Eleven of the 15 vulnerabilities in a joint advisory on 2023’s top routinely exploited flaws were initially exploited as zero-days, according to CISA, NSA, the FBI and international partners. The advisory was published on 12 November 2024; “2023” refers to the year the agencies collected the vulnerability data. It names vulnerabilities associated with Citrix, Cisco and Fortinet, but the available official details support a closer account of the Citrix entries than of the Cisco and Fortinet flaws.
What the 2023 advisory says
The annual advisory ranks the 15 CVEs its authoring agencies collected as the year’s top routinely exploited vulnerabilities. Its headline zero-day finding is a count of vulnerabilities exploited before a fix was available—not a claim that all 15 were zero-days or that every flaw was used in the same way.
In its announcement, the NSA said 11 of the 15 CVEs were initially exploited as zero-days. For comparison, it said only two of the top exploited vulnerabilities in the 2022 report were zero-days. That comparison concerns the reports’ top-exploited sets, not every vulnerability exploited across either year. NSA announcement, 12 November 2024.
What is established about the named vendor flaws
Citrix: CVE-2023-3519 and CVE-2023-4966
The joint advisory extract identifies CVE-2023-3519 as affecting Citrix NetScaler ADC and NetScaler Gateway. Joint advisory PDF hosted by the FBI.
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A separate CISA notice identifies CVE-2023-4966, commonly called Citrix Bleed, as affecting NetScaler ADC and Gateway and describes active, targeted exploitation. CISA also says it added the vulnerability to its Known Exploited Vulnerabilities catalog. For product impact and remediation, operators should follow the current vendor instructions and CISA guidance rather than rely on the brief advisory summary. CISA guidance on CVE-2023-4966.
Cisco and Fortinet: names are not enough to establish technical details
The report is associated with Cisco IOS XE Web UI CVE-2023-20198 and Fortinet FortiOS/FortiProxy SSL-VPN CVE-2023-27997. However, the available official extracts do not establish enough detail to describe their consequences, affected versions, exploitation timelines or fixes confidently. Consult the relevant Cisco and Fortinet vendor advisories before making decisions about exposure or remediation. The report’s inclusion alone does not show that these flaws were exploited in the same manner, against the same targets, or with the same operational consequences.
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What defenders should do
The NSA recommends timely software patching and centralized patch management, alongside defensive tools such as endpoint detection and response (EDR), web application firewalls and network protocol analyzers. It also advises organizations to ask software providers about secure-by-design programs. These measures support vulnerability management; they do not replace applying relevant vendor patches or mitigations.
For organizations running affected Citrix products, use current Citrix instructions together with CISA’s guidance for CVE-2023-4966. Verify the exact product and version against vendor notices, prioritize applicable fixes, and ensure patch status is tracked centrally. The NSA’s cybersecurity technical director, Jeffrey Dickerson, said: “Network defenders should pay careful attention to trends and take immediate action to ensure vulnerabilities are patched and mitigated. Exploitation will likely continue in 2024 and 2025.” NSA announcement.
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