How Japan's Government and Financial Institutions Responded to Claude Mythos — FSA and Megabank Reactions and Corporate Preparedness

The Japanese Government and Financial Institutions' Response to Claude Mythos refers to a series of coordinated public-private defense measures undertaken by the Financial Services Agency (FSA), the Bank of Japan, and major banks in response to "Claude Mythos" — a cybersecurity-specialized AI that Anthropic has restricted from public release due to misuse risks.

This article examines how Japan responded to Claude Mythos, from the perspective of decision-makers in management and DX promotion. By the end, readers will understand: (1) why Japan moved under government leadership, (2) the specific responses of the FSA and major banks, and (3) the preparations non-financial companies should take. The capabilities of the Mythos model itself, the vulnerabilities it discovered, and the details of Project Glasswing are covered in a separate article: Claude Mythos and Project Glasswing.

Claude Mythos is a general-purpose AI announced by Anthropic in April 2026, with exceptional cybersecurity capabilities. It is reported to have reached a level where it can independently discover vulnerabilities that have long been dormant in operating systems, browsers, and open-source software, and automatically generate exploit code. Because these capabilities can be repurposed for attacks, Anthropic has withheld public release and is providing access only to a limited set of partners (Anthropic).

The key point is that this is not a "convenient business AI" — it is a capability that fundamentally transforms both offense and defense in security. The revelation that unknown vulnerabilities exist in vast numbers within the foundational software everyone relies on, and that attacks exploiting them could be automated, is what put nations and financial infrastructure on alert.

The subject of this article lies beyond that. The question is: "When an AI this powerful emerges, how do nations and financial infrastructure brace themselves?" The model's capabilities, its discovery track record, and the mechanics of Project Glasswing are left to the separate article Claude Mythos and Project Glasswing; here, the focus is exclusively on the response of the Japanese government and financial institutions.

If a Mythos-class AI falls into the hands of attackers, it could strike directly at financial infrastructure. Because the damage would not be confined to a single company but would ripple through the nation's entire credit order, Japan chose not to leave this to individual companies — the government stepped to the forefront.

The financial system serves as the lifeblood of society, handling payments, remittances, and market transactions. If it were halted or tampered with through a cyberattack, the damage would not stop at a single company — it would cascade into market disruption and a crisis of confidence. For example, if a bank's payment system were compromised, it could trigger a chain reaction of suspended transfers, ATM outages, and delayed interbank settlements, ultimately affecting the cash flow of business partners. If securities settlement were halted, the market itself could become dysfunctional. A failure in finance does not end as "just that bank's problem."

What Mythos revealed is the reality that unknown vulnerabilities lie dormant in vast numbers even within foundational software long considered "secure." If attackers were to acquire equivalent capabilities, attacks exploiting those vulnerabilities could be automated at massive scale and high speed. This is precisely why the Japanese government treated this not as a security issue for a single private company, but as a matter concerning the stability of the entire financial infrastructure. The recognition that AI capabilities are directly tied to the risk facing the nation's core systems is what drove the government to take the lead.

Because Mythos is not publicly available, it is tempting to think, "Our company can't use it, so it's not our concern." But that misses the point. The issue is that vulnerabilities have been confirmed in the foundational software that both companies and financial institutions depend on — and that equivalent capabilities could eventually spread to the attacking side as well.

A concrete example makes this clear. If unknown vulnerabilities remain in the server OS, VPN appliances, or payment processing libraries your company uses, there will be little defense if an attacker with Mythos-class capabilities exploits them. The question is not "when will an attack come?" but "assuming an attack will come, how many vulnerabilities have we already closed?"

In other words, Japan's response is not about "adopting a convenient AI" — it is a defensive story about how to fortify defenses in advance as the targeted side. The judgment that now, while defenders still have a window to get ahead and patch vulnerabilities before attacks arrive, is the time to act — this is what drove the swift response from the government and the financial sector. The consistent posture throughout is not to wait passively for damage, but to prepare proactively.

