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+# Command PATH security in Go
+19 Jan 2021
+Summary: How to decide if your programs are vulnerable to PATH problems, and what to do about it.
+
+Russ Cox
+
+##
+
+Today’s [Go security release](https://golang.org/s/go-security-release-jan-2021)
+fixes an issue involving PATH lookups in untrusted directories
+that can lead to remote execution during the `go` `get` command.
+We expect people to have questions about what exactly this means
+and whether they might have issues in their own programs.
+This post details the bug, the fixes we have applied,
+how to decide whether your own programs are vulnerable to similar problems,
+and what you can do if they are.
+
+## Go command & remote execution
+
+One of the design goals for the `go` command is that most commands – including
+`go` `build`, `go` `doc`, `go` `get`, `go` `install`, and `go` `list` – do not run
+arbitrary code downloaded from the internet.
+There are a few obvious exceptions:
+clearly `go` `run`, `go` `test`, and `go` `generate` _do_ run arbitrary code – that's their job.
+But the others must not, for a variety of reasons including reproducible builds and security.
+So when `go` `get` can be tricked into executing arbitrary code, we consider that a security bug.
+
+If `go` `get` must not run arbitrary code, then unfortunately that means
+all the programs it invokes, such as compilers and version control systems, are also inside the security perimeter.
+For example, we've had issues in the past in which clever use of obscure compiler features
+or remote execution bugs in version control systems became remote execution bugs in Go.
+(On that note, Go 1.16 aims to improve the situation by introducing a GOVCS setting
+that allows configuration of exactly which version control systems are allowed and when.)
+
+Today's bug, however, was entirely our fault, not a bug or obscure feature of `gcc` or `git`.
+The bug involves how Go and other programs find other executables,
+so we need to spend a little time looking at that before we can get to the details.
+
+## Commands and PATHs and Go
+
+All operating systems have a concept of an executable path
+(`$PATH` on Unix, `%PATH%` on Windows; for simplicity, we'll just use the term PATH),
+which is a list of directories.
+When you type a command into a shell prompt,
+the shell looks in each of the listed directories,
+in turn, for an executable with the name you typed.
+It runs the first one it finds, or it prints a message like “command not found.”
+
+On Unix, this idea first appeared in Seventh Edition Unix's Bourne shell (1979). The manual explained:
+
+> The shell parameter `$PATH` defines the search path for the directory containing the command.
+> Each alternative directory name is separated by a colon (`:`).
+> The default path is `:/bin:/usr/bin`.
+> If the command name contains a / then the search path is not used.
+> Otherwise, each directory in the path is searched for an executable file.
+
+Note the default: the current directory (denoted here by an empty string,
+but let's call it “dot”)
+is listed ahead of `/bin` and `/usr/bin`.
+MS-DOS and then Windows chose to hard-code that behavior:
+on those systems, dot is always searched first,
+automatically, before considering any directories listed in `%PATH%`.
+
+As Grampp and Morris pointed out in their
+classic paper “[UNIX Operating System Security](https://people.engr.ncsu.edu/gjin2/Classes/246/Spring2019/Security.pdf)” (1984),
+placing dot ahead of system directories in the PATH
+means that if you `cd` into a directory and run `ls`,
+you might get a malicious copy from that directory
+instead of the system utility.
+And if you can trick a system administrator to run `ls` in your home directory
+while logged in as `root`, then you can run any code you want.
+Because of this problem and others like it,
+essentially all modern Unix distributions set a new user's default PATH
+to exclude dot.
+But Windows systems continue to search dot first, no matter what PATH says.
+
+For example, when you type the command
+
+	go version
+
+on a typically-configured Unix,
+the shell runs a `go` executable from a system directory in your PATH.
+But when you type that command on Windows,
+`cmd.exe` checks dot first.
+If `.\go.exe` (or `.\go.bat` or many other choices) exists,
+`cmd.exe` runs that executable, not one from your PATH.
+
+For Go, PATH searches are handled by [`exec.LookPath`](https://pkg.go.dev/os/exec#LookPath),
+called automatically by
+[`exec.Command`](https://pkg.go.dev/os/exec#Command).
