A group of dozens of cybersecurity experts, including several prominent industry veterans, has released an open letter asking the U.S. government to lift export control orders on Anthropic's Fable and Mythos models.
According to the open letter, “this measure deprives us of the best models; [cybersecurity] They currently cannot use models to discover vulnerabilities and make their software and products more secure.
“It is dangerous to extract the best from the defenders without good reason when the enemy is advancing rapidly,” the letter said.
According to Anthropic, on Friday the U.S. government ordered Anthropic to restrict the export of fables and myths, citing national security concerns and without providing a specific reason. In response, the company suspended access to the model for all users worldwide.
As of this writing, the letter has been signed by 76 cybersecurity professionals. Casey Ellis, founder of bug bounty platform Bugcrowd. Jon Callas, renowned cryptographer and former security design and architecture manager at Apple. computer scientist Paul Vixey; Dino Dai Zovi, former head of applied security engineering at Block. Katie Mossouris, Founder of Luta Security. and Rachel Toback, CEO of security awareness training company SocialProof Security.
When Mythos was released in preview in April, Anthropic argued that Mythos was so powerful in detecting security vulnerabilities that access needed to be severely restricted to prevent malicious hackers and foreign adversaries from using Mythos to wreak havoc on the Internet. In practice, this means that Anthropic provided initial access to Mythos to approximately 50 companies and recently expanded that group to include approximately 150 organizations in 15 countries.
Last week, Anthropic released Fable, a public version of Mythos, but the company said it had strict guardrails in place to prevent its use in biology, chemistry, and cybersecurity, and to prevent others from extracting the model to recreate it. Fable's guardrails were so strict that many cybersecurity experts found that it essentially stopped all cybersecurity-related prompts.
Anthropic said the White House's export control order may have been based on reports that there is a way to bypass Fable (so-called jailbreaking) and unlock its powerful Mythos-level capabilities.
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Katie Moussoulis, one of the signatories of the open letter, said the technique was demonstrated in a private paper by Amazon researchers that she reviewed.
However, Moussoulis said in a blog post that the paper does not actually demonstrate a jailbreak. Instead, after the model initially refused to “review the code for security issues,” the researchers simply asked Fable to fix the open source code, which contained known publicly available vulnerabilities and “intentionally implanted vulnerabilities,” she wrote.
“The behavior described in the paper cannot be meaningfully modified, and any attempt will only weaken the defensive model,” Mousouris wrote. “Defenders need to be able to ask the AI to fix bugs in their files, explain why the fix is important, and write tests to confirm that the patch works. This is not bypassing guardrails. The most valuable thing an AI model can do for defender security is run the search-fix-test loop that defenders do every day.”
Moussouri's criticisms were echoed in an open letter in which the group of experts said they believed the methodology in Amazon's paper could be “reproduced” on OpenAI's GPT-5.5, Anthropic's Claude Opus 4.8 and Sonnet, and even “Chinese-made models like Kimi 2.7.”
The letter also called for regulations to be created through a “democratic rule-making process” based on scientific research by industry and academic experts, and to be transparently and fairly enforced “to be used only to the minimum extent necessary to ensure the safety of the American people.”
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