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Python : Add query to detect PAM authorization bypass
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 a expired login or a password can still access the system. This PR includes a qhelp describing the issue, a query which detects instances where a call to `pam_acc_mgmt` does not follow a call to `pam_authenticate` and it's corresponding tests. This PR has multiple detections. Some of the public one I can find are : * [CVE-2022-0860](https://nvd.nist.gov/vuln/detail/CVE-2022-0860) found in [cobbler/cobbler](https://www.github.com/cobbler/cobbler) * [fredhutch/motuz](https://www.huntr.dev/bounties/d46f91ca-b8ef-4b67-a79a-2420c4c6d52b/)
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‎python/ql/src/experimental/Security/CWE-285/PamAuthorization.qhelp‎

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<!DOCTYPE qhelp PUBLIC "-//Semmle//qhelp//EN" "qhelp.dtd">
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<qhelp>
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<overview>
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<p>
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Using only a call to
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<code>pam_authenticate</code>
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to check the validity of a login can lead to authorization bypass vulnerabilities.
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</p>
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<p>
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A
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<code>pam_authenticate</code>
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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 a expired login or a password can still access the system.
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</p>
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</overview>
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<recommendation>
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<p>
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A call to
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<code>pam_authenticate</code>
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should be followed by a call to
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<code>pam_acct_mgmt</code>
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to check if a user is allowed to login.
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</p>
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</recommendation>
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<example>
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<p>
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In the following example, the code only checks the credentials of a user. Hence, in this case, a user expired with expired creds can still login. This can be verified by creating a new user account, expiring it with
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<code>chage -E0 `username` </code>
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and then trying to log in.
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</p>
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<sample src="PamAuthorizationBad.py" />
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<p>
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This can be avoided by calling
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<code>pam_acct_mgmt</code>
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call to verify access as has been done in the snippet shown below.
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</p>
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<sample src="PamAuthorizationGood.py" />
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</example>
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<references>
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<li>
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Man-Page:
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<a href="https://man7.org/linux/man-pages/man3/pam_acct_mgmt.3.html">pam_acct_mgmt</a>
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</li>
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</references>
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</qhelp>

‎python/ql/src/experimental/Security/CWE-285/PamAuthorization.ql‎

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/**
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* @name Authorization bypass due to incorrect usage of PAM
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* @description Using only the `pam_authenticate` call to check the validity of a login can lead to a authorization bypass.
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* @kind problem
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* @problem.severity warning
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* @id py/pam-auth-bypass
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* @tags security
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* external/cwe/cwe-285
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*/
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import python
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import semmle.python.ApiGraphs
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import experimental.semmle.python.Concepts
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import semmle.python.dataflow.new.TaintTracking
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private class LibPam extends API::Node {
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LibPam() {
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exists(
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API::Node cdll, API::Node find_library, API::Node libpam, API::CallNode cdll_call,
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API::CallNode find_lib_call, StrConst str
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|
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API::moduleImport("ctypes").getMember("CDLL") = cdll and
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find_library = API::moduleImport("ctypes.util").getMember("find_library") and
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cdll_call = cdll.getACall() and
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find_lib_call = find_library.getACall() and
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DataFlow::localFlow(DataFlow::exprNode(str), find_lib_call.getArg(0)) and
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str.getText() = "pam" and
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cdll_call.getArg(0) = find_lib_call and
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libpam = cdll_call.getReturn()
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|
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libpam = this
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)
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}
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override string toString() { result = "libpam" }
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}
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class PamAuthCall extends API::Node {
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PamAuthCall() { exists(LibPam pam | pam.getMember("pam_authenticate") = this) }
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override string toString() { result = "pam_authenticate" }
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}
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class PamActMgt extends API::Node {
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PamActMgt() { exists(LibPam pam | pam.getMember("pam_acct_mgmt") = this) }
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override string toString() { result = "pam_acct_mgmt" }
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}
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from PamAuthCall p, API::CallNode u, Expr handle
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where
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u = p.getACall() and
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handle = u.asExpr().(Call).getArg(0) and
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not exists(PamActMgt pam |
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DataFlow::localFlow(DataFlow::exprNode(handle),
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DataFlow::exprNode(pam.getACall().asExpr().(Call).getArg(0)))
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)
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select u, "This PAM authentication call may be lead to an authorization bypass."

