryptographic attacks associated with legacy TLS versions, such as BEAST, POODLE, or CBC padding oracle attacks. No verified public exploit is available at the time of publication.

Detection Methods for CVE-2026-4942

Indicators of Compromise

TLS sessions established on IBM i endpoints using protocol versions that were administratively disabled in SSLCONFIG or system TLS policy.
Unexpected TLS 1.0 or TLS 1.1 handshake completions logged against IBM i services when policy mandates TLS 1.2 or higher.
Repeated abnormal or malformed ClientHello messages from a single source preceding successful legacy-version handshakes.

Detection Strategies

Inspect TLS handshake metadata at network sensors and compare negotiated protocol versions against the intended server policy for each IBM i host.
Correlate IBM i audit journal QAUDJRN entries for TLS session establishment with expected protocol baselines and alert on deviations.
Deploy JA3/JA3S fingerprinting to identify anomalous client fingerprints that consistently trigger legacy protocol negotiation.

Monitoring Recommendations

Forward IBM i security audit journals and network TLS telemetry to a centralized analytics platform for continuous baseline comparison.
Alert on any negotiated TLS version below the organization-defined minimum, per host and per service port.
Track the source IP addresses and ASN reputation of clients initiating downgraded handshakes to prioritize investigation.

How to Mitigate CVE-2026-4942

Immediate Actions Required

Apply the IBM PTF group updates referenced in the IBM Support advisory for CVE-2026-4942 as soon as they are available for each affected release.
Inventory all IBM i 7.3, 7.4, 7.5, and 7.6 systems and identify every service that terminates TLS, including Telnet SSL, FTP SSL, HTTP Server, and Db2 connections.
Restrict network reachability of TLS-enabled IBM i services to trusted management segments until patches are applied.

Patch Information

Refer to the IBM Support Page for CVE-2026-4942 for the specific PTF numbers and installation instructions for each affected IBM i release. Administrators should validate the fix by attempting a handshake with a disabled protocol version and confirming the server rejects it.

Workarounds

Terminate TLS on a hardened reverse proxy or load balancer in front of IBM i services and enforce the minimum TLS version there.
Use network access controls and firewall rules to permit TLS connections only from clients known to support modern protocol versions.
Increase logging verbosity for TLS handshakes on affected hosts to enable rapid detection of any successful downgrade attempts until patching completes.

bash

# Verify negotiated TLS version against an IBM i service
# Attempt to force a disabled protocol; a patched server should refuse the handshake
openssl s_client -connect ibmi.example.com:992 -tls1_1 </dev/null
openssl s_client -connect ibmi.example.com:992 -tls1 </dev/null

# Confirm only TLS 1.2 or 1.3 is accepted
openssl s_client -connect ibmi.example.com:992 -tls1_2 </dev/null