Add check against directory traversal for docs_dir config value

Signed-off-by: Jussi Hallila <jussi@hallila.com>

Author: Xantier

.changeset/cold-rockets-mix.md

@@ -0,0 +1,5 @@
+---
+'@backstage/techdocs-common': patch
+---
+
+Adding additional checks on tech docs to prevent folder traversal via mkdocs.yml docs_dir value.

packages/techdocs-common/src/stages/generate/__fixtures__/mkdocs_invalid_doc_dir2.yml

@@ -0,0 +1,3 @@
+site_name: Test site name
+site_description: Test site description
+docs_dir: ../../etc/

packages/techdocs-common/src/stages/generate/helpers.test.ts

@@ -344,19 +344,28 @@ describe('helpers', () => {
       mockFs.restore();
     });
 
+    const inputDir = resolvePath(__filename, '../__fixtures__/');
     it('should return true on when no docs_dir present', async () => {
-      await expect(validateMkdocsYaml('/mkdocs.yml')).resolves.toBeUndefined();
+      await expect(
+        validateMkdocsYaml(inputDir, '/mkdocs.yml'),
+      ).resolves.toBeUndefined();
     });
 
     it('should return false on absolute doc_dir path', async () => {
       await expect(
-        validateMkdocsYaml('/mkdocs_invalid_doc_dir.yml'),
+        validateMkdocsYaml(inputDir, '/mkdocs_invalid_doc_dir.yml'),
+      ).rejects.toThrow();
+    });
+
+    it('should return false on doc_dir path that traverses directory structure backwards', async () => {
+      await expect(
+        validateMkdocsYaml(inputDir, '/mkdocs_invalid_doc_dir2.yml'),
       ).rejects.toThrow();
     });
 
     it('should validate files with custom yaml tags', async () => {
       await expect(
-        validateMkdocsYaml('/mkdocs_with_extensions.yml'),
+        validateMkdocsYaml(inputDir, '/mkdocs_with_extensions.yml'),
       ).resolves.toBeUndefined();
     });
   });

packages/techdocs-common/src/stages/generate/helpers.ts

@@ -18,12 +18,12 @@ import { Entity } from '@backstage/catalog-model';
 import { spawn } from 'child_process';
 import fs from 'fs-extra';
 import yaml, { DEFAULT_SCHEMA, Type } from 'js-yaml';
-import { isAbsolute, normalize } from 'path';
 import { PassThrough, Writable } from 'stream';
 import { Logger } from 'winston';
 import { ParsedLocationAnnotation } from '../../helpers';
 import { RemoteProtocol } from '../prepare/types';
 import { SupportedGeneratorKey } from './types';
+import { resolve as resolvePath } from 'path';
 
 // TODO: Implement proper support for more generators.
 export function getGeneratorKey(entity: Entity): SupportedGeneratorKey {
@@ -158,9 +158,13 @@ const MKDOCS_SCHEMA = DEFAULT_SCHEMA.extend([
  * Validating mkdocs config file for incorrect/insecure values
  * Throws on invalid configs
  *
+ * @param {string} inputDir base dir to be used as a docs_dir path validity check
  * @param {string} mkdocsYmlPath Absolute path to mkdocs.yml or equivalent of a docs site
  */
-export const validateMkdocsYaml = async (mkdocsYmlPath: string) => {
+export const validateMkdocsYaml = async (
+  inputDir: string,
+  mkdocsYmlPath: string,
+) => {
   let mkdocsYmlFileString;
   try {
     mkdocsYmlFileString = await fs.readFile(mkdocsYmlPath, 'utf8');
@@ -173,9 +177,14 @@ export const validateMkdocsYaml = async (mkdocsYmlPath: string) => {
   const mkdocsYml: any = yaml.load(mkdocsYmlFileString, {
     schema: MKDOCS_SCHEMA,
   });
-  if (mkdocsYml.docs_dir && isAbsolute(normalize(mkdocsYml.docs_dir))) {
+
+  if (
+    mkdocsYml.docs_dir &&
+    !resolvePath(inputDir, mkdocsYml.docs_dir).startsWith(inputDir)
+  ) {
     throw new Error(
-      "docs_dir configuration value in mkdocs can't be an absolute directory path for security reasons. Use relative paths instead which are resolved relative to your mkdocs.yml file location.",
+      `docs_dir configuration value in mkdocs can't be an absolute directory or start with ../ for security reasons.
+       Use relative paths instead which are resolved relative to your mkdocs.yml file location.`,
     );
   }
 };

packages/techdocs-common/src/stages/generate/techdocs.ts

@@ -89,7 +89,7 @@ export class TechdocsGenerator implements GeneratorBase {
       );
     }
 
-    await validateMkdocsYaml(mkdocsYmlPath);
+    await validateMkdocsYaml(inputDir, mkdocsYmlPath);
 
     // Directories to bind on container
     const mountDirs = {