GHSA-x6jw-m9v5-85vh simple-git unsafe-operation guard does not block trailer command configuration

• npm simple-git: >= 3.15.0, < 4.0.1 → fixed in 4.0.1

Affected product The default `blockUnsafeOperationsPlugin` in `simple-git` when an application permits untrusted values to reach `SimpleGitOptions.config` or Git inline configuration arguments such as `-c =`. Summary `trailer..cmd` is not recognized as unsafe by the default guard. Therefore, a configured inline value reaches Git without a `GitPluginError`. Git documents `trailer..cmd` as a shell command invoked by `git interpret-trailers`. An application that relies on the default plugin to reject unsafe configuration can therefore execute a command supplied through an untrusted trailer-command configuration value. Technical details `simple-git/src/lib/git-factory.ts` installs `commandConfigPrefixingPlugin` before `blockUnsafeOperationsPlugin`. The prefixing plugin in `simple-git/src/lib/plugins/command-config-prefixing-plugin.ts` turns every `SimpleGitOptions.config` entry into `-c =` before the unsafe-operation plugin evaluates the final argv. In `[email protected]`, `blockUnsafeOperationsPlugin` delegates to `@simple-git/argv-parser`. `packages/argv-parser/src/vulnerabilities/detect-vulnerable-config-writes.ts` compares parsed configuration writes against `preventUnsafeConfig`. That list has no matcher for `trailer..cmd`, so the invocation is allowed. Git v2.39.5's `Documentation/git-interpret-trailers.txt` states that `trailer..cmd` specifies a shell command called to generate or modify a trailer. Preconditions The application must use an affected `simple-git` version with the default unsafe-operation plugin active and must pass attacker-controlled data into instance configuration or Git command arguments that configure `trailer..cmd`. The invoked Git binary must support the documented trailer-command behavior, and the application must execute `git interpret-trailers` with the attacker-controlled configuration in scope. The command runs with the operating-system identity and permissions of the Node.js process. Verification Use an isolated test environment and a harmless executable test helper that records only its invocation. Control: Configure `core.editor=` through `SimpleGitOptions.config` and invoke a benign Git task. The default plugin should throw `GitPluginError` before spawning Git because `core.editor` is present in `preventUnsafeConfig`. Bypass: Configure `trailer.audit.cmd=` through the same option and invoke Git with the equivalent argv shape: `git -c trailer.audit.cmd= interpret-trailers --trailer audit: ` A vulnerable build does not raise `GitPluginError`; Git invokes the test helper while processing the trailer. Confirm the helper invocation, then remove test artifacts. Impact An attacker who controls the stated configuration input can cause Git to execute a shell command as the Node.js application process. The impact is bounded by that process's filesystem, network, and service permissions. Applications that do not expose untrusted configuration or command arguments to `simple-git` are outside this threat model. Affected versions Commit `6b3c631eadea81f80ed10f6dec7d19a9db4d7084` introduced the default unsafe-operation plugin, and `[email protected]` is the first release confirmed to contain it. Its implementation only rejected `protocol.allow` configuration, leaving trailer-command configuration unblocked. The latest `simple-git` release, `3.36.0`, still lacks a trailer-command matcher. The current `main` branch also lacks one. No released remediation was identified. Remediation Default-deny configuration keys that can trigger executable behavior, or add a dedicated unsafe category that rejects `trailer..cmd` before spawning Git unless the application explicitly opts in. Evaluate `trailer..command` alongside `.cmd`, because Git documents it as related command behavior. Add parser and integration tests for leading `-c`, configured instance prefixes, and `git config` write forms, asserting that no Git child process is spawned without an explicit unsafe opt-in. Evidence • `[email protected]` was released on 2022-11-12 and contains the initial unsafe-operation plugin. • `[email protected]` was released on 2026-04-12; its parser source does not match `trailer..cmd`. • `main` retains the missing matcher in `packages/argv-parser/src/vulnerabilities/detect-vulnerable-config-writes.ts`. • Git v2.39.5 documents the trailer command behavior in `Documentation/git-interpret-trailers.txt`. • PR #1167 expanded other configuration checks but did not add a trailer-command matcher and is not release-backed as a remediation. • This review verified repository, release, and source artifacts through GitHub; it did not independently execute the runtime reproduction.