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Excessive privileges - Temporary Files

Description#

Opening temporary files without appropriate measures or controls can leave the file, its contents and any function that consumes it vulnerable to attack.

Impact#

  • Get access to the created temporary files.
  • Tamper data contained in the created temporary files.

Recommendation#

Create the temporary files in a different directory than the default provided by the Operative system and ensure it has 0600 permission mask.

Threat#

Anonymous attacker with local access to the server.

Score#

Default score using CVSS 3.1. It may change depending on the context of the vulnerability.

Base#

  • Attack vector: L
  • Attack complexity: H
  • Privileges required: H
  • User interaction: R
  • Scope: U
  • Confidentiality: L
  • Integrity: L
  • Availability: N

Temporal#

  • Exploit code madurity: P
  • Remediation level: U
  • Report confidence: C

Result#

  • Vector string: CVSS:3.1/AV:L/AC:H/PR:H/UI:R/S:U/C:L/I:L/A:N/E:P/RL:U/RC:C
  • Score:
    • Base: 2.9
    • Temporal: 2.8
  • Severity:
    • Base: Low
    • Temporal: Low

Details#

This java method has the following signature:

public static File createTempFile(
// The prefix string defines the files name;
// must be at least three characters long
String prefix,
// The suffix string defines the file's extension;
// if null the suffix ".tmp" will be used
String suffix,
// The directory in which the file is to be created.
// For default temporary-file directory null is to passed.
File directory,
)

The first two arguments do not affect security of the created file.

In Linux, files and directories are different entities, they have their own permissions and are isolated from other files and directories.

Protections you apply to the directory do not affect the files inside of it. For example, if I protect the directory I can prevent an attacker from executing the ls command, but nothing impedes the attacker from executing cat directory/file.

To prevent access to the file we must protect the file, not the directory.

Additionally, we make sure that permissions are set atomically. If we do not, the following situation can happen:

  • At moment A we File.createTempFile() a file.
  • At moment B we add secure permissions to the file.

After moment B the file is secured. However, between moment A and moment B the file has insecure permissions and an attacker had enough opportunity to get control over it.

Vulnerable implementation#

The java.io.File.createTempFile method creates files with write permissions in groups and other:

import java.io.File;
public class Test {
public static void main(String[] args){
try {
System.out.println(File.createTempFile("xxx", null));
}
catch (Exception e) {}
}
}
/*
* $ ls -al $(javac Test.java && java Test)
*
* -rw-r--r-- 1 fluid fluid 0 Aug 28 14:44 /tmp/xxx948760279845756007.tmp
*/

Secure implementation#

  • Use java.nio.file.Files.createTempFile.
  • Use the attrs argument (an optional list of file attributes to set atomically when creating the file).

Requirements#