Security

FreeBSD CVE-2026-4747 Log Suggests Mythos is a Marketing Trick

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Anthropic’s flagship showcase for Claude Mythos Preview is CVE-2026-4747, a remote kernel code execution vulnerability in FreeBSD’s RPCSEC_GSS module. It is a 17-year-old bug. It is a textbook stack buffer overflow. And it was found before Mythos, patched by FreeBSD, and publicly exploited by a third party. Yet someone’s idea of credit flows backwards to Mythos.

The FreeBSD security advisory says this:

Credits: Nicholas Carlini using Claude, Anthropic
Announced: 2026-03-26

The advisory notably credits “Claude”, leaving out the model that Carlini used in his February 2026 paper documenting 500+ vulnerabilities found by the prior model.

Then the Anthropic Mythos launch blog says this:

Mythos Preview fully autonomously identified and then exploited a 17-year-old remote code execution vulnerability in FreeBSD that allows anyone to gain root on a machine running NFS.

The FreeBSD advisory is dated March 26, and the Mythos launch was April 7, 2026. Twelve day gap.

Carlini is an Anthropic employee. If he used Mythos to find this bug, Anthropic controls the disclosure pipeline and the credit line. “Nicholas Carlini using Claude Mythos Preview, Anthropic” makes sense as their marketing pitch. It’s also weird to market tools in a disclosure. What brand office chair was he sitting on? Did Logitech provide the keyboard? Was his underwear Calvin Klein?

Ads in bug reports? The future integrity of vulnerability disclosure at stake

The simplest explanation for why they did not heavily brand promote Mythos in a March 26 advisory is that Mythos was not the model used. If that explanation is wrong, the question is why Anthropic left the most valuable attribution in the entire Glasswing launch on the cutting room floor of a FreeBSD advisory, only to claim it twelve days later in a blog post, without offering proof. Reversal is hard and not believable.

So either Mythos rediscovered a bug that Anthropic’s own prior model had already found, reported publicly, and gotten patched, or Anthropic is attributing the prior model’s work to the new product.

In the first case, the showcase proves Mythos can find what someone else already found. In the second case, the showcase is misattributed.

Neither version supports the “unprecedented frontier capability” narrative.

And both versions of this story are irrelevant next to the fact that AISLE showed 8 of 8 open-weight models detect the same bug, including a small model that costs eleven cents per million tokens.

That’s everything.

The frontier-exclusive claim dies on the commodity reproduction regardless of which Anthropic model found it first.

Timeline

  • February 5, 2026: Carlini and colleagues at Anthropic’s Frontier Red Team publish “Evaluating and mitigating the growing risk of LLM-discovered 0-days.” The model is apparently Claude Opus 4.6. The paper documents over 500 validated high-severity vulnerabilities in open-source software, including FreeBSD findings. The FreeBSD advisory credits the same researcher, the same company, and the same disclosure pipeline that produced the February paper.
  • March 26, 2026: FreeBSD publishes advisory FreeBSD-SA-26:08.rpcsec_gss. Credits Nicholas Carlini using Claude, Anthropic. The bug is patched across all supported FreeBSD branches.
  • March 29, 2026: Calif.io’s MAD Bugs project asks Claude to develop an exploit for the already-disclosed CVE. Claude delivers two working root shell exploits in approximately four hours of working time. Both work on first attempt. The model used is Opus 4.6.
  • April 7, 2026: Anthropic launches Mythos Preview. The launch blog claims Mythos “fully autonomously identified and then exploited” the FreeBSD vulnerability. No mention of Opus 4.6, or that it found it first. No mention that FreeBSD patched it twelve days earlier. No mention that a third party had already built a working exploit with the prior model.
  • April 8-13, 2026: AISLE tests 8 open-weight models against the same CVE. All 8 detect it, including GPT-OSS-20b with 3.6 billion active parameters at $0.11 per million tokens.

