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Merge pull request github#7252 from museljh/feature/cwe-338
Python: CWE-338 insecureRandomness
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‎python/ql/src/experimental/Security/CWE-338/InsecureRandomness.qhelp

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<!DOCTYPE qhelp PUBLIC
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"-//Semmle//qhelp//EN"
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"qhelp.dtd">
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<qhelp>
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<overview>
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<p>
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Using a cryptographically weak pseudo-random number generator to generate a security-sensitive value,
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such as a password, makes it easier for an attacker to predict the value.
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</p>
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<p>
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Pseudo-random number generators generate a sequence of numbers that only approximates the properties
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of random numbers. The sequence is not truly random because it is completely determined by a
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relatively small set of initial values, the seed. If the random number generator is
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cryptographically weak, then this sequence may be easily predictable through outside observations.
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</p>
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</overview>
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<recommendation>
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<p>
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Use a cryptographically secure pseudo-random number generator if the output is to be used in a
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security sensitive context. As a rule of thumb, a value should be considered "security sensitive"
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if predicting it would allow the attacker to perform an action that they would otherwise be unable
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to perform. For example, if an attacker could predict the random password generated for a new user,
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they would be able to log in as that new user.
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</p>
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<p>
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For Python, <code>secrets</code> provides a cryptographically secure pseudo-random
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number generator. <code>random</code> is not cryptographically secure, and should be avoided in
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security contexts.
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</p>
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</recommendation>
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<example>
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<p>
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The example below uses the <code>random</code> package instead of <code>secrets</code> to generate a password:
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</p>
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<sample src="examples/InsecureRandomness.py" />
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<p>
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Instead, use <code>secrets</code>:
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</p>
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<sample src="examples/InsecureRandomnessGood.py" />
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</example>
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<references>
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<li>Wikipedia. <a href="http://en.wikipedia.org/wiki/Pseudorandom_number_generator">Pseudo-random number generator</a>.</li>
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<li>OWASP: <a href="https://owasp.org/www-community/vulnerabilities/Insecure_Randomness">Insecure Randomness</a>.</li>
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<li>OWASP: <a href="https://cheatsheetseries.owasp.org/cheatsheets/Cryptographic_Storage_Cheat_Sheet.html#secure-random-number-generation">Secure Random Number Generation</a>.</li>
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</references>
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</qhelp>

‎python/ql/src/experimental/Security/CWE-338/InsecureRandomness.ql

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/**
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* @name Insecure randomness
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* @description Using a cryptographically weak pseudo-random number generator to generate a
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* security-sensitive value may allow an attacker to predict what value will
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* be generated.
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* @kind path-problem
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* @problem.severity warning
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* @security-severity 7.8
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* @precision high
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* @id py/insecure-randomness
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* @tags security
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* external/cwe/cwe-338
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*/
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import python
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import experimental.semmle.python.security.InsecureRandomness::InsecureRandomness
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import semmle.python.dataflow.new.DataFlow
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import DataFlow::PathGraph
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from Configuration cfg, DataFlow::PathNode source, DataFlow::PathNode sink
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where cfg.hasFlowPath(source, sink)
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select sink.getNode(), source, sink, "Cryptographically insecure $@ in a security context.",
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source.getNode(), "random value"

‎python/ql/src/experimental/Security/CWE-338/examples/InsecureRandomness.py

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import random
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def generatePassword():
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# BAD: the random is not cryptographically secure
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return random.random()

‎python/ql/src/experimental/Security/CWE-338/examples/InsecureRandomnessGood.py

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import secrets
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def generatePassword():
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# GOOD: the random is cryptographically secure
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secret_generator = secrets.SystemRandom()
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return secret_generator.random()