The Minister for Financial Services declared Mythos an "imminent crisis" and convened an emergency public-private meeting with the Bank of Japan and major bank executives at the Financial Services Agency, leading to the establishment of a public-private working group and the granting of access rights.

The Japanese government's response was swift. The Minister for Financial Services characterized Claude Mythos as an "imminent crisis" and convened an emergency public-private meeting at the Financial Services Agency, bringing together the Governor of the Bank of Japan and the heads of major banks (Ledge.ai).

The series of responses began moving immediately after Anthropic disclosed Mythos. The fact that top officials from financial regulators, the central bank, and major banks gathered in one place within such a short time of a powerful AI's emergence speaks to the depth of the government's sense of urgency. Normally, institutional responses to new technologies are deliberated over time, but in this case, the judgment that "we cannot afford to wait" came first.

The backdrop is a strong sense of crisis that a cyberattack on the financial system could directly trigger market turmoil and a loss of confidence in credit. Rather than treating this as a single company's problem, the government took the lead in framing it as a matter concerning the stability of the entire financial infrastructure. This can be seen as a symbolic event in which AI capabilities were brought into national-level risk discussions.

The government announced the establishment of a public-private working group to address cyber risks. Approximately 36 organizations are set to participate, including the Bank of Japan, the Tokyo Stock Exchange, major banks, regional banks, and Anthropic's Japanese subsidiary, with the agenda covering not only cyber defense research but also the development of AI ethics guidelines (Ledge.ai).

What is noteworthy is that rather than imposing regulations to contain the threat, the approach has stakeholders sharing defensive knowledge at the same table and working to standardize responses. Starting from the premise that no single company can defend against AI threats alone, the initiative is steering toward building a framework in which "the entire industry defends together."

Equally important is that the development of AI ethics guidelines is being pursued alongside defensive research. The scope extends beyond defensive countermeasures against attacks to encompass establishing rules for how powerful AI should be used responsibly. This aligns with the broader direction of Japan's AI policy, which seeks to simultaneously advance "promotion of utilization" and "assurance of trustworthiness." The breadth of this response is evident in the ambition to go beyond symptomatic treatment of threats and to put in place a comprehensive framework for how such AI is used.

In addition, the Minister for Financial Services announced a policy to grant access to Claude Mythos to Japanese government bodies and financial institutions (Nikkei). This is a symbolic choice. Rather than keeping a powerful AI that could be repurposed for attacks at arm's length on the grounds that it is dangerous, it reflects a judgment that "those on the defensive side are precisely the ones who should possess this weapon."

If the defensive side uses Mythos to proactively close vulnerabilities in their own systems, damage can be limited even if the attacking side acquires equivalent capabilities. Japan's government approach aligns in direction with Anthropic's policy of providing limited access with the aim of giving the defensive side a head start. The defining characteristic of this response lies in choosing to "distribute it quickly to the right side" rather than "keeping it at bay through regulation."

That said, there are those who take a cautious view of financial institutions themselves possessing capabilities that could be used offensively. That is precisely why restricting access to security use cases and establishing usage logs and governance frameworks must go hand in hand with granting access. Distributing a powerful tool necessarily entails the responsibility of managing it. Only when a mechanism exists to track who used it, for what purpose, and in what manner can it safely function as "a weapon for the defensive side."

While the three major banks gain early access to Mythos, the Financial Services Agency has requested that regional banks put countermeasures in place. As threats transcend national borders, responses are also being discussed at the G7.

According to reports, the three megabanks — Mitsubishi UFJ, Mizuho, and Sumitomo Mitsui — are expected to gain access within a short timeframe, positioning them as the first Japanese companies to leverage Mythos (Nikkei).

Large banks face vast systems with broad attack surfaces, yet they are better positioned to secure specialized talent and budgets. The anticipated sequence is for well-resourced major institutions to reinforce their defenses first, then share the lessons learned across the industry.

In practical terms, the primary use case is expected to be "attacker-perspective auditing" — proactively identifying vulnerabilities lurking in their own systems and applications. The idea is to use capabilities like those of Mythos to scan enormous volumes of code that human reviewers cannot keep pace with, patching vulnerabilities before attackers can exploit them. How effectively the early adopters build their defenses will serve as a benchmark for other financial institutions and industries that follow.