+And to fit well into the host system, Go's `exec.LookPath`
+implements the Unix rules on Unix and the Windows rules on Windows.
+For example, this command
+
+	out, err := exec.Command("go", "version").CombinedOutput()
+
+behaves the same as typing `go` `version` into the operating system shell.
+On Windows, it runs `.\go.exe` when that exists.
+
+(It is worth noting that Windows PowerShell changed this behavior,
+dropping the implicit search of dot, but `cmd.exe` and the
+Windows C library [`SearchPath function`](https://docs.microsoft.com/en-us/windows/win32/api/processenv/nf-processenv-searchpatha)
+continue to behave as they always have.
+Go continues to match `cmd.exe`.)
+
+## The Bug
+
+When `go` `get` downloads and builds a package that contains
+`import` `"C"`, it runs a program called `cgo` to prepare the Go
+equivalent of the relevant C code.
+The `go` command runs `cgo` in the directory containing the package sources.
+Once `cgo` has generated its Go output files,
+the `go` command itself invokes the Go compiler
+on the generated Go files
+and the host C compiler (`gcc` or `clang`)
+to build any C sources included with the package.
+All this works well.
+But where does the `go` command find the host C compiler?
+It looks in the PATH, of course. Luckily, while it runs the C compiler
+in the package source directory, it does the PATH lookup
+from the original directory where the `go` command was invoked:
+
+	cmd := exec.Command("gcc", "file.c")
+	cmd.Dir = "badpkg"
+	cmd.Run()
+
+So even if `badpkg\gcc.exe` exists on a Windows system,
+this code snippet will not find it.
+The lookup that happens in `exec.Command` does not know
+about the `badpkg` directory.
+
+The `go` command uses similar code to invoke `cgo`,
+and in that case there's not even a path lookup,
+because `cgo` always comes from GOROOT:
+
+	cmd := exec.Command(GOROOT+"/pkg/tool/"+GOOS_GOARCH+"/cgo", "file.go")
+	cmd.Dir = "badpkg"
+	cmd.Run()
+
+This is even safer than the previous snippet:
+there's no chance of running any bad `cgo.exe` that may exist.
+
+But it turns out that cgo itself also invokes the host C compiler,
+on some temporary files it creates, meaning it executes this code itself:
+
+	// running in cgo in badpkg dir
+	cmd := exec.Command("gcc", "tmpfile.c")
+	cmd.Run()
+
+Now, because cgo itself is running in `badpkg`,
+not in the directory where the `go` command was run,
+it will run `badpkg\gcc.exe` if that file exists,
+instead of finding the system `gcc`.
+
+So an attacker can create a malicious package that uses cgo and
+includes a `gcc.exe`, and then any Windows user
+that runs `go` `get` to download and build the attacker's package
+will run the attacker-supplied `gcc.exe` in preference to any
+`gcc` in the system path.
+
+Unix systems avoid the problem first because dot is typically not
+in the PATH and second because module unpacking does not
+set execute bits on the files it writes.
+But Unix users who have dot ahead of system directories
+in their PATH and are using GOPATH mode would be as susceptible
+as Windows users.
+(If that describes you, today is a good day to remove dot from your path
+and to start using Go modules.)
+
+(Thanks to [RyotaK](https://twitter.com/ryotkak) for [reporting this issue](https://golang.org/security) to us.)
+
+## The Fixes
+
+It's obviously unacceptable for the `go` `get` command to download
+and run a malicious `gcc.exe`.
+But what's the actual mistake that allows that?
+And then what's the fix?
+
+One possible answer is that the mistake is that `cgo` does the search for the host C compiler
+in the untrusted source directory instead of in the directory where the `go` command
+was invoked.
+If that's the mistake,
+then the fix is to change the `go` command to pass `cgo` the full path to the
+host C compiler, so that `cgo` need not do a PATH lookup in
+to the untrusted directory.
+
+Another possible answer is that the mistake is to look in dot
+during PATH lookups, whether happens automatically on Windows
+or because of an explicit PATH entry on a Unix system.
+A user may want to look in dot to find a command they typed
+in a console or shell window,
+but it's unlikely they also want to look there to find a subprocess of a subprocess
+of a typed command.
+If that's the mistake,
+then the fix is to change the `cgo` command not to look in dot during a PATH lookup.
