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Merge pull request #90 from GitHubSecurityLab/cors-csharp
Add CORS query to C# pack
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‎csharp/src/security/CWE-942/CorsMisconfiguration.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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A server can send the
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<code>"Access-Control-Allow-Credentials"</code> CORS header to control
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when a browser may send user credentials in Cross-Origin HTTP
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requests.
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</p>
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<p>
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When the <code>Access-Control-Allow-Credentials</code> header
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is <code>"true"</code>, the <code>Access-Control-Allow-Origin</code>
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header must have a value different from <code>"*"</code> in order to
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make browsers accept the header. Therefore, to allow multiple origins
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for Cross-Origin requests with credentials, the server must
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dynamically compute the value of the
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<code>"Access-Control-Allow-Origin"</code> header. Computing this
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header value from information in the request to the server can
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therefore potentially allow an attacker to control the origins that
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the browser sends credentials to.
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</p>
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</overview>
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<recommendation>
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<p>
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When the <code>Access-Control-Allow-Credentials</code> header
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value is <code>"true"</code>, a dynamic computation of the
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<code>Access-Control-Allow-Origin</code> header must involve
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sanitization if it relies on user-controlled input.
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</p>
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<p>
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Since the <code>"null"</code> origin is easy to obtain for an
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attacker, it is never safe to use <code>"null"</code> as the value of
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the <code>Access-Control-Allow-Origin</code> header when the
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<code>Access-Control-Allow-Credentials</code> header value is
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<code>"true"</code>.
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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 example below, the server allows the browser to send
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user credentials in a Cross-Origin request. The request header
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<code>origins</code> controls the allowed origins for such a
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Cross-Origin request.
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</p>
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<sample src="examples/CorsBad.cs"/>
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<p>
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This is not secure, since an attacker can choose the value of
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the <code>origin</code> request header to make the browser send
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credentials to their own server. The use of a allowlist containing
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allowed origins for the Cross-Origin request fixes the issue:
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</p>
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<sample src="examples/CorsGood.cs"/>
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</example>
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<references>
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<li>Mozilla Developer Network: <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Origin">CORS, Access-Control-Allow-Origin</a>.</li>
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<li>Mozilla Developer Network: <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Credentials">CORS, Access-Control-Allow-Credentials</a>.</li>
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<li>PortSwigger: <a href="http://blog.portswigger.net/2016/10/exploiting-cors-misconfigurations-for.html">Exploiting CORS Misconfigurations for Bitcoins and Bounties</a></li>
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<li>W3C: <a href="https://w3c.github.io/webappsec-cors-for-developers/#resources">CORS for developers, Advice for Resource Owners</a></li>
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</references>
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</qhelp>

‎csharp/src/security/CWE-942/CorsMisconfiguration.ql‎

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/**
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* @name Credentialed CORS Misconfiguration
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* @description Allowing any origin while allowing credentials may result in security issues as third party website may be able to
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* access private resources.
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* @kind problem
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* @problem.severity error
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* @security-severity 7.5
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* @precision high
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* @id cs/web/cors-misconfiguration
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* @tags security
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* external/cwe/cwe-942
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*/
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import csharp
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import CorsMisconfigurationLib
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/**
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* Holds if the application allows an origin using "*" origin.
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*/
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private predicate allowAnyOrigin(MethodCall m) {
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m.getTarget()
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.hasFullyQualifiedName("Microsoft.AspNetCore.Cors.Infrastructure.CorsPolicyBuilder",
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"AllowAnyOrigin")
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}
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/**
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* Holds if the application uses a vulnerable CORS policy.
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*/
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private predicate hasDangerousOrigins(MethodCall m) {
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m.getTarget()
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.hasFullyQualifiedName("Microsoft.AspNetCore.Cors.Infrastructure.CorsPolicyBuilder",
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"WithOrigins") and
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exists(StringLiteral idStr |
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idStr.getValue().toLowerCase().matches(["null", "*"]) and
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TaintTracking::localExprTaint(idStr, m.getAnArgument())
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)
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}
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from MethodCall add_policy, MethodCall child
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where
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(
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usedPolicy(add_policy) and
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// Misconfigured origin affects used policy
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getCallableFromExpr(add_policy.getArgument(1)).calls*(child.getTarget())
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or
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add_policy
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.getTarget()
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.hasFullyQualifiedName("Microsoft.AspNetCore.Cors.Infrastructure.CorsOptions",
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"AddDefaultPolicy") and
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// Misconfigured origin affects added default policy
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getCallableFromExpr(add_policy.getArgument(0)).calls*(child.getTarget())
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) and
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(hasDangerousOrigins(child) or allowAnyOrigin(child))
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select add_policy, "The following CORS policy may allow requests from 3rd party websites"

