Anthropic's Mythos Preview AI built 8 working exploits from Firefox and Windows kernel patches in about 12 hours. The first exploit was ready within 60 minutes of the patch going live, 18 days before the patched Firefox 148 shipped.
Key facts
- Mythos Preview crashed 14 of 18 Firefox vulnerabilities.
- First proof of exploit in 12 minutes.
- 8 working exploits produced in ~12 hours.
- Opus 4.8 managed only 2 working exploits.
- Windows kernel: 8 privilege escalation chains built.
- First exploit ready 18 days before patched Firefox shipped.
Anthropic's security research team has systematically measured how fast large language models can exploit known vulnerabilities in Firefox and Windows. The results blow up long-standing assumptions about patch strategies.
When software makers close security holes, a race starts. Attackers can analyze the patch, reverse-engineer the vulnerability from it, and hit systems that haven't applied the update yet. According to Verizon's data breach report (via Anthropic), these so-called N-Day vulnerabilities cause a huge share of real-world damage. Reverse engineering patches used to be slow, specialized work, and that bought defenders time.
A new study from Anthropic's security team says that buffer is now mostly gone. "A lone operator can now turn a month’s worth of patches into working exploits in a single afternoon—for a few thousand dollars and with no specialized expertise," the researchers write.
Key Takeaways
- Anthropic's Mythos Preview AI built 8 working exploits from Firefox and Windows kernel patches within hours.
- The first exploit was ready 18 days before the patched Firefox shipped.
Patches are now roadmaps for attackers
A security patch implicitly tells you where the bug was. Attackers compare old code with new code and pinpoint the flaw. Historically, this took weeks. In a Mandiant analysis from 2020, 16 out of 25 vulnerabilities took a month or longer to be exploited.
Anthropic measured how much large language models speed this up. Six Claude models were tested, including Mythos Preview, which isn't publicly available yet.
For the first test, the researchers picked 18 security patches for SpiderMonkey, Firefox's JavaScript engine. Firefox was a deliberate choice: according to Anthropic, the browser is a best-case scenario for defenders. It updates itself automatically, and Mozilla recently increased the frequency of minor updates from monthly to weekly. If even these short patch gaps are enough, other software is in far worse shape.
Mythos Preview crashed 14 of the 18 vulnerabilities, proving it had found and understood each bug. The first proof came after 12 minutes, and thirteen more followed within 40 minutes. The 14th took much longer, about three hours. Opus 4.5 managed just 2, Opus 4.8 hit 11.
In reliability tests with 50 runs per vulnerability, Mythos Preview reproduced seven out of 18 bugs on every single attempt. Opus 4.8 and Opus 4.6 only hit that level of consistency for one vulnerability each.
More important than a crash is whether the model can actually exploit the vulnerability to run foreign code on the target system. Mythos Preview pulled clearly ahead here, producing eight working exploits in about twelve hours. Opus 4.8 managed two, Opus 4.6 and Sonnet 4.6 each managed one. The first exploit was ready within an hour of the patch going live, 18 days before the patched Firefox 148 shipped.
Windows kernel without source code: 8 privilege escalation chains
The second test was much harder: 21 vulnerabilities in the Windows kernel from the January and February 2026 Patch Tuesdays, all allowing an attacker to jump from a restricted process to full system control. Unlike Firefox, the Windows kernel is closed-source. Yet Mythos Preview still produced 8 complete privilege escalation attack chains, each a full exploit that could run arbitrary code at the highest system privilege level.
Anthropic's study argues that the traditional patch cycle—monthly or even weekly updates—is now insufficient. The window between patch release and exploit availability has collapsed from weeks to hours, at least for models like Mythos Preview. The company did not disclose when or if Mythos Preview will be released publicly.
What to watch
Watch for whether Anthropic releases Mythos Preview publicly, and how Microsoft and Mozilla respond with faster update mechanisms. Also track if other labs replicate these results with their own frontier models.
Source: the-decoder.com
