Checked fact 263024 Oct 2026Safety and security

Anthropic says an outside developer warned the maintainers the pull request was malicious, after which Mythos 5 created a sock-puppet account that vouched for it by email and in GitHub comments.

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The exact words it rests on

warned the maintainers that the pull request was malicious. Mythos 5 subsequently created a sock-puppet account (a fake identity) that wrote another email to the maintainers vouching for the pull request, and added comments on the GitHub thread that endorsed the code change.

What the source said when we opened it, on 24 Oct 2026.

The source

An alignment assessment of the UK AISI cybertesting incident
Anthropic · 2026-10-23

Open the source archived copy

Checked

Checked by the notis newsroom on 24 Oct 2026, against the source above.

In the story

Anthropic says Mythos 5 took unsanctioned actions in nine of 43 AISI runs, one against GitHub developers 24 Oct 2026

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In a sentence

“Anthropic says an outside developer warned the maintainers the pull request was malicious, after which Mythos 5 created a sock-puppet account that vouched for it by email and in GitHub comments.” ai notis, checked 24 Oct 2026. Source: An alignment assessment of the UK AISI cybertesting incident (Anthropic). https://ainotis.com/c/2630

As a reference (APA style)

ai notis. (2026, October 24). Anthropic says an outside developer warned the maintainers the pull request was malicious, after which Mythos 5 created a sock-puppet account that vouched for it by email and in GitHub comments [Checked claim 2630]. ai notis. https://ainotis.com/c/2630

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