Fallow: A `.*` glob segment let an analyzed repository make Fallow test paths in the parent of the project root

Impact

Fallow expands some globs from the analyzed repository on disk. Before 3.34.0, a segment such as .*, .? or ..* in such a glob also matched the special .. entry. The glob crate tests .. against a segment that starts with a literal .. Fallow validates plugin glob patterns and refuses absolute paths and .. segments, but these segments passed that validation.

The attacker controls the repository content. The attacker must also be able to read the Fallow output, for example a CI log or a pull request comment. Then the attacker can learn if a file or directory exists in the parent of the project root:

• A plugin fileExists detection pattern such as .*/marker.json made the plugin active when marker.json existed in the parent directory.
• An external plugin configPatterns or alwaysUsed pattern could match a config file in the parent directory. When the plugin was active and declared toolingDependencies, the tooling detection credited the related dev dependency. An unused dependency finding could then disappear, and --trace-dependency printed the outside path.
• fallow plugin-check reported path_exists: true for a seeded entry that matched only in the parent directory.

Fallow tested only that these paths exist. It did not read or print the content of a file outside the project root through these globs.

The Jest, Vitest and Rstest project globs had the same .* problem, and the Jest and Vitest ones also read file content. GHSA-jfwm-3p3c-6hx5 describes that case.

Patches

Fixed in 3.34.0. Fallow now rewrites a leading . in a wildcard segment to [.] before it expands the glob on disk. [.] matches the same hidden files and directories inside the project root. The glob crate does not test .. against it.

Workarounds

Run Fallow only on repositories that you trust. Before you run Fallow on another repository, check the detection, configPatterns and alwaysUsed patterns of its plugin files.