Arbitrary File Write

Affecting yarn package, versions <1.21.1

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Overview

yarn is a package for dependency management.

Affected versions of this package are vulnerable to Arbitrary File Write. The package install functionality can be abused to generate arbitrary symlinks on the host filesystem by using specially crafted bin keys. Existing files could be overwritten depending on the current user permission set.

Details

A Directory Traversal attack (also known as path traversal) aims to access files and directories that are stored outside the intended folder. By manipulating files with "dot-dot-slash (../)" sequences and its variations, or by using absolute file paths, it may be possible to access arbitrary files and directories stored on file system, including application source code, configuration, and other critical system files.

Directory Traversal vulnerabilities can be generally divided into two types:

  • Information Disclosure: Allows the attacker to gain information about the folder structure or read the contents of sensitive files on the system.

st is a module for serving static files on web pages, and contains a vulnerability of this type. In our example, we will serve files from the public route.

If an attacker requests the following URL from our server, it will in turn leak the sensitive private key of the root user.

curl http://localhost:8080/public/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/root/.ssh/id_rsa

Note %2e is the URL encoded version of . (dot).

  • Writing arbitrary files: Allows the attacker to create or replace existing files. This type of vulnerability is also known as Zip-Slip.

One way to achieve this is by using a malicious zip archive that holds path traversal filenames. When each filename in the zip archive gets concatenated to the target extraction folder, without validation, the final path ends up outside of the target folder. If an executable or a configuration file is overwritten with a file containing malicious code, the problem can turn into an arbitrary code execution issue quite easily.

The following is an example of a zip archive with one benign file and one malicious file. Extracting the malicious file will result in traversing out of the target folder, ending up in /root/.ssh/ overwriting the authorized_keys file:

2018-04-15 22:04:29 .....           19           19  good.txt
2018-04-15 22:04:42 .....           20           20  ../../../../../../root/.ssh/authorized_keys

Remediation

Upgrade yarn to version 1.21.1 or higher.

References

CVSS Score

2.6
low severity
  • Attack Vector
    Network
  • Attack Complexity
    High
  • Privileges Required
    Low
  • User Interaction
    Required
  • Scope
    Unchanged
  • Confidentiality
    Low
  • Integrity
    None
  • Availability
    None
CVSS:3.0/AV:N/AC:H/PR:L/UI:R/S:U/C:L/I:N/A:N/E:P/RL:O/RC:C
Credit
Daniel Ruf
CVE
CVE-2019-10773
CWE
CWE-22
Snyk ID
SNYK-JS-YARN-537806
Disclosed
11 Dec, 2019
Published
13 Dec, 2019