What version of Go are you using (go version)?

$ go version
go version go1.11.5 windows/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

$ go env
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\User\AppData\Local\go-build
set GOEXE=.exe
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOOS=windows
set GOPATH=C:\Users\User\go
set GOPROXY=
set GORACE=
set GOROOT=C:\Go
set GOTMPDIR=
set GOTOOLDIR=C:\Go\pkg\tool\windows_amd64
set GCCGO=gccgo
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=C:\Users\User\project\go.mod
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=C:\Users\User\AppData\Local\Temp\go-build112371658=/tmp/go-build -gno-record-gcc-switches

What did you do?

Starting with a default Windows install (GOROOT=C:\Go, GOPATH=%USERPROFILE%\go):

1. Create a file in %USERPROFILE% with any of these prefixes:

   go/src/cmd/go/internal/modcmd/vendor.go

   Lines 143 to 153 in aab0b70

   	var metaPrefixes = []string{
   	"AUTHORS",
   	"CONTRIBUTORS",
   	"COPYLEFT",
   	"COPYING",
   	"COPYRIGHT",
   	"LEGAL",
   	"LICENSE",
   	"NOTICE",
   	"PATENTS",
   	}

   
   I used "LEGAL.txt".

2. Create a new folder in %USERPROFILE%.
   I used "project".

3. Create a main.go in that folder that will import modules.
   I used the code from https://github.com/golang/go/wiki/Modules#quick-start

4. Initialize the module (go mod init mymodule)

5. Vendor the module (go mod vendor)

You should see an error at this point: go vendor: open C:\Users\LEGAL.txt: Access is denied.
This error is given because the vendoring code is attemping to copy LEGAL.txt all the way up the directory tree.
If you change your GOPATH to somewhere that has more parent folders and follow the above instructions (create project folder at same level as GOPATH and include a LEGAL.txt in one of the parent folders), you will see it will copy it up the entire directory tree until it hits an access denied error. It will also overwrite any files with the same name in any of these folders.

What did you expect to see?

The command should not modify files outside of Go-related directories.

What did you see instead?

Go copies and overwrites files outside of Go-related directories.