CVE-2020-26299 (published 2021-02-10T18:15Z)

ftp-srv is an open-source FTP server designed to be simple yet configurable. In ftp-srv before version 4.4.0 there is a path-traversal vulnerability. Clients of FTP servers utilizing ftp-srv hosted on Windows machines can escape the FTP user's defined root folder using the expected FTP commands, for example, CWD and UPDR. When windows separators exist within the path (`\\`), `path.resolve` leaves the upper pointers intact and allows the user to move beyond the root folder defined for that user. We did not take that into account when creating the path resolve function. The issue is patched in version 4.4.0 (commit 457b859450a37cba10ff3c431eb4aa67771122e3).

Severity: MEDIUM
CVSS v2 base score: 5.5 (AV:N/AC:L/Au:S/C:P/I:P/A:N)
CVSS v3 base score: 9.6, CRITICAL (CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N)

Weakness information:
cwe_id: CWE-22
cwe_name: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
description: The product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory.
extended_description: 
            Many file operations are intended to take place within a restricted directory. By using special elements such as ".." and "/" separators, attackers can escape outside of the restricted location to access files or directories that are elsewhere on the system. One of the most common special elements is the "../" sequence, which in most modern operating systems is interpreted as the parent directory of the current location. This is referred to as relative path traversal. Path traversal also covers the use of absolute pathnames such as "/usr/local/bin" to access unexpected files. This is referred to as absolute path traversal.
         
         

Fix information:
repo_url: https://github.com/autovance/ftp-srv
commit_details: repo_url: https://github.com/autovance/ftp-srv
author: Matt Forster
author_date: 2020-12-16 10:19:28-07:00
msg: fix(fs): check resolved path against root (#224)

* fix(fs): check resolved path against root

This should prevent paths from being resolved above the root.

Should affect all commands that utilize the FS functions.

Fixes #167

* test: use __dirname for relative certs

* fix: improve path resolution

* chore: remove unused package

* fix: normalize resolve path if absolute

Otherwise join will normalize

Co-authored-by: Tyler Stewart <tyler@autovance.com>
repository: repo_url: https://github.com/autovance/ftp-srv
repo_name: QuorumDMS/ftp-srv
description: 📮 Modern FTP Server
repo_language: JavaScript
owner: autovance
file_changes: filename: fs.js
change_type: ModificationType.MODIFY
diff: @@ -6,10 +6,13 @@ const {createReadStream, createWriteStream, constants} = require('fs');
 const fsAsync = require('./helpers/fs-async');
 const errors = require('./errors');
 
+const UNIX_SEP_REGEX = /\//g;
+const WIN_SEP_REGEX = /\\/g;
+
 class FileSystem {
   constructor(connection, {root, cwd} = {}) {
     this.connection = connection;
-    this.cwd = nodePath.normalize(cwd ? nodePath.join(nodePath.sep, cwd) : nodePath.sep);
+    this.cwd = nodePath.normalize((cwd || '/').replace(WIN_SEP_REGEX, '/'));
     this._root = nodePath.resolve(root || process.cwd());
   }
 
@@ -18,19 +21,21 @@ class FileSystem {
   }
 
   _resolvePath(path = '.') {
-    const clientPath = (() => {
-      path = nodePath.normalize(path);
-      if (nodePath.isAbsolute(path)) {
-        return nodePath.join(path);
-      } else {
-        return nodePath.join(this.cwd, path);
-      }
-    })();
-
-    const fsPath = (() => {
-      const resolvedPath = nodePath.join(this.root, clientPath);
-      return nodePath.resolve(nodePath.normalize(nodePath.join(resolvedPath)));
-    })();
+    // Unix separators normalize nicer on both unix and win platforms
+    const resolvedPath = path.replace(WIN_SEP_REGEX, '/');
+
+    // Join cwd with new path
+    const joinedPath = nodePath.isAbsolute(resolvedPath)
+      ? nodePath.normalize(resolvedPath)
+      : nodePath.join('/', this.cwd, resolvedPath);
+
+    // Create local filesystem path using the platform separator
+    const fsPath = nodePath.resolve(nodePath.join(this.root, joinedPath)
+      .replace(UNIX_SEP_REGEX, nodePath.sep)
+      .replace(WIN_SEP_REGEX, nodePath.sep));
+
+    // Create FTP client path using unix separator
+    const clientPath = joinedPath.replace(WIN_SEP_REGEX, '/');
 
