Claude Mythos Audited Symfony and Found 19 Vulnerabilities (Symfony Blog)

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Claude Mythos Audited Symfony and Found 19 Vulnerabilities

May 21, 2026

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Javier Eguiluz

Claude Mythos Preview is a new general-purpose AI language model by Anthropic.<br>This model performs strongly across the board, but it is especially strong at<br>computer security tasks.

This model is not publicly available yet, but Anthropic is making it available<br>to selected tech projects via their Project Glasswing. Through this initiative,<br>Claude Mythos has found thousands of security vulnerabilities , including<br>some in every major operating system and web browser.

Symfony recently teamed up with The PHP Foundation and Anthropic to build<br>the official MCP SDK for PHP applications. That's why we reached out to some<br>folks at Anthropic, and they were kind enough to provide us with a one-off<br>analysis of Symfony's and Twig's code by Claude Mythos Preview .

Security Analysis Results

A few days later, and following Symfony's security disclosure process, we<br>received a ZIP file with all their findings. In total, Claude Mythos reported<br>19 security vulnerabilities in Symfony and Twig codebases. The Symfony<br>Core Team reviewed every report manually, and all 19 findings turned out to be<br>real vulnerabilities, with no false positives .

Each vulnerability was reported in a separate file containing:

The CWE, affected files, component, and version

A summary of the problem with the vulnerable code highlighted

Step-by-step exploitation instructions and impact analysis

A reproducer

A suggested fix

We've already fixed every one of these issues in our latest security<br>releases. Details are available in the security advisories blog category.

The Future of Code Security

In 2011, Symfony organized a crowdfunding campaign to pay for an external<br>security audit of Symfony code and, in 2019, Symfony set up a bug bounty program<br>with the support of the European Commission.

In 2026, models like Claude Mythos Preview and initiatives like Project Glasswing<br>are revolutionizing the way code security is audited. Thanks to Anthropic for<br>giving us a chance to be part of it.

We're also grateful to every security researcher who recently reported issues to<br>us, whether using other AI tools or through careful manual review.

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