‎python/ql/src/experimental/Security/CWE-285/PamAuthorizationBad.py‎

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def authenticate(self, username, password, service='login', encoding='utf-8', resetcreds=True):
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libpam = CDLL(find_library("pam"))
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pam_authenticate = libpam.pam_authenticate
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pam_acct_mgmt = libpam.pam_acct_mgmt
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pam_authenticate.restype = c_int
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pam_authenticate.argtypes = [PamHandle, c_int]
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pam_acct_mgmt.restype = c_int
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pam_acct_mgmt.argtypes = [PamHandle, c_int]
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handle = PamHandle()
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conv = PamConv(my_conv, 0)
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retval = pam_start(service, username, byref(conv), byref(handle))
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retval = pam_authenticate(handle, 0)
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return retval == 0

‎python/ql/src/experimental/Security/CWE-285/PamAuthorizationGood.py‎

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def authenticate(self, username, password, service='login', encoding='utf-8', resetcreds=True):
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libpam = CDLL(find_library("pam"))
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pam_authenticate = libpam.pam_authenticate
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pam_acct_mgmt = libpam.pam_acct_mgmt
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pam_authenticate.restype = c_int
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pam_authenticate.argtypes = [PamHandle, c_int]
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pam_acct_mgmt.restype = c_int
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pam_acct_mgmt.argtypes = [PamHandle, c_int]
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handle = PamHandle()
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conv = PamConv(my_conv, 0)
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retval = pam_start(service, username, byref(conv), byref(handle))
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retval = pam_authenticate(handle, 0)
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if retval == 0:
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retval = pam_acct_mgmt(handle, 0)
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return retval == 0

‎python/ql/test/experimental/query-tests/Security/CWE-285/PamAuthorization.expected‎

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| bad.py:92:18:92:44 | ControlFlowNode for pam_authenticate() | This PAM authentication call may be lead to an authorization bypass. |

‎python/ql/test/experimental/query-tests/Security/CWE-285/PamAuthorization.qlref‎

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experimental/Security/CWE-285/PamAuthorization.ql

‎python/ql/test/experimental/query-tests/Security/CWE-285/bad.py‎

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from ctypes import CDLL, POINTER, Structure, CFUNCTYPE, cast, byref, sizeof
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from ctypes import c_void_p, c_size_t, c_char_p, c_char, c_int
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from ctypes import memmove
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from ctypes.util import find_library
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class PamHandle(Structure):
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_fields_ = [ ("handle", c_void_p) ]
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def __init__(self):
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Structure.__init__(self)
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self.handle = 0
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class PamMessage(Structure):
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"""wrapper class for pam_message structure"""
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_fields_ = [ ("msg_style", c_int), ("msg", c_char_p) ]
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def __repr__(self):
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return "<PamMessage %i '%s'>" % (self.msg_style, self.msg)
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class PamResponse(Structure):
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"""wrapper class for pam_response structure"""
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_fields_ = [ ("resp", c_char_p), ("resp_retcode", c_int) ]
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def __repr__(self):
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return "<PamResponse %i '%s'>" % (self.resp_retcode, self.resp)
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conv_func = CFUNCTYPE(c_int, c_int, POINTER(POINTER(PamMessage)), POINTER(POINTER(PamResponse)), c_void_p)
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class PamConv(Structure):
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"""wrapper class for pam_conv structure"""
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_fields_ = [ ("conv", conv_func), ("appdata_ptr", c_void_p) ]
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# Various constants
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PAM_PROMPT_ECHO_OFF = 1
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PAM_PROMPT_ECHO_ON = 2
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PAM_ERROR_MSG = 3
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PAM_TEXT_INFO = 4
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PAM_REINITIALIZE_CRED = 8
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libc = CDLL(find_library("c"))
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libpam = CDLL(find_library("pam"))
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calloc = libc.calloc
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calloc.restype = c_void_p
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calloc.argtypes = [c_size_t, c_size_t]
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# bug #6 (@NIPE-SYSTEMS), some libpam versions don't include this function
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if hasattr(libpam, 'pam_end'):
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pam_end = libpam.pam_end
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pam_end.restype = c_int
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pam_end.argtypes = [PamHandle, c_int]
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pam_start = libpam.pam_start
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pam_start.restype = c_int
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pam_start.argtypes = [c_char_p, c_char_p, POINTER(PamConv), POINTER(PamHandle)]
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pam_setcred = libpam.pam_setcred
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pam_setcred.restype = c_int
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pam_setcred.argtypes = [PamHandle, c_int]
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pam_strerror = libpam.pam_strerror
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pam_strerror.restype = c_char_p
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pam_strerror.argtypes = [PamHandle, c_int]
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pam_authenticate = libpam.pam_authenticate
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pam_authenticate.restype = c_int
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pam_authenticate.argtypes = [PamHandle, c_int]
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pam_acct_mgmt = libpam.pam_acct_mgmt
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pam_acct_mgmt.restype = c_int
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pam_acct_mgmt.argtypes = [PamHandle, c_int]
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class pam():
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code = 0
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reason = None
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def __init__(self):
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pass
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def authenticate(self, username, password, service='login', encoding='utf-8', resetcreds=True):
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@conv_func
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def my_conv(n_messages, messages, p_response, app_data):
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return 0
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cpassword = c_char_p(password)
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handle = PamHandle()
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conv = PamConv(my_conv, 0)
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retval = pam_start(service, username, byref(conv), byref(handle))
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retval = pam_authenticate(handle, 0)
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auth_success = retval == 0
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return auth_success

