CWE-939 自定义URL方案处理程序中的授权不正确

Improper Authorization in Handler for Custom URL Scheme

结构: Simple

Abstraction: Base

状态: Incomplete

被利用可能性: unkown

基本描述

The software uses a handler for a custom URL scheme, but it does not properly restrict which actors can invoke the handler using the scheme.

扩展描述

Mobile platforms and other architectures allow the use of custom URL schemes to facilitate communication between applications. In the case of iOS, this is the only method to do inter-application communication. The implementation is at the developer's discretion which may open security flaws in the application. An example could be potentially dangerous functionality such as modifying files through a custom URL scheme.

相关缺陷

  • cwe_Nature: ChildOf cwe_CWE_ID: 862 cwe_View_ID: 699 cwe_Ordinal: Primary

  • cwe_Nature: ChildOf cwe_CWE_ID: 862 cwe_View_ID: 1000 cwe_Ordinal: Primary

适用平台

Paradigm: {'cwe_Name': 'Mobile', 'cwe_Prevalence': 'Undetermined'}

可能的缓解方案

Architecture and Design

策略:

Utilize a user prompt pop-up to authorize potentially harmful actions such as those modifying data or dealing with sensitive information. When designing functionality of actions in the URL scheme, consider whether the action should be accessible to all mobile applications, or if a whitelist of applications to interface with is appropriate.

示例代码

例

This iOS application uses a custom URL scheme. The replaceFileText action in the URL scheme allows an external application to interface with the file incomingMessage.txt and replace the contents with the text field of the query string.

External Application

good Objective-C

NSString stringURL = @"appscheme://replaceFileText?file=incomingMessage.txt&text=hello";
NSURL url = [NSURL URLWithString:stringURL];
[[UIApplication sharedApplication] openURL:url];

Application URL Handler

bad


- (BOOL)application:(UIApplication )application handleOpenURL:(NSURL )url {
if (!url) {
return NO;
}
NSString action = [url host];
if([action isEqualToString: @"replaceFileText"]) {
NSDictionary dict = [self parseQueryStringExampleFunction:[url query]];
//this function will write contents to a specified file
FileObject *objectFile = [self writeToFile:[dict objectForKey: @"file"] withText:[dict objectForKey: @"text"]];
}
return YES;
}

The handler has no restriction on who can use its functionality. The handler can be invoked using any method that invokes the URL handler such as the following malicious iframe embedded on a web page opened by Safari.

attack HTML

<iframe src="appscheme://replaceFileText?file=Bookmarks.dat&text=listOfMaliciousWebsites">

The attacker can host a malicious website containing the iframe and trick users into going to the site via a crafted phishing email. Since Safari automatically executes iframes, the user is not prompted when the handler executes the iframe code which automatically invokes the URL handler replacing the bookmarks file with a list of malicious websites. Since replaceFileText is a potentially dangerous action, an action that modifies data, there should be a sanity check before the writeToFile:withText: function.

例

These Android and iOS applications intercept URL loading and perform special actions if a particular URL scheme is used, thus allowing the Javascript within the WebView to communicate with the application:

bad Java

@Override
public boolean shouldOverrideUrlLoading(WebView view, String url){
if (url.substring(0,14).equalsIgnoreCase("examplescheme:")){
if(url.substring(14,25).equalsIgnoreCase("getUserInfo")){
writeDataToView(view, UserData);
return false;
}
else{
return true;
}
}
}

bad Objective-C

-(BOOL) webView:(UIWebView )exWebView shouldStartLoadWithRequest:(NSURLRequest )exRequest navigationType:(UIWebViewNavigationType)exNavigationType
{
NSURL URL = [exRequest URL];
if ([[URL scheme] isEqualToString:@"exampleScheme"])
{
NSString functionString = [URL resourceSpecifier];
if ([functionString hasPrefix:@"specialFunction"])
{

// Make data available back in webview.
UIWebView *webView = [self writeDataToView:[URL query]];
}
return NO;
}
return YES;
}

A call into native code can then be initiated by passing parameters within the URL:

attack JavaScript

window.location = examplescheme://method?parameter=value

Because the application does not check the source, a malicious website loaded within this WebView has the same access to the API as a trusted site.

分析过的案例

标识 说明 链接
CVE-2013-5725 URL scheme has action replace which requires no user prompt and allows remote attackers to perform undesired actions. https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2013-5725
CVE-2013-5726 URL scheme has action follow and favorite which allows remote attackers to force user to perform undesired actions. https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2013-5726

引用