CWE-939: Improper Authorization in Handler for Custom URL Scheme

Extended Description

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.

Code Examples & CVEs

Demonstrative Examples

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

GoodObjective-C

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

External Application

BadObjective-C

- (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;}

External Application

AttackHTML

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

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

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

BadJava

// Android                      @Overridepublic 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;}}                      }

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

BadObjective-C

// iOS                      -(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:

AttackJavaScript

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

Observed CVE Examples (2)

CVE-2013-5725

URL scheme has action replace which requires no user prompt and allows remote attackers to perform undesired actions.

View Details

CVE-2013-5726

URL scheme has action follow and favorite which allows remote attackers to force user to perform undesired actions.

View Details

Source-backed guidance

Additional facts reviewed against primary or authoritative security sources.

Verify controls for CWE-939 with SSDF evidence

Use NIST SSDF verification and vulnerability-response practices to detect CWE-939, Improper Authorization in Handler for Custom URL Scheme, throughout the product lifecycle. Derive review questions, static or dynamic checks, and negative tests from the CWE's causal behavior; define the components and lifecycle stages each check covers; and retain findings with enough evidence to distinguish the root cause from symptoms and impacts. Track escapes and false negatives, then improve the verification plan after every confirmed occurrence.

NIST SP 800-218 Secure Software Development Framework — National Institute of Standards and Technology

Address Improper Authorization in Handler for Custom URL Scheme during Architecture and Design

MITRE associates mitigation with Architecture and Design; documented detection approaches include Automated Static Analysis; recorded impacts include Gain Privileges or Assume Identity, Varies by Context, and Bypass Protection Mechanism. Use these source-defined anchors to turn CWE-939 into implementation, review, and verification checks for the affected component.

CWE-939: Improper Authorization in Handler for Custom URL Scheme — MITRE CWE

Triage CWE-939 against known exploitation evidence

Use CISA's Known Exploited Vulnerabilities catalog to test whether a vulnerability mapped to CWE-939, Improper Authorization in Handler for Custom URL Scheme, has evidence of exploitation in the wild. Confirm the CVE-to-CWE root-cause mapping independently before attaching the example, then capture the affected product, required action, and remediation deadline. A missing KEV match is not evidence that the weakness is unexploited, and a KEV entry must not be generalized to every occurrence of this CWE.

Known Exploited Vulnerabilities Catalog — Cybersecurity and Infrastructure Security Agency

Apply precise root-cause mapping to CWE-939

Apply MITRE's full root-cause mapping guidance when using CWE-939, Improper Authorization in Handler for Custom URL Scheme. Separate weakness language from attacker prerequisites and technical impact, check the entry's abstraction and vulnerability-mapping notes, and prefer the most specific Base or Variant supported by the evidence. Record the rejected alternatives and require an independent review before the mapping is used for remediation trends or program metrics.

CVE to CWE Root Cause Mapping Guidance — MITRE CWE

Validate CWE-939 with root-cause mapping checks

Apply MITRE's root-cause mapping quick tips to CWE-939, Improper Authorization in Handler for Custom URL Scheme. Confirm the finding describes the causal weakness rather than an impact or attack pattern, compare the abstraction and mapping notes with plausible alternatives, and have a second reviewer challenge the selection. Preserve the evidence and reasoning so recurring defects can be measured against one consistent identifier.

CVE to CWE Root Cause Mapping Quick Tips — MITRE CWE

Frequently Asked Questions

What is CWE-939: Improper Authorization in Handler for Custom URL Scheme?+

CWE-939: Improper Authorization in Handler for Custom URL Scheme is a Common Weakness Enumeration (CWE) entry maintained by MITRE. The product uses a handler for a custom URL scheme, but it does not properly restrict which actors can invoke the handler using the scheme. 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.

What are the security consequences of Improper Authorization in Handler for Custom URL Scheme?+

If exploited, CWE-939 (Improper Authorization in Handler for Custom URL Scheme) it can compromise Access Control and Other, leading to outcomes such as Gain Privileges or Assume Identity, Varies by Context and Bypass Protection Mechanism.

How do you prevent or mitigate Improper Authorization in Handler for Custom URL Scheme?+

Recommended mitigations for CWE-939 include: 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 an allowlist of applications to interface with is appropriate.

What are real-world examples of Improper Authorization in Handler for Custom URL Scheme?+

MITRE documents real CVEs mapped to CWE-939, including CVE-2013-5725 and CVE-2013-5726. You can look up the full details of each CVE, including CVSS scores and remediation guidance, on our CVE Lookup tool.

What is the difference between a CWE and a CVE?+

A CWE (Common Weakness Enumeration) like CWE-939 describes a category of software weakness — the underlying flaw type. A CVE (Common Vulnerabilities and Exposures) identifies a specific, real-world vulnerability in a particular product. In short, a CWE is the kind of mistake, and a CVE is an instance of that mistake being found in software.

CWE-939: Improper Authorization in Handler for Custom URL Scheme

Extended Description

Technical Details

Applicable To

Security Consequences

Scope

Impact

Mitigation Strategies

Phase

Description

Detection Methods

Code Examples & CVEs

Demonstrative Examples

Observed CVE Examples (2)

CWE Relationships

Parent Weaknesses

Source-backed guidance

Verify controls for CWE-939 with SSDF evidence

Address Improper Authorization in Handler for Custom URL Scheme during Architecture and Design

Triage CWE-939 against known exploitation evidence

Apply precise root-cause mapping to CWE-939

Validate CWE-939 with root-cause mapping checks

Frequently Asked Questions

Learn More