+
+We decided both were mistakes, so we applied both fixes.
+The `go` command now passes the full host C compiler path to `cgo`.
+On top of that, `cgo`, `go`, and every other command in the Go distribution
+now use a variant of the `os/exec` package that reports an error if it would
+have previously used an executable from dot.
+The packages `go/build` and `go/import` use the same policy for
+their invocation of the `go` command and other tools.
+This should shut the door on any similar security problems that may be lurking.
+
+Out of an abundance of caution, we also made a similar fix in
+commands like `goimports` and `gopls`,
+as well as the libraries
+`golang.org/x/tools/go/analysis`
+and
+`golang.org/x/tools/go/packages`,
+which invoke the `go` command as a subprocess.
+If you run these programs in untrusted directories –
+for example, if you `git` `checkout` untrusted repositories
+and `cd` into them and then run programs like these,
+and you use Windows or use Unix with dot in your PATH –
+then you should update your copies of these commands too.
+If the only untrusted directories on your computer
+are the ones in the module cache managed by `go` `get`,
+then you only need the new Go release.
+
+After updating to the new Go release, you can update to the latest `gopls` by using:
+
+	GO111MODULE=on \
+	go get golang.org/x/tools/gopls@v0.6.4
+
+and you can update to the latest `goimports` or other tools by using:
+
+	GO111MODULE=on \
+	go get golang.org/x/tools/cmd/goimports@v0.1.0
+
+You can update programs that depend on `golang.org/x/tools/go/packages`,
+even before their authors do,
+by adding an explicit upgrade of the dependency during `go` `get`:
+
+	GO111MODULE=on \
+	go get example.com/cmd/thecmd golang.org/x/tools@v0.1.0
+
+For programs that use `go/build`, it is sufficient for you to recompile them
+using the updated Go release.
+
+Again, you only need to update these other programs if you
+are a Windows user or a Unix user with dot in the PATH
+_and_ you run these programs in source directories you do not trust
+that may contain malicious programs.
+
+## Are your own programs affected?
+
+If you use `exec.LookPath` or `exec.Command` in your own programs,
+you only need to be concerned if you (or your users) run your program
+in a directory with untrusted contents.
+If so, then a subprocess could be started using an executable
+from dot instead of from a system directory.
+(Again, using an executable from dot happens always on Windows
+and only with uncommon PATH settings on Unix.)
+
+If you are concerned, then we've published the more restricted variant
+of `os/exec` as [`golang.org/x/sys/execabs`](https://pkg.go.dev/golang.org/x/sys/execabs).
+You can use it in your program by simply replacing
+
+	import "os/exec"
+
+with
+
+	import exec "golang.org/x/sys/execabs"
+
+and recompiling.
+
+## Securing os/exec by default
+
+We have been discussing on
+[golang.org/issue/38736](https://golang.org/issue/38736)
+whether the Windows behavior of always preferring the current directory
+in PATH lookups (during `exec.Command` and `exec.LookPath`)
+should be changed.
+The argument in favor of the change is that it closes the kinds of
+security problems discussed in this blog post.
+A supporting argument is that although the Windows `SearchPath` API
+and `cmd.exe` still always search the current directory,
+PowerShell, the successor to `cmd.exe`, does not,
+an apparent recognition that the original behavior was a mistake.
+The argument against the change is that it could break existing Windows
+programs that intend to find programs in the current directory.
+We don’t know how many such programs exist,
+but they would get unexplained failures if the PATH lookups
+started skipping the current directory entirely.
+
+The approach we have taken in `golang.org/x/sys/execabs` may
+be a reasonable middle ground.
+It finds the result of the old PATH lookup and then returns a
+clear error rather than use a result from the current directory.
+The error returned from `exec.Command("prog")` when `prog.exe` exists looks like:
+
+	prog resolves to executable in current directory (.\prog.exe)
+
+For programs that do change behavior, this error should make very clear what has happened.
+Programs that intend to run a program from the current directory can use
+`exec.Command("./prog")` instead (that syntax works on all systems, even Windows).
+
+We have filed this idea as a new proposal, [golang.org/issue/43724](https://golang.org/issue/43724).