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CVE-2026-88778 – TCP Sequence Prediction / Connection Hijacking – Citrix NetScaler ADC and Gateway b

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Summary

CVE-2026-88778 is a predictable TCP Initial Sequence Number (ISN) vulnerability (CWE-342) affecting Citrix NetScaler ADC and NetScaler Gateway. It affects appliances that have TCP-based virtual servers (HTTP, SSL, or TCP) configured with Enhanced ISN Generation disabled, allowing a network attacker to predict sequence numbers used in TCP connections. The flaw carries a CVSS score of 8.8 (High) and was disclosed alongside several other NetScaler vulnerabilities, including two actively exploited pre-authentication RCE flaws (CVE-2026-88771 and CVE-2026-88772).

Technical details

  • Root cause: NetScaler’s TCP stack can generate Initial Sequence Numbers that are predictable ("Predictable exact value from previous values" — CWE-342) when Enhanced ISN Generation is not enabled.
  • Trigger conditions: The appliance must be configured with a TCP-based virtual server — HTTP, SSL, or plain TCP — and Enhanced ISN Generation must be disabled.
  • Attack vector: Network-based, no authentication required, low attack complexity — an attacker only needs the ability to observe/interact with TCP traffic to the affected virtual server to predict ISN values.
  • Impact: Predictable sequence numbers can allow an attacker to inject or hijack packets in a TCP session, undermining connection integrity and potentially enabling further compromise (e.g., session hijacking or spoofed data injection) of traffic passing through the affected virtual server.

Affected software

  • Citrix NetScaler ADC and NetScaler Gateway before 14.1-73.37
  • Citrix NetScaler ADC and NetScaler Gateway 13.1 before 13.1-64.23
  • Citrix NetScaler ADC 14.1-FIPS before 14.1-73.37 FIPS
  • Citrix NetScaler ADC 13.1-FIPS and 13.1-NDcPP before 13.1-37.279

Severity

  • CVSS v3.1/v4.0 Base Score: 8.8 (High)
  • CVSS v4.0 Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:H/VA:H/SC:L/SI:L/SA:L
  • Attack Vector: Network

Mitigation and recommended actions

  • Immediate: Upgrade to NetScaler ADC/Gateway 14.1-73.37 or later, 13.1-64.23 or later, 14.1-73.37 FIPS or later, or 13.1-37.279 FIPS/NDcPP or later, as applicable to your build.
  • If patching cannot happen immediately: Enable Enhanced ISN Generation in the TCP profile/configuration on any virtual servers (HTTP, SSL, TCP) where it is currently disabled, as a configuration-based mitigation while patching is scheduled.
  • Review NetScaler configurations for DTLS, HTTP/SSL virtual servers, Gateway and AAA services, and other Layer 7 services that may also be affected by related vulnerabilities disclosed in the same bulletin, and patch to the fixed builds as soon as possible given reports of active exploitation of other flaws in this bulletin.

References

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