Meanwhile, there are moves by the Financial Services Agency (FSA) to urge regional banks — which have more limited resources — to prepare countermeasures against misuse (Security Measures Lab). This is where a structural challenge lies. Because attackers target the weakest link, a disparity in defensive capabilities means that institutions slower to respond could become the point of breach.

For regional banks, the realistic path forward is not to independently operate Mythos-level systems, but to first solidify the fundamentals — prompt patch application, understanding of their own assets, and establishing monitoring frameworks. Beyond that, the key will be how they incorporate knowledge shared through public-private frameworks and leverage cooperative utilization mechanisms.

If the capability gap between large and smaller institutions widens, the security of the entire system will be dragged down by its weakest point. The inclusion of regional banks in public-private working groups reflects a recognition that raising the baseline across the entire industry is essential. The defensive principle that security is determined by "the weakest point" applies here in full.

Capabilities like those of Mythos transcend national borders. Attack infrastructure is often located overseas, which means the effectiveness of any single country's countermeasures has its limits. In fact, responses are being discussed at the G7 Finance Ministers and Central Bank Governors' Meeting (Business+IT), and countries are moving toward building a coordinated defensive posture.

Even if each country protects its own financial institutions, international remittances and markets are interconnected — meaning the country or institution with the most lagging countermeasures can become the weak point for the entire system. Even if one country's defenses are airtight, a breach at a connected counterpart makes spillover unavoidable. The G7 discussions reflect the need for a joint response to this kind of "connected risk."

This signifies that the emergence of powerful AI has elevated the issue from a concern for individual companies or nations to an item on the international security agenda. Japan's domestic actions must also be understood within the context of this international coordination. Domestic response and international cooperation are becoming inseparable — two sides of the same coin.

While the current activity is centered on finance, this is not an issue that companies outside the financial sector can afford to ignore. The vulnerabilities Mythos uncovers exist not in industry-specific software, but in the operating systems, browsers, and open-source components used by every organization. Before equivalent capabilities eventually proliferate more widely, defenders should use this window — while they still have the advantage — to solidify their foundations.

The necessary preparations are not extraordinary; they lie in rigorously executing the basics. First, maintain the capacity to promptly update OS, browsers, and libraries. As discoveries increase, so will remediation patches, meaning the speed of application directly determines defensive strength. Second, understand what components make up your organization's software — i.e., maintain a Software Bill of Materials (SBOM). Without knowing what is used and where, you cannot act even when a vulnerability is disclosed. Third, establish usage rules and log management for generative AI, bringing internal AI use itself under governance.

Furthermore, these foundational measures align with the direction of rule-making being set out by the government. The AI Business Operator Guidelines from the Ministry of Internal Affairs and Communications (MIC) and the Ministry of Economy, Trade and Industry (METI) call for incorporating human oversight into the autonomous decision-making of AI agents, as well as managing input and output logs. A framework that governs and records internal AI use serves both as a Mythos countermeasure and as the foundation for compliance with these guidelines. Rather than running "security measures" and "AI governance compliance" as separate tracks, it is more efficient to satisfy both through the same framework. For broader trends in AI cybersecurity, see also Latest Trends in AI Cybersecurity.

Surrounding these developments, there are misconceptions that can easily lead to flawed management decisions. Two of the most representative ones are worth addressing.

Because this movement is centered on finance, it would be a mistake to assume "this doesn't concern us since we're not in finance." The reason the government moved on finance first is simply that the potential impact is greatest and the urgency is highest. The vulnerabilities themselves exist across every organization that uses software—manufacturing, retail, healthcare, and government alike.

In manufacturing, for example, production control systems; in healthcare, electronic medical records; in government, systems handling resident data—all may harbor the same flaws in foundational software as financial institutions. It is not uncommon for small and medium-sized enterprises connected through supply chains to be used as entry points for breaching large corporations. Finance simply moved first; the risk itself knows no industry or organizational size.

