fix: take the shim header's helpers off the caller's PATH - #60
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The header of every POSIX bin shim resolved `readlink`, `dirname`, `sed`,
and `uname` by bare name. A shim runs with `node_modules/.bin` at the
front of `PATH` by design, which is where a dependency's own bins live,
so a dependency could supply any of those four names. The shim runs it
before reaching its target, and its answer decides the directory the
shim execs from.
`readlink`, `sed`, and `uname` now go through `command -p`, which
searches the POSIX system default path rather than the caller's.
`dirname` is dropped for `${link%/*}`, which removes two forks per shim
invocation. `${link%/*}` needs a separator to strip, so `$0` is
normalized to `./name` when a `PATH` lookup left it a bare name.
`cygpath` and `wslpath` are left resolving through `PATH`. `command -p`
helps only if the helper is on the system default path, and that is not
verifiable for two tools that exist only on Cygwin, MSYS2, and WSL, where
no CI job runs. A `command -p cygpath` that came up empty would leave
`basedir_win` a POSIX path that `node.exe` cannot open.
The header is byte-identical to the one pnpm v12 generates after
pnpm/pnpm#14845, so pacquet keeps recognizing
shims this package wrote.
Ref pnpm/pnpm#14837
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W5f2VB7CDEj1hu1SEy5chh
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📝 WalkthroughWalkthroughThe Unix shim now uses system-path helper commands, shell parameter expansion, and bare-link normalization during symlink resolution. POSIX end-to-end tests and shim snapshots cover the updated behavior. ChangesUnix shim resolution
Priority: ➖ Normal Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🟡 Moderate · up to Cygwin, MSYS, and WSL shim execution can still run a caller-selected path-conversion helper. Resolve those helpers through the system path while retaining the existing unavailable-helper fallback before merging. 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
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PR Summary by QodoHarden POSIX shim helper resolution against PATH hijacking
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Files changed (4)
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Inline comments:
In `@src/index.ts`:
- Around line 570-571: Update the basedir conversion logic around the cygpath
and wslpath availability checks to use command -p for both helper lookup and
invocation, avoiding caller-provided PATH overrides. Preserve the existing
conversion-failure fallback behavior.
In `@test/e2e.test.js`:
- Line 254: Update the test around spawnSync to invoke tsc-link by command name
rather than the absolute link path, and add binDir after decoyDir in PATH so
command resolution exercises the bare-command normalization branch. Preserve the
decoy helpers before binDir.
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🪛 ast-grep (0.45.3)
test/e2e.test.js
[warning] 207-207: Filesystem path is not a string literal; a request-/variable-derived path can enable path traversal. Validate and normalize the path before use.
Context: fs.writeFileSync(file, body, 'utf8')
Note: [CWE-22] Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal').
(detect-non-literal-fs-filename)
[warning] 221-221: Filesystem path is not a string literal; a request-/variable-derived path can enable path traversal. Validate and normalize the path before use.
Context: fs.writeFileSync(hijackTarget, 'console.log("hijacked")\n', 'utf8')
Note: [CWE-22] Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal').
(detect-non-literal-fs-filename)
[warning] 240-240: Filesystem path is not a string literal; a request-/variable-derived path can enable path traversal. Validate and normalize the path before use.
Context: fs.writeFileSync(target, 'console.log("tsc-output")\n', 'utf8')
Note: [CWE-22] Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal').
(detect-non-literal-fs-filename)
[warning] Importing child_process exposes a command-execution surface; ensure any command/argument built from input is validated, and prefer execFile/spawn with an argument array over exec.
Context: import { spawn, spawnSync } from 'node:child_process'
Note: [CWE-78] Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection').
