fix(git): Add validation for Git subdirectory paths to prevent traversal

Signed-off-by: Suleiman Dibirov <idsulik@gmail.com>

pkg/remote/git.go

@@ -26,6 +26,7 @@ import (
 	"path/filepath"
 	"regexp"
 	"strconv"
+	"strings"
 
 	"github.com/compose-spec/compose-go/v2/cli"
 	"github.com/compose-spec/compose-go/v2/loader"
@@ -113,6 +114,9 @@ func (g gitRemoteLoader) Load(ctx context.Context, path string) (string, error)
 		g.known[path] = local
 	}
 	if ref.SubDir != "" {
+		if err := validateGitSubDir(local, ref.SubDir); err != nil {
+			return "", err
+		}
 		local = filepath.Join(local, ref.SubDir)
 	}
 	stat, err := os.Stat(local)
@@ -129,6 +133,41 @@ func (g gitRemoteLoader) Dir(path string) string {
 	return g.known[path]
 }
 
+// validateGitSubDir ensures a subdirectory path is contained within the base directory
+// and doesn't escape via path traversal. Unlike validatePathInBase for OCI artifacts,
+// this allows nested directories but prevents traversal outside the base.
+func validateGitSubDir(base, subDir string) error {
+	cleanSubDir := filepath.Clean(subDir)
+
+	if filepath.IsAbs(cleanSubDir) {
+		return fmt.Errorf("git subdirectory must be relative, got: %s", subDir)
+	}
+
+	if cleanSubDir == ".." || strings.HasPrefix(cleanSubDir, "../") || strings.HasPrefix(cleanSubDir, "..\\") {
+		return fmt.Errorf("git subdirectory path traversal detected: %s", subDir)
+	}
+
+	if len(cleanSubDir) >= 2 && cleanSubDir[1] == ':' {
+		return fmt.Errorf("git subdirectory must be relative, got: %s", subDir)
+	}
+
+	targetPath := filepath.Join(base, cleanSubDir)
+	cleanBase := filepath.Clean(base)
+	cleanTarget := filepath.Clean(targetPath)
+
+	// Ensure the target starts with the base path
+	relPath, err := filepath.Rel(cleanBase, cleanTarget)
+	if err != nil {
+		return fmt.Errorf("invalid git subdirectory path: %w", err)
+	}
+
+	if relPath == ".." || strings.HasPrefix(relPath, "../") || strings.HasPrefix(relPath, "..\\") {
+		return fmt.Errorf("git subdirectory escapes base directory: %s", subDir)
+	}
+
+	return nil
+}
+
 func (g gitRemoteLoader) resolveGitRef(ctx context.Context, path string, ref *gitutil.GitRef) error {
 	if !commitSHA.MatchString(ref.Ref) {
 		cmd := exec.CommandContext(ctx, "git", "ls-remote", "--exit-code", ref.Remote, ref.Ref)

pkg/remote/git_test.go

@@ -0,0 +1,175 @@
+/*
+   Copyright 2020 Docker Compose CLI authors
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package remote
+
+import (
+	"testing"
+
+	"gotest.tools/v3/assert"
+)
+
+func TestValidateGitSubDir(t *testing.T) {
+	base := "/tmp/cache/compose/abc123def456"
+
+	tests := []struct {
+		name    string
+		subDir  string
+		wantErr bool
+	}{
+		{
+			name:    "valid simple directory",
+			subDir:  "examples",
+			wantErr: false,
+		},
+		{
+			name:    "valid nested directory",
+			subDir:  "examples/nginx",
+			wantErr: false,
+		},
+		{
+			name:    "valid deeply nested directory",
+			subDir:  "examples/web/frontend/config",
+			wantErr: false,
+		},
+		{
+			name:    "valid current directory",
+			subDir:  ".",
+			wantErr: false,
+		},
+		{
+			name:    "valid directory with redundant separators",
+			subDir:  "examples//nginx",
+			wantErr: false,
+		},
+		{
+			name:    "valid directory with dots in name",
+			subDir:  "examples/nginx.conf.d",
+			wantErr: false,
+		},
+		{
+			name:    "path traversal - parent directory",
+			subDir:  "..",
+			wantErr: true,
+		},
+		{
+			name:    "path traversal - multiple parent directories",
+			subDir:  "../../../etc/passwd",
+			wantErr: true,
+		},
+		{
+			name:    "path traversal - deeply nested escape",
+			subDir:  "../../../../../../../tmp/pwned",
+			wantErr: true,
+		},
+		{
+			name:    "path traversal - mixed with valid path",
+			subDir:  "examples/../../etc/passwd",
+			wantErr: true,
+		},
+		{
+			name:    "path traversal - at the end",
+			subDir:  "examples/..",
+			wantErr: false, // This resolves to "." which is the current directory, safe
+		},
+		{
+			name:    "path traversal - in the middle",
+			subDir:  "examples/../../../etc/passwd",
+			wantErr: true,
+		},
+		{
+			name:    "path traversal - windows style",
+			subDir:  "..\\..\\..\\windows\\system32",
+			wantErr: true,
+		},
+		{
+			name:    "absolute unix path",
+			subDir:  "/etc/passwd",
+			wantErr: true,
+		},
+		{
+			name:    "absolute windows path",
+			subDir:  "C:\\windows\\system32\\config\\sam",
+			wantErr: true,
+		},
+		{
+			name:    "absolute path with home directory",
+			subDir:  "/home/user/.ssh/id_rsa",
+			wantErr: true,
+		},
+		{
+			name:    "normalized path that would escape",
+			subDir:  "./../../etc/passwd",
+			wantErr: true,
+		},
+		{
+			name:    "directory name with three dots",
+			subDir:  ".../config",
+			wantErr: false,
+		},
+		{
+			name:    "directory name with four dots",
+			subDir:  "..../config",
+			wantErr: false,
+		},
+		{
+			name:    "directory name with five dots",
+			subDir:  "...../etc/passwd",
+			wantErr: false, // ".....'' is a valid directory name, not path traversal
+		},
+		{
+			name:    "directory name starting with two dots and letter",
+			subDir:  "..foo/bar",
+			wantErr: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := validateGitSubDir(base, tt.subDir)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("validateGitSubDir(%q, %q) error = %v, wantErr %v",
+					base, tt.subDir, err, tt.wantErr)
+			}
+		})
+	}
+}
+
+// TestValidateGitSubDirSecurityScenarios tests specific security scenarios
+func TestValidateGitSubDirSecurityScenarios(t *testing.T) {
+	base := "/var/cache/docker-compose/git/1234567890abcdef"
+
+	// Test the exact vulnerability scenario from the issue
+	t.Run("CVE scenario - /tmp traversal", func(t *testing.T) {
+		maliciousPath := "../../../../../../../tmp/pwned"
+		err := validateGitSubDir(base, maliciousPath)
+		assert.ErrorContains(t, err, "path traversal")
+	})
+
+	// Test variations of the attack
+	t.Run("CVE scenario - /etc traversal", func(t *testing.T) {
+		maliciousPath := "../../../../../../../../etc/passwd"
+		err := validateGitSubDir(base, maliciousPath)
+		assert.ErrorContains(t, err, "path traversal")
+	})
+
+	// Test that legitimate nested paths still work
+	t.Run("legitimate nested path", func(t *testing.T) {
+		validPath := "examples/docker-compose/nginx/config"
+		err := validateGitSubDir(base, validPath)
+		assert.NilError(t, err)
+	})
+}