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Techskills Linuxsecurity 1 3 Auditing User Passwords

This document discusses techniques for auditing and enforcing strong user passwords in Linux. It describes how to set password policies that expire passwords after a certain number of days and warn users before expiration using chage and /etc/login.defs. It also explains how to require complex passwords using the /etc/security/pwquality.conf file and criteria like length, uppercase/lowercase letters, numbers, and special characters. Finally, it provides instructions for using John the Ripper to test password strength through a brute force attack on the hashed passwords.

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54 views2 pages

Techskills Linuxsecurity 1 3 Auditing User Passwords

This document discusses techniques for auditing and enforcing strong user passwords in Linux. It describes how to set password policies that expire passwords after a certain number of days and warn users before expiration using chage and /etc/login.defs. It also explains how to require complex passwords using the /etc/security/pwquality.conf file and criteria like length, uppercase/lowercase letters, numbers, and special characters. Finally, it provides instructions for using John the Ripper to test password strength through a brute force attack on the hashed passwords.

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DDDD
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Linux Security Techniques - 1.

0 Monitoring and Auditing Security

============================================================

Filename: techskills-linuxsecurity-1-3-auditing_user_passwords
Title: Auditing User Passwords
Subtitle: Linux Security Techniques

1.3 Auditing User Passwords


How can we set a password policy for our users?

Can be set individually using chage


Display current info
chage -l jdoe
Define password expiration policy
chage <username>
Modify policy
chage -m <mindays> -M <maxdays> -E <expiredate> -W <warndays> jdoe
chage -M 90 -m 1 -W 7 jdoe

Can we make that affect all users?

Can be set globally by editing /etc/login.defs


Only applied at account creation
Not retroactive
Settings
PASS_MAX_DAYS 90
PASS_MIN_DAYS 1
PASS_WARN_AGE 7
PASS_MIN_LEN 5

What about requiring a complex password?

Password complexity
/etc/security/pwquality.conf
minlen- Password length (in credits)
lcredit - Lower case characters
ucredit - Upper case characters
ocredit - Other characters
dcredit - Digits
Minlen defines "credits" not length
1 credit for each character
Additional credits for other criteria
Use a -1 to indicate one or more of a character
Does not count for credits
Example: minlen=8 lcredit=1 ucredit=1 ocredit=1 dcredit=1
Password: 12345678
Passes
One credit for each character (+8)
One credit for each number (dcredit) (+8)
Adds up to 16 credits
Example: minlen=8 lcredit=1 ucredit=-1 ocredit=-1 dcredit=-1
Password: 12345678
Fails
One credit for each character (+8)
Penalty for missing an upper case character (-1)
Penalty for missing a lower case character (-1)
Penalty for missing a special character (-1)
Adds up to 5
Password: P@ssw0rd
One credit for each character (+8)
No penalty for missing an upper case character (+0)
No penalty for missing a lower case character (+0)
No penalty for missing a special character (+0)
Adds up to 8

How do we know if our users are actually using a strong password?


Passwords are stored as non-reversible hashes
Hashed passwords can't reveal complexity
echo -n 1234567890 | sha256sum
The only way to test is a brute force attack
John the Ripper
Open source password utility
Performs a dictionary attack followed by permutations
Using the tool
1. Install John the Ripper
yum install -y wget make gcc
wget http://www.openwall.com/john/j/john-1.8.0.tar.gz
tar -xvzf ./john-1.8.0.tar.gz
cd ./john-1.8.0/src
make clean linux-x86-64
make for full list
cd ../run
./john --test
2. Export your hashed passwords to a text file
./unshadow /etc/passwd /etc/shadow > users.txt
3. Cleanup unnecessary records from the file
vi ./users.txt
Delete any line with no password (!! or *)
:g/!!/d
:g/:\*:/d
4. Run the attack
./john ./users.txt

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