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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

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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_authenticatepam_authenticate.restype=c_intpam_authenticate.argtypes=[PamHandle,c_int]defauthenticate(username,password,service='login'):defmy_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)returnretval==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_authenticatepam_authenticate.restype=c_intpam_authenticate.argtypes=[PamHandle,c_int]pam_acct_mgmt=libpam.pam_acct_mgmtpam_acct_mgmt.restype=c_intpam_acct_mgmt.argtypes=[PamHandle,c_int]defauthenticate(username,password,service='login'):defmy_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)ifretval==0:retval=pam_acct_mgmt(handle,0)returnretval==0

References

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