‎python/ql/src/experimental/semmle/python/security/InsecureRandomness.qll

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/**
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* Provides a taint tracking configuration for reasoning about random
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* values that are not cryptographically secure.
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*
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* Note, for performance reasons: only import this file if
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* `InsecureRandomness::Configuration` is needed, otherwise
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* `InsecureRandomnessCustomizations` should be imported instead.
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*/
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private import python
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import semmle.python.dataflow.new.DataFlow
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import semmle.python.dataflow.new.TaintTracking
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/**
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* A taint tracking configuration for random values that are not cryptographically secure.
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*/
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module InsecureRandomness {
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import InsecureRandomnessCustomizations::InsecureRandomness
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/**
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* A taint-tracking configuration for reasoning about random values that are
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* not cryptographically secure.
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*/
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class Configuration extends TaintTracking::Configuration {
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Configuration() { this = "InsecureRandomness" }
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override predicate isSource(DataFlow::Node source) { source instanceof Source }
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override predicate isSink(DataFlow::Node sink) { sink instanceof Sink }
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override predicate isSanitizer(DataFlow::Node node) { node instanceof Sanitizer }
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override predicate isSanitizerGuard(DataFlow::BarrierGuard guard) {
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guard instanceof SanitizerGuard
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}
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}
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}

‎python/ql/src/experimental/semmle/python/security/InsecureRandomnessCustomizations.qll

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/**
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* Provides default sources, sinks and sanitizers for reasoning about random values that are
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* not cryptographically secure, as well as extension points for adding your own.
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*/
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private import python
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private import semmle.python.ApiGraphs
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private import semmle.python.Concepts
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private import semmle.python.dataflow.new.BarrierGuards
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private import semmle.python.dataflow.new.internal.DataFlowPrivate
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/**
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* Provides default sources, sinks and sanitizers for reasoning about random values that are
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* not cryptographically secure, as well as extension points for adding your own.
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*/
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module InsecureRandomness {
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/**
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* A data flow source for random values that are not cryptographically secure.
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*/
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abstract class Source extends DataFlow::Node { }
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/**
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* A data flow sink for random values that are not cryptographically secure.
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*/
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abstract class Sink extends DataFlow::Node { }
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/**
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* A sanitizer for random values that are not cryptographically secure.
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*/
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abstract class Sanitizer extends DataFlow::Node { }
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/**
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* A sanitizer guard for random values that are not cryptographically secure.
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*/
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abstract class SanitizerGuard extends DataFlow::BarrierGuard { }
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/**
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* A random source that is not sufficient for security use. So far this is only made up
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* of the math package's rand function, more insufficient random sources can be added here.
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*/
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class InsecureRandomSource extends Source {
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InsecureRandomSource() {
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this =
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API::moduleImport("random")
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.getMember([
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"betavariate", "choice", "choices", "expovariate", "gammavariate", "gauss",
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"getrandbits", "getstate", "lognormvariate", "normalvariate", "paretovariate",
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"randbytes", "randint", "random", "randrange", "sample", "seed", "setstate",
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"shuffle", "triangular", "uniform", "vonmisesvariate", "weibullvariate"
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])
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.getACall()
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}
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}
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/**
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* A use in a function that heuristically deals with unsafe random numbers or random strings.
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*/
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class RandomFnSink extends Sink {
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RandomFnSink() {
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exists(DataFlowCallable randomFn |
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randomFn
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.getName()
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.regexpMatch("(?i).*(gen(erate)?|make|mk|create).*(nonce|salt|pepper|Password).*")
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|
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this.getEnclosingCallable() = randomFn
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)
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}
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}
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/**
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* A cryptographic key, considered as a sink for random values that are not cryptographically
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* secure.
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*/
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class CryptoKeySink extends Sink {
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CryptoKeySink() {
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exists(Cryptography::CryptographicOperation operation | this = operation.getAnInput())
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}
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}
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}

‎python/ql/test/experimental/query-tests/Security/CWE-338/InsecureRandomness.expected

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edges
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nodes
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| InsecureRandomness.py:5:12:5:26 | ControlFlowNode for Attribute() | semmle.label | ControlFlowNode for Attribute() |
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subpaths
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#select
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| InsecureRandomness.py:5:12:5:26 | ControlFlowNode for Attribute() | InsecureRandomness.py:5:12:5:26 | ControlFlowNode for Attribute() | InsecureRandomness.py:5:12:5:26 | ControlFlowNode for Attribute() | Cryptographically insecure $@ in a security context. | InsecureRandomness.py:5:12:5:26 | ControlFlowNode for Attribute() | random value |

‎python/ql/test/experimental/query-tests/Security/CWE-338/InsecureRandomness.py

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import random
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def generatePassword():
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# BAD: the random is not cryptographically secure
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return random.random()

‎python/ql/test/experimental/query-tests/Security/CWE-338/InsecureRandomness.qlref

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experimental/Security/CWE-338/InsecureRandomness.ql

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