‎csharp/src/security/CWE-942/CorsMisconfigurationCredentials.ql‎

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/**
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* @name Credentialed CORS Misconfiguration
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* @description Allowing any origin while allowing credentials may result in security issues as third party website may be able to
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* access private resources.
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* @kind problem
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* @problem.severity error
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* @security-severity 7.5
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* @precision high
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* @id cs/web/cors-misconfiguration-credentials
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* @tags security
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* external/cwe/cwe-942
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*/
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import csharp
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import CorsMisconfigurationLib
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/** A call to `CorsPolicyBuilder.AllowCredentials`. */
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class AllowsCredentials extends MethodCall {
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AllowsCredentials() {
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this.getTarget()
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.hasFullyQualifiedName("Microsoft.AspNetCore.Cors.Infrastructure.CorsPolicyBuilder",
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"AllowCredentials")
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}
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}
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from MethodCall add_policy, MethodCall setIsOriginAllowed, AllowsCredentials allowsCredentials
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where
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(
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getCallableFromExpr(add_policy.getArgument(1)).calls*(setIsOriginAllowed.getTarget()) and
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usedPolicy(add_policy) and
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getCallableFromExpr(add_policy.getArgument(1)).calls*(allowsCredentials.getTarget())
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or
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add_policy
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.getTarget()
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.hasFullyQualifiedName("Microsoft.AspNetCore.Cors.Infrastructure.CorsOptions",
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"AddDefaultPolicy") and
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getCallableFromExpr(add_policy.getArgument(0)).calls*(setIsOriginAllowed.getTarget()) and
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getCallableFromExpr(add_policy.getArgument(0)).calls*(allowsCredentials.getTarget())
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) and
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setIsOriginAllowedReturnsTrue(setIsOriginAllowed)
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select add_policy,
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"The following CORS policy may allow credentialed requests from 3rd party websites"

‎csharp/src/security/CWE-942/CorsMisconfigurationLib.qll‎

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import csharp
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import DataFlow
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import security.JsonWebTokenHandler.JsonWebTokenHandlerLib
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/**
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* Gets the actual callable corresponding to the expression `e`.
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*/
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Callable getCallableFromExpr(Expr e) {
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exists(Expr dcArg | dcArg = e.(DelegateCreation).getArgument() |
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result = dcArg.(CallableAccess).getTarget() or
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result = dcArg.(AnonymousFunctionExpr)
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)
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or
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result = e
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}
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/**
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* Holds if SetIsOriginAllowed always returns true. This sets the Access-Control-Allow-Origin to the requester
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*/
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predicate setIsOriginAllowedReturnsTrue(MethodCall mc) {
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mc.getTarget()
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.hasFullyQualifiedName("Microsoft.AspNetCore.Cors.Infrastructure.CorsPolicyBuilder",
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"SetIsOriginAllowed") and
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mc.getArgument(0) instanceof CallableAlwaysReturnsTrue
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}
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/**
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* Holds if UseCors is called with the relevant cors policy
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*/
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predicate usedPolicy(MethodCall add_policy) {
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exists(MethodCall uc |
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uc.getTarget()
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.hasFullyQualifiedName("Microsoft.AspNetCore.Builder.CorsMiddlewareExtensions", "UseCors") and
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(
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// Same hardcoded name
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uc.getArgument(1).getValue() = add_policy.getArgument(0).getValue() or
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// Same variable access
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uc.getArgument(1).(VariableAccess).getTarget() =
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add_policy.getArgument(0).(VariableAccess).getTarget() or
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DataFlow::localExprFlow(add_policy.getArgument(0), uc.getArgument(1))
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)
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) and
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add_policy
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.getTarget()
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.hasFullyQualifiedName("Microsoft.AspNetCore.Cors.Infrastructure.CorsOptions", "AddPolicy")
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}