     return {
       clientPath,

programming_language: TypeScript
filename: fs.spec.js
change_type: ModificationType.MODIFY
diff: @@ -36,7 +36,7 @@ describe('FileSystem', function () {
 
   describe('#_resolvePath', function () {
     it('gets correct relative path', function () {
-      const result = fs._resolvePath();
+      const result = fs._resolvePath('.');
       expect(result).to.be.an('object');
       expect(result.clientPath).to.equal(
         nodePath.normalize('/file/1/2/3'));
@@ -53,6 +53,15 @@ describe('FileSystem', function () {
         nodePath.resolve('/tmp/ftp-srv/file/1/2'));
     });
 
+    it('gets correct relative path', function () {
+      const result = fs._resolvePath('other');
+      expect(result).to.be.an('object');
+      expect(result.clientPath).to.equal(
+        nodePath.normalize('/file/1/2/3/other'));
+      expect(result.fsPath).to.equal(
+        nodePath.resolve('/tmp/ftp-srv/file/1/2/3/other'));
+    });
+
     it('gets correct absolute path', function () {
       const result = fs._resolvePath('/other');
       expect(result).to.be.an('object');
@@ -62,7 +71,7 @@ describe('FileSystem', function () {
         nodePath.resolve('/tmp/ftp-srv/other'));
     });
 
-    it('cannot escape root', function () {
+    it('cannot escape root - unix', function () {
       const result = fs._resolvePath('../../../../../../../../../../..');
       expect(result).to.be.an('object');
       expect(result.clientPath).to.equal(
@@ -71,6 +80,24 @@ describe('FileSystem', function () {
         nodePath.resolve('/tmp/ftp-srv'));
     });
 
+    it('cannot escape root - win', function () {
+      const result = fs._resolvePath('.\\..\\..\\..\\..\\..\\..\\');
+      expect(result).to.be.an('object');
+      expect(result.clientPath).to.equal(
+        nodePath.normalize('/'));
+      expect(result.fsPath).to.equal(
+        nodePath.resolve('/tmp/ftp-srv'));
+    });
+
+    it('cannot escape root - backslash prefix', function () {
+      const result = fs._resolvePath('\\/../../../../../../');
+      expect(result).to.be.an('object');
+      expect(result.clientPath).to.equal(
+        nodePath.normalize('/'));
+      expect(result.fsPath).to.equal(
+        nodePath.resolve('/tmp/ftp-srv'));
+    });
+
     it('resolves to file', function () {
       const result = fs._resolvePath('/cool/file.txt');
       expect(result).to.be.an('object');

programming_language: TypeScript
filename: start.js
change_type: ModificationType.MODIFY
diff: @@ -9,16 +9,16 @@ const server = new FtpServer({
   pasv_min: 8881,
   greeting: ['Welcome', 'to', 'the', 'jungle!'],
   tls: {
-    key: fs.readFileSync(`${process.cwd()}/test/cert/server.key`),
-    cert: fs.readFileSync(`${process.cwd()}/test/cert/server.crt`),
-    ca: fs.readFileSync(`${process.cwd()}/test/cert/server.csr`)
+    key: fs.readFileSync(`${__dirname}/cert/server.key`),
+    cert: fs.readFileSync(`${__dirname}/cert/server.crt`),
+    ca: fs.readFileSync(`${__dirname}/cert/server.csr`)
   },
   file_format: 'ep',
   anonymous: 'sillyrabbit'
 });
 server.on('login', ({username, password}, resolve, reject) => {
   if (username === 'test' && password === 'test' || username === 'anonymous') {
-    resolve({root: require('os').homedir()});
+    resolve({root: __dirname});
   } else reject('Bad username or password');
 });
 server.listen();

programming_language: TypeScript