If anything, general companies without dedicated security departments or budgets—unlike financial institutions—tend to deprioritize the basics, such as updates and asset management. Reframing "finance has acted" as a signal to "inspect our own foundations" is the first practical step toward preparedness.

As public-private frameworks take shape, it is tempting to think "if the government is handling it, we can afford to wait." However, what the government and working groups are building are cross-industry frameworks and guidelines—they will not directly patch the vulnerabilities in each company's own systems. Updating the software your organization uses and maintaining an accurate inventory of assets are responsibilities that ultimately rest with each individual company.

In practical terms, there are steps you can take even while waiting for government frameworks to be finalized. Decide in advance how many days after a critical vulnerability is disclosed you will apply a patch. Test whether you can actually restore operations from your critical data backups—not just assume you can. These self-contained measures can be started right now.

Government action provides a tailwind, but it is not a substitute. In fact, now that vigilance is heightened across the entire country, it is an ideal time to advance your own preparedness. Rather than waiting for frameworks to be established, acting first on basic measures will ultimately prove to be the fastest path to protection.

Below is a compilation of frequently asked questions from those working in management and DX promotion regarding Claude Mythos and Japan's response.

If the financial system were halted by a cyberattack, market disruption and a crisis of confidence could ripple across the entire country. The Minister for Financial Services described Mythos as a "clear and present danger" and convened an emergency public-private meeting at the Financial Services Agency with the Bank of Japan and the major banks (Ledge.ai). The very fact that financial leaders gathered within a short period of a powerful AI's announcement reflects the intensity of the sense of crisis. Finance is the lifeblood of society—if it stops, the real economy is directly affected. This response is symbolic of a judgment that AI capabilities have become directly linked to national-level risk.

At this time, that is not possible. Mythos is available on a limited basis, and in Japan the process of granting access to government bodies and financial institutions is still underway (Nikkei). It is not yet accessible to general companies in the way standard Claude is, and no timeline for general availability has been indicated. For this reason, rather than building plans on the assumption that your organization will use Mythos, it is more realistic to take the stance of "securing your own defenses first, on the assumption that the attacking side may already have access to equivalent capabilities." For details on the model and its availability framework, please refer to the separate article Claude Mythos and Project Glasswing.

Start with the basics. It is best to begin with these three points: (1) establish a practice of promptly updating your OS, browsers, and software in use; (2) maintain an inventory of the software components your organization relies on; and (3) verify backup procedures and recovery processes for critical data. No major investment is required. Simply deciding on a single rule—such as "within how many days will we apply a patch after a critical vulnerability is disclosed"—can dramatically improve response speed. Regardless of organizational size, the most effective defense is having a system that can respond to disclosed vulnerabilities quickly and reliably.

The emergence of Claude Mythos has heralded an era in which powerful AI can also serve as a tool for attack. Rather than leaving this to individual companies, Japan's Financial Minister declared it "a clear and present danger," and the Financial Services Agency, the Bank of Japan, and the major banks moved together in a government-led, swift response involving both the public and private sectors. The granting of access rights, the establishment of public-private working groups, requests to regional banks, and discussions at the G7—all of these align with a consistent objective: for the defenders to get ahead before the attacks come.

What is notable is that Japan chose not to "keep AI at a distance through regulation," but rather to "distribute it quickly to the right parties and defend as an entire industry." By pairing the granting of access rights with logging and governance frameworks, and by running defensive research and AI ethics guideline development in parallel, the approach aims not merely to treat the symptoms of threats, but to put in place the entire framework for how AI is used.

For those in management and DX promotion roles, the key takeaway is neither to be overly fearful nor to wait for the government to act. Treat the financial sector's moves as a signal to inspect your own organization's foundations, and use this window—while the defenders still have the advantage—to solidify the basics: understanding your foundational software, accelerating updates, and establishing governance over AI use. The capabilities of Mythos itself and the mechanics of Project Glasswing are explained in detail in a separate article, Claude Mythos and Project Glasswing. Our company is also committed to helping organizations achieve both security and governance in the age of AI. If you have any concerns about your preparedness, please feel free to reach out to us.