‎csharp/src/security/CWE-942/examples/CorsBad.cs‎

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using Leaf.Middlewares;
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using Microsoft.AspNetCore.Builder;
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using Microsoft.AspNetCore.Hosting;
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using Microsoft.Extensions.Configuration;
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using Microsoft.Extensions.DependencyInjection;
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using Microsoft.Extensions.Hosting;
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namespace Leaf
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{
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public class Startup
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{
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public Startup(IConfiguration configuration)
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{
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Configuration = configuration;
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}
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public IConfiguration Configuration { get; }
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// This method gets called by the runtime. Use this method to add services to the container.
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public void ConfigureServices(IServiceCollection services)
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{
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services.AddControllers();
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//services.AddTransient<MySqlConnection>(_ => new MySqlConnection(Configuration["ConnectionStrings:Default"]));
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services.AddControllersWithViews()
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.AddNewtonsoftJson(options =>
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options.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore
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);
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services.AddCors(options => {
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options.AddPolicy("AllowPolicy", builder => builder
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.WithOrigins("null")
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.AllowCredentials()
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.AllowAnyMethod()
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.AllowAnyHeader());
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});
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}
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// This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
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public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
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{
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app.UseRouting();
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app.UseCors("AllowPolicy");
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app.UseRequestResponseLogging();
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if (env.IsDevelopment())
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{
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app.UseDeveloperExceptionPage();
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}
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app.UseHttpsRedirection();
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app.UseAuthorization();
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app.UseEndpoints(endpoints =>
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{
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endpoints.MapControllers();
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});
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}
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}
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}

‎csharp/src/security/CWE-942/examples/CorsGood.cs‎

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using Leaf.Middlewares;
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using Microsoft.AspNetCore.Builder;
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using Microsoft.AspNetCore.Hosting;
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using Microsoft.Extensions.Configuration;
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using Microsoft.Extensions.DependencyInjection;
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using Microsoft.Extensions.Hosting;
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namespace Leaf
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{
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public class Startup
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{
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public Startup(IConfiguration configuration)
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{
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Configuration = configuration;
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}
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public IConfiguration Configuration { get; }
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// This method gets called by the runtime. Use this method to add services to the container.
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public void ConfigureServices(IServiceCollection services)
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{
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services.AddControllers();
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//services.AddTransient<MySqlConnection>(_ => new MySqlConnection(Configuration["ConnectionStrings:Default"]));
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services.AddControllersWithViews()
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.AddNewtonsoftJson(options =>
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options.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore
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);
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services.AddCors(options => {
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options.AddPolicy("AllowPolicy", builder => builder
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.WithOrigins("http://example.com")
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.AllowCredentials()
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.AllowAnyMethod()
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.AllowAnyHeader());
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});
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}
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// This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
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public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
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{
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app.UseRouting();
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app.UseCors("AllowPolicy");
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app.UseRequestResponseLogging();
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if (env.IsDevelopment())
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{
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app.UseDeveloperExceptionPage();
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}
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app.UseHttpsRedirection();
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app.UseAuthorization();
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app.UseEndpoints(endpoints =>
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{
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endpoints.MapControllers();
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});
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}
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}
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}

‎csharp/test/security/CWE-942/CorsMiconfigurationCredentials.cs‎

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using Microsoft.AspNetCore.Builder;
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using Microsoft.AspNetCore.Mvc;
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using System;
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using Microsoft.Extensions.DependencyInjection;
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public class Startup {
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public void ConfigureServices(string[] args) {
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var builder = WebApplication.CreateBuilder(args);
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var MyAllowSpecificOrigins = "_myAllowSpecificOrigins";
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builder.Services.AddCors(options => {
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options.AddPolicy(MyAllowSpecificOrigins,
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policy => {
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policy.SetIsOriginAllowed(test => true).AllowCredentials().AllowAnyHeader().AllowAnyMethod();
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});
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});
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var app = builder.Build();
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app.MapGet("/", () => "Hello World!");
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app.UseCors(MyAllowSpecificOrigins);
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app.Run();
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}
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}

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