(detect-child-process)
The kernel and execvp hand the interpreter the path they resolved, so `$0` is bare only when a shell is given the name itself. Cover that branch of the header directly instead of through a PATH lookup, which cannot reach it. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01W5f2VB7CDEj1hu1SEy5chh
The guard that normalizes a bare `$0` to `./name` looked only for a forward slash. The header converts backslashes to slashes before it takes the directory, so a Windows-form `$0` such as `C:\foo\bar` has no forward slash yet and the guard mistook it for a bare name. It became `./C:\foo\bar`, and the directory came out `./C:/foo`, which resolves against the working directory instead of the drive root. `dirname` answered `C:/foo`. The guard now counts a backslash as a separator too, matching `@zkochan/cmd-shim`, which carries this same header for pnpm 11's shims. The placeholder bin keeps the narrower guard. It has no backslash-to-slash conversion and cannot run on Windows, so a backslash there is an ordinary character in a file name, which is how `dirname` treats it as well. See pnpm/cmd-shim#60
A warm reinstall decided a shim was current from its `# cmd-shim-target=` marker alone whenever no NODE_PATH block was expected, which is the ordinary case. The marker says nothing about the header, so a shim written before the helpers moved to `command -p` still matched and stayed on disk. Upgrading pnpm therefore left every existing shim resolving `readlink`, `sed`, and `uname` on the caller's PATH, where a dependency's own bins come first, and the fix reached only shims whose target had moved. The marker-only branch now also requires `is_sh_shim_hardened`, which looks for the header's `command -p readlink` line. A shim that lacks it is rewritten even though its target has not moved. Shims that `@zkochan/cmd-shim` wrote are rewritten once until pnpm/cmd-shim#60 ships the same header, which is the safe direction: those shims have the bug. `generate_sh_shim_header_carries_the_hardened_helper_line` pins the line the check looks for against the header that emits it, so reformatting one cannot silently strand the other.
The guard that normalizes a bare `$0` to `./name` looked only for a forward slash. The header converts backslashes to slashes before it takes the directory, so a Windows-form `$0` such as `C:\foo\bar` has no forward slash yet and the guard mistook it for a bare name. It became `./C:\foo\bar`, and the directory came out `./C:/foo`, which resolves against the working directory instead of the drive root. `dirname` answered `C:/foo`. The guard now counts a backslash as a separator too, matching `@zkochan/cmd-shim`, which carries this same header for pnpm 11's shims. The placeholder bin keeps the narrower guard. It has no backslash-to-slash conversion and cannot run on Windows, so a backslash there is an ordinary character in a file name, which is how `dirname` treats it as well. See pnpm/cmd-shim#60
A warm reinstall decided a shim was current from its `# cmd-shim-target=` marker alone whenever no NODE_PATH block was expected, which is the ordinary case. The marker says nothing about the header, so a shim written before the helpers moved to `command -p` still matched and stayed on disk. Upgrading pnpm therefore left every existing shim resolving `readlink`, `sed`, and `uname` on the caller's PATH, where a dependency's own bins come first, and the fix reached only shims whose target had moved. The marker-only branch now also requires `is_sh_shim_hardened`, which looks for the header's `command -p readlink` line. A shim that lacks it is rewritten even though its target has not moved. Shims that `@zkochan/cmd-shim` wrote are rewritten once until pnpm/cmd-shim#60 ships the same header, which is the safe direction: those shims have the bug. `generate_sh_shim_header_carries_the_hardened_helper_line` pins the line the check looks for against the header that emits it, so reformatting one cannot silently strand the other.
1 participant
Summary
The header of every POSIX bin shim this package generates resolved
readlink,dirname,sed, andunameby bare name, so the caller'sPATHdecided which of them ran. A shim runs withnode_modules/.binat the front ofPATH, which is where a dependency's own bins live, so a dependency could supply any of those four names. The shim runs it before reaching its target, and its answer decides the directory the shim finally execs from.readlink,sed, andunamenow go throughcommand -p, which searches the POSIX system default path instead of the caller's.dirnameis gone: directories come from${link%/*}, so the header also forks two fewer processes per shim invocation.${link%/*}needs a separator to strip, so$0is normalized to./namefirst when aPATHlookup left it a bare name.This is the
@zkochan/cmd-shimhalf of pnpm/pnpm#14837. pnpm v12 carries its own copy of this header and fixes it in pnpm/pnpm#14845; the header here is byte-identical to that one, so pacquet keeps recognizing shims this package wrote. pnpm v11 generates its shims through this package, so it needs a release of this fix and a dependency bump.Not changed:
cygpathandwslpathThe same header reaches
cygpathandwslpaththroughPATH.command -ponly helps if the helper is on the system default path, which is verifiable forreadlink,sed, anduname(CI runs the new test on Linux) but not for two tools that exist only on Cygwin, MSYS2, and WSL. Acommand -p cygpaththat came up empty on Git Bash would leavebasedir_wina POSIX path thatnode.execannot open, breaking every shim there. That wants a check on one of those hosts before it changes.Tests
The new e2e test runs a generated shim through a relative symlink with decoy
readlink,dirname,sed, andunamefirst onPATH, plus a secondnode_modulestree for them to point at and a plantednode.exein the shim's bin directory for theunamedecoy. It fails on the current header with the shim printinghijacked.The snapshot files were updated by exact text replacement of the header block, so the Windows-only entries are preserved.
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Summary by CodeRabbit
Bug Fixes
Tests