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PAM authorization bypass due to incorrect usage

ID: py/pam-auth-bypass
Kind: path-problem
Security severity: 8.1
Severity: warning
Precision: high
Tags:
   - security
   - external/cwe/cwe-285
Query suites:
   - python-code-scanning.qls
   - python-security-extended.qls
   - python-security-and-quality.qls

Click to see the query in the CodeQL repository

Using only a call to pam_authenticate to check the validity of a login can lead to authorization bypass vulnerabilities.

A pam_authenticate only verifies the credentials of a user. It does not check if a user has an appropriate authorization to actually login. This means a user with an expired login or a password can still access the system.

Recommendation

A call to pam_authenticate should be followed by a call to pam_acct_mgmt to check if a user is allowed to login.

Example

In the following example, the code only checks the credentials of a user. Hence, in this case, a user with expired credentials can still login. This can be verified by creating a new user account, expiring it with chage -E0 `username` and then trying to log in.

libpam                    = CDLL(find_library("pam"))

pam_authenticate          = libpam.pam_authenticate
pam_authenticate.restype  = c_int
pam_authenticate.argtypes = [PamHandle, c_int]

def authenticate(username, password, service='login'):
    def my_conv(n_messages, messages, p_response, app_data):
        """
        Simple conversation function that responds to any prompt where the echo is off with the supplied password
        """
        ...

    handle = PamHandle()
    conv   = PamConv(my_conv, 0)
    retval = pam_start(service, username, byref(conv), byref(handle))

    retval = pam_authenticate(handle, 0)
    return retval == 0

This can be avoided by calling pam_acct_mgmt call to verify access as has been done in the snippet shown below.

libpam                    = CDLL(find_library("pam"))

pam_authenticate          = libpam.pam_authenticate
pam_authenticate.restype  = c_int
pam_authenticate.argtypes = [PamHandle, c_int]

pam_acct_mgmt          = libpam.pam_acct_mgmt
pam_acct_mgmt.restype  = c_int
pam_acct_mgmt.argtypes = [PamHandle, c_int]

def authenticate(username, password, service='login'):
    def my_conv(n_messages, messages, p_response, app_data):
        """
        Simple conversation function that responds to any prompt where the echo is off with the supplied password
        """
        ...

    handle = PamHandle()
    conv   = PamConv(my_conv, 0)
    retval = pam_start(service, username, byref(conv), byref(handle))

    retval = pam_authenticate(handle, 0)
    if retval == 0:
        retval = pam_acct_mgmt(handle, 0)
    return retval == 0

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