Sandboxed git fetch cannot use the `gh auth git-credential` helper: `~/Library/Keychains` stays denied

[dgtjn]

What happened

Since c9bfdc3 ("Extend sandboxing to package processing"), upgrading a formula whose url is a private GitHub repository (url "https://github.com/<owner>/<private-repo>.git", tag:, revision:) fails when the new tag has to be fetched:

fatal: could not read Username for 'https://github.com': terminal prompts disabled

git is configured with the GitHub CLI as its credential helper (credential.https://github.com.helper=!/opt/homebrew/bin/gh auth git-credential, as written by gh auth setup-git), and gh auth status reports the token is stored in the (keyring), which is the default on macOS.

Why

GitDownloadStrategy#allow_fetch_credentials recognises the gh auth git-credential helper and opens GH_CONFIG_DIR / ~/.config/gh to the sandbox. That is not enough when gh keeps its token in the macOS Keychain: the sandbox's home deny list in sandbox.rb still includes Library/Keychains, so gh auth git-credential get returns nothing and git falls back to a prompt it cannot show.

Reproduced outside brew with a profile containing only (deny file-read* (subpath "<home>/Library/Keychains")): under sandbox-exec, gh auth git-credential get returns no credential; without the profile it returns one.

Workaround

brew skips the fetch when the tag is already in the cached clone, so fetching it from a normal shell first lets the upgrade through:

git -C "$(brew --cache)/<name>--git" fetch origin tag <tag>

A first install (no cached clone yet) has no such workaround.

Environment

• Homebrew 7.0.6-21-g3f12c3e (on main because homebrew.devcmdrun is set; 7.0.6 itself is not affected)
• macOS 27.0, Apple Silicon
• gh with the token in the keyring, git credential helper set by gh auth setup-git
• Seen on two Macs

Suggestion

When the gh helper is detected and gh stores its token in the keyring, allow the sandboxed git process to reach the Keychain (read access to the login keychain, plus whatever mach lookup securityd needs). Or document that the helper is only supported with gh auth login --insecure-storage.

[github-actions]

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[Yarden-zamir]

I can reproduce this on Homebrew 7.0.6-70-gce46735 (macOS, Apple Silicon). A formula with a private url "https://github.com/<owner>/<repo>.git", tag:, revision: fails on upgrade with the same could not read Username error. gh auth status shows the token in the keyring. A plain git fetch of the same cached clone outside brew works.

Library/Homebrew/sandbox.rb still lists Library/Keychains in the home deny list after d2b5dc5 ("Relax Git download credential sandboxing"). GIT_CREDENTIAL_PATHS opens .config/gh, but gh keeps the token in the Keychain.

Workaround for tap maintainers: a CurlDownloadStrategy subclass that adds Authorization: Bearer #{GitHub::API.github_cli_token} at fetch time, with the tag archive URL and a sha256. Curl downloads run outside the fetch sandbox, so gh auth token can read the Keychain.

@dgtjn, the bot reopens this issue if you restore the sections of the current template that it lists above.

(Drafted with Claude Code, model Claude Opus 5.5. I reviewed it and will answer questions myself.)

[MikeMcQuaid]

@Yarden-zamir please open your own issue, thanks.