‎python/ql/test/experimental/query-tests/Security/CWE-285/good.py‎

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from ctypes import CDLL, POINTER, Structure, CFUNCTYPE, cast, byref, sizeof
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from ctypes import c_void_p, c_size_t, c_char_p, c_char, c_int
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from ctypes import memmove
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from ctypes.util import find_library
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class PamHandle(Structure):
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_fields_ = [ ("handle", c_void_p) ]
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def __init__(self):
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Structure.__init__(self)
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self.handle = 0
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class PamMessage(Structure):
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"""wrapper class for pam_message structure"""
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_fields_ = [ ("msg_style", c_int), ("msg", c_char_p) ]
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def __repr__(self):
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return "<PamMessage %i '%s'>" % (self.msg_style, self.msg)
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class PamResponse(Structure):
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"""wrapper class for pam_response structure"""
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_fields_ = [ ("resp", c_char_p), ("resp_retcode", c_int) ]
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def __repr__(self):
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return "<PamResponse %i '%s'>" % (self.resp_retcode, self.resp)
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conv_func = CFUNCTYPE(c_int, c_int, POINTER(POINTER(PamMessage)), POINTER(POINTER(PamResponse)), c_void_p)
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class PamConv(Structure):
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"""wrapper class for pam_conv structure"""
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_fields_ = [ ("conv", conv_func), ("appdata_ptr", c_void_p) ]
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# Various constants
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PAM_PROMPT_ECHO_OFF = 1
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PAM_PROMPT_ECHO_ON = 2
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PAM_ERROR_MSG = 3
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PAM_TEXT_INFO = 4
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PAM_REINITIALIZE_CRED = 8
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libc = CDLL(find_library("c"))
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libpam = CDLL(find_library("pam"))
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calloc = libc.calloc
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calloc.restype = c_void_p
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calloc.argtypes = [c_size_t, c_size_t]
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# bug #6 (@NIPE-SYSTEMS), some libpam versions don't include this function
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if hasattr(libpam, 'pam_end'):
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pam_end = libpam.pam_end
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pam_end.restype = c_int
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pam_end.argtypes = [PamHandle, c_int]
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pam_start = libpam.pam_start
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pam_start.restype = c_int
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pam_start.argtypes = [c_char_p, c_char_p, POINTER(PamConv), POINTER(PamHandle)]
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pam_setcred = libpam.pam_setcred
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pam_setcred.restype = c_int
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pam_setcred.argtypes = [PamHandle, c_int]
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pam_strerror = libpam.pam_strerror
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pam_strerror.restype = c_char_p
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pam_strerror.argtypes = [PamHandle, c_int]
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pam_authenticate = libpam.pam_authenticate
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pam_authenticate.restype = c_int
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pam_authenticate.argtypes = [PamHandle, c_int]
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pam_acct_mgmt = libpam.pam_acct_mgmt
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pam_acct_mgmt.restype = c_int
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pam_acct_mgmt.argtypes = [PamHandle, c_int]
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class pam():
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code = 0
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reason = None
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def __init__(self):
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pass
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def authenticate(self, username, password, service='login', encoding='utf-8', resetcreds=True):
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@conv_func
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def my_conv(n_messages, messages, p_response, app_data):
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return 0
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cpassword = c_char_p(password)
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handle = PamHandle()
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conv = PamConv(my_conv, 0)
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retval = pam_start(service, username, byref(conv), byref(handle))
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retval = pam_authenticate(handle, 0)
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if retval == 0:
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retval = pam_acct_mgmt(handle, 0)
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auth_success = retval == 0
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return auth_success

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