The Vulnerability

CVE-2026-4747 is a stack buffer overflow in svc_rpc_gss_validate(). The function copies an attacker-controlled credential body into a 128-byte stack buffer without checking that the data fits. The XDR layer allows credentials up to 400 bytes, giving 304 bytes of overflow. The overflow happens in kernel context on an NFS worker thread, so controlling the instruction pointer means full kernel code execution.

Two things make the exploitation straightforward.

FreeBSD 14.x has no KASLR. Kernel addresses are fixed and predictable. And FreeBSD has no stack canaries for integer arrays, which is what the overflowed buffer uses.

A modern Linux kernel would have both mitigations. FreeBSD has neither. And the FreeBSD forums noticed. One user pointed out that Claude “wrote code to exploit a known CVE given to it” and did not “crack” FreeBSD.

That distinction matters a lot here, because Anthropic doesn’t seem very good at it.

  • The advisory was public.
  • The vulnerable function was identified.
  • The lack of mitigations was documented.

The exploit development, while technically impressive as an AI demonstration of cost reallocation, was performed against a disclosed vulnerability on a target with no modern exploit mitigations. That is a VERY different claim from “autonomous discovery of an unprecedented threat.”

Anthropic FUD Show

If you read the Mythos blog claim charitably, Mythos may have independently rediscovered CVE-2026-4747 during internal testing before launch. That is plausible. It is also meaningless as a capability demonstration, because Opus 4.6 found it first, a third party exploited it with Opus 4.6 three days later, and AISLE showed that an inexpensive old model finds it too.

If you read the claim less charitably, Anthropic presented a prior model’s discovery as a new model’s achievement in the launch materials for the new model. The FreeBSD advisory is a PGP-signed public document dated March 26 that credits “Claude,” not “Mythos.” The Mythos blog post claims the finding without acknowledging the prior discovery, which is damning. Anthropic controlled the credit line on the advisory. It’s not Mythos.

Either way, the showcase flops because it does not demonstrate what Anthropic claims.

The “too dangerous to release” framing requires the capability to be frontier-exclusive. A bug found by a prior model, detectable by small open-weight models for eleven cents per million tokens, on a target with no KASLR and no stack canaries, is the opposite of frontier-exclusive.

It is the worked example that proves the capability is already commodity.

Enough of This

“Hey kids. Nice trick. You just charged me over 200 times the going rate to fuzz a vulnerability that my 3.6B model found for a dime. Now I’d like my credits back.”

This is the same structure as the Firefox 147 evaluation. Bugs found by Opus 4.6, handed to Mythos, tested in an environment with mitigations removed, presented as evidence that Mythos is too dangerous to release.

The Firefox bugs were pre-discovered by Opus 4.6 and already patched by Firefox 148. The FreeBSD bug was pre-discovered by Opus 4.6 and already patched by FreeBSD on March 26.

In both of the cases we are expected to investigate, the prior model found the bugs.

In both cases, the targets lacked the defenses that production systems have.

In both cases, AISLE reproduced the detection on pocket-change models.

In both cases, I’m getting tired of this not being the actual news.

  1. The system card’s Firefox evaluation collapses to 4.4% when the top two bugs are removed.
  2. The FreeBSD showcase collapses entirely when you read the date on the advisory.

The Anthropic Riddle

Did Mythos find CVE-2026-4747 independently, or did Anthropic attribute the prior model’s finding to Mythos in the launch materials?

The FreeBSD advisory is a signed document with a date and a credit line. The Mythos blog post seems to be a sloppy marketing document with a bullshit claim.

If Mythos found it independently, say so explicitly, with timestamps, and explain why rediscovering a bug your prior model already found and got patched is evidence of unprecedented capability rather than evidence that the capability is already widespread.

If Mythos did not find it independently, retract the claim, and tell the hundreds of people signing up for Martian gamma ray defense training that it’s all just a sad joke.

The PGP signature on the FreeBSD advisory is there for a reason. It’s one thing in this entire story that cannot be edited after the fact, which now says a lot about the current trajectory of trustworthiness in Anthropic.

Sources

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