BREW-breezy-GHSA-8w8g-wq8h-fq33 — Dulwich: Symlink write-through in checkout(paths=[]) via raw os.open bypasses all symlink protections

Affected: Homebrew: breezy

## Summary

Dulwich's `porcelain.checkout(paths=[...])` code path writes files using raw `os.open(file_path, O_WRONLY|O_CREAT|O_TRUNC, mode)` followed by `f.write(obj.data)`. This code path does NOT call `build_file_from_blob()` at all, completely bypassing any symlink protections (including the unreleased d09f8af fix). `os.open` without `O_NOFOLLOW` follows symlinks at both the target file and intermediate directories, allowing arbitrary file writes.

## Root Cause

At `dulwich/porcelain/__init__.py:5661-5675`, the `checkout(paths=[...])` implementation:

```python file_path = _checked_worktree_path(r, path) os.makedirs(os.path.dirname(file_path), exist_ok=True) flags = os.O_WRONLY | os.O_CREAT | os.O_TRUNC with os.fdopen(os.open(file_path, flags, mode), "wb") as f: f.write(obj.data) ```

`_checked_worktree_path()` (line 601-631) only performs name validation — checking that the path doesn't start with `/` or `\\` and that components pass `INVALID_DOTNAMES` checks. It performs zero filesystem symlink detection.

## Impact

An attacker can craft a malicious repository that, when a victim clones it and runs `checkout(paths=[...])`, writes attacker-controlled content (with attacker-controlled permissions) to any filesystem location accessible to the user. Writing to `.git/hooks/post-checkout` achieves RCE on the next git checkout.

## Attack Scenario

1. Attacker creates a repository where HEAD has `trigger` as a symlink (mode 120000, content `../../.git/hooks/post-checkou…

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BREW-breezy-GHSA-8w8g-wq8h-fq33 — Dulwich: Symlink write-through in checkout(paths=[]) via raw os.open bypasses all symlink protections