100 Cybersecurity Experts Demand US Reverse Its Ban on Fable 5

Three days after the US government ordered Anthropic to shut down Fable 5 and Mythos 5, about 100 leading cybersecurity specialists published an open letter demanding that the ban be lifted. In their assessment, the decision does not make the country safer. It weakens the people who defend it and gives attackers a tactical advantage.

The defenders' argument

The letter's position is blunt. Taking the best AI tools away from cyber defense professionals while adversaries keep using them with no restrictions at all is not a security measure but deliberate sabotage of one's own infrastructure. That is how the specialists describe the core of the problem.

"Taking the best AI tools away from defenders while adversaries keep

building them is not security, it is sabotage," the open letter states.

Fable 5 and Mythos 5 had already become standard tools in the arsenal of many specialized teams. The models are used for automated malicious code analysis, real-time vulnerability detection, faster incident response and monitoring of critical infrastructure. Forcing them offline does not make networks safer. It simply shifts the tactical advantage toward those who attack.

Who signed

The letter carries the signatures of about 100 specialists: researchers from major technology companies, analysts from government agencies and independent experts. The scale and diversity of the signatories point to a systemic position within the community, not narrow professional lobbying. Their areas of work include:

• threat and malware analysis
• protection of critical infrastructure and industrial control systems
• zero-day vulnerability and exploit research
• incident response, forensics and attack investigations
• academic research on the security of AI systems

The authors are not theorists. They are practitioners who had already built Fable 5 into real workflows and overnight lost access to a tool that had become an industry standard for them.

The logic of the ban and its contradictions

The US authorities' decision was driven by concerns that Fable 5 and Mythos 5 could be used to create cyberweapons or automate attacks. It is the classic dual-use dilemma: the same capabilities help both defend and attack.

The problem is that malicious actors, whether state hackers, organized groups or insiders, are not bound by the American regulatory sphere. They can easily switch to comparable models from providers in other jurisdictions. The ban on Anthropic creates a clear asymmetry: legitimate defenders lose their tools, while illegitimate attackers do not.

Furthermore, Fable 5 was developed with an emphasis on safety and alignment, which is what made it attractive to the professional community. If regulatory pressure falls specifically on responsible AI companies, market incentives will start working against the goals of regulation itself.

What this means

The Fable 5 situation exposes a core contradiction in the approach to AI regulation. Security-motivated restrictions often hit precisely those who provide that security. If regulators keep systematically restricting access to frontier models for legitimate players, the gap between attackers' and defenders' capabilities will only widen.

The open letter from 100 experts is a serious signal to the authorities about the real cost of such decisions.