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Fix passwordless login for sensitive accounts, only deny passwordless privilege escalation

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Aaron Rainbolt 2025-11-10 22:40:15 -06:00
parent fb587f78fd
commit 3070aa5d1f
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GPG key ID: A709160D73C79109
1 changed files with 43 additions and 6 deletions
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49
usr/libexec/security-misc/block-unsafe-logins#security-misc-shared
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@ -9,7 +9,12 @@ set -o errtrace
set -o pipefail
if [ -z "${PAM_USER:-}" ]; then
true "$0: ERROR: Environment variable PAM_USER is unset!"
true "$0: ERROR: Environment variable 'PAM_USER' is unset!"
## 'exit 0' here to let the appropriate PAM module handle this.
exit 0
fi
if [ -z "${PAM_SERVICE:-}" ]; then
true "$0: ERROR: Environment variable 'PAM_SERVICE' is unset!"
## 'exit 0' here to let the appropriate PAM module handle this.
exit 0
fi
@ -33,6 +38,43 @@ fi
true "INFO: session type: user session"
if [ "$PAM_USER" = 'sysmaint' ]; then
printf '%s\n' 'ERROR: Rejecting sysmaint account in user session!'
exit 1
fi
## Threat model: If a legitimate privilege escalation executable exists on the
## system that uses PAM, and we do not yet lock down its permissions with
## permission-hardener, we should prevent the use of that application to
## escalate privileges to any passwordless account that is a member of a group
## considered to be "sensitive" while in a user session. For instance, if
## account 'user' is a member of group 'sudo', and a su-like executable
## exists on the system that account 'nginx' can execute, 'nginx' should not
## be able to escalate privileges to account 'user'.
##
## Out of scope:
## - Malicious privilege escalation tools. These would need to run as root or
## be SUID, and such an executable most likely would grant root access
## without authentication or by authenticating the attacker rather than a
## legitimate user.
## - Legitimate privilege escalation tools that don't use PAM. Preventing
## these from causing issues would require manipulating password locks in
## /etc/passwd on bootup, which is dangerous and likely impossible to do
## correctly.
## - Legitimate login tools that use PAM, such as login, greetd, sshd. These
## tools do not permit privilege escalation from one user to another, and
## passwordless login is expected to work even for sensitive accounts.
login_service_list=( 'login' 'greetd' 'sshd' )
for login_service in "${login_service_list[@]}"; do
if [ "$PAM_SERVICE" = "$login_service" ]; then
true "INFO: Login service '$PAM_SERVICE' is considered safe, allowing authentication to proceed."
exit 0
fi
done
true "INFO: Login service '$PAM_SERVICE' is potentially unsafe, checking if account is sensitive and passwordless."
if ! output="$(/usr/libexec/helper-scripts/get-user-list)"; then
printf '%s\n' 'ERROR: Failed to get user list!'
exit 1
@ -60,11 +102,6 @@ if [ "${#passwd_status_list[@]}" = '0' ] \
exit 1
fi
if [ "$PAM_USER" = 'sysmaint' ]; then
printf '%s\n' 'ERROR: Rejecting sysmaint account in user session!'
exit 1
fi
interactive_user_idx='-1'
for user_idx in "${!user_list[@]}"; do
if [ "${user_list[user_idx]}" = "$PAM_USER" ]; then