CWE-927: Use of Implicit Intent for Sensitive Communication
Weakness ID: 927
Vulnerability Mapping:
ALLOWEDThis CWE ID may be used to map to real-world vulnerabilities Abstraction: VariantVariant - a weakness that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource.
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Description
The Android application uses an implicit intent for transmitting sensitive data to other applications.
Extended Description
Since an implicit intent does not specify a particular application to receive the data, any application can process the intent by using an Intent Filter for that intent. This can allow untrusted applications to obtain sensitive data. There are two variations on the standard broadcast intent, ordered and sticky.
Ordered broadcast intents are delivered to a series of registered receivers in order of priority as declared by the Receivers. A malicious receiver can give itself a high priority and cause a denial of service by stopping the broadcast from propagating further down the chain. There is also the possibility of malicious data modification, as a receiver may also alter the data within the Intent before passing it on to the next receiver. The downstream components have no way of asserting that the data has not been altered earlier in the chain.
Sticky broadcast intents remain accessible after the initial broadcast. An old sticky intent will be broadcast again to any new receivers that register for it in the future, greatly increasing the chances of information exposure over time. Also, sticky broadcasts cannot be protected by permissions that may apply to other kinds of intents.
In addition, any broadcast intent may include a URI that references data that the receiving component does not normally have the privileges to access. The sender of the intent can include special privileges that grant the receiver read or write access to the specific URI included in the intent. A malicious receiver that intercepts this intent will also gain those privileges and be able to read or write the resource at the specified URI.
Relationships
This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.
Relevant to the view "Research Concepts" (CWE-1000)
Nature
Type
ID
Name
ChildOf
Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource.
Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource.
The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.
Phase
Note
Architecture and Design
Applicable Platforms
This listing shows possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.
Languages
Class: Not Language-Specific (Undetermined Prevalence)
Technologies
Class: Mobile (Undetermined Prevalence)
Common Consequences
This table specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.
Scope
Impact
Likelihood
Confidentiality
Technical Impact: Read Application Data
Other applications, possibly untrusted, can read the data that is offered through the Intent.
Integrity
Technical Impact: Varies by Context
The application may handle responses from untrusted applications on the device, which could cause it to perform unexpected or unauthorized actions.
Demonstrative Examples
Example 1
This application wants to create a user account in several trusted applications using one broadcast intent:
This application assumes only the trusted applications will be listening for the action. A malicious application can register for this action and intercept the user's login information, as below:
(attack code)
Example Language: Java
IntentFilter filter = new IntentFilter("com.example.CreateUser"); MyReceiver receiver = new MyReceiver(); registerReceiver(receiver, filter);
When a broadcast contains sensitive information, create an allowlist of applications that can receive the action using the application's manifest file, or programmatically send the intent to each individual intended receiver.
Example 2
This application interfaces with a web service that requires a separate user login. It creates a sticky intent, so that future trusted applications that also use the web service will know who the current user is:
(bad code)
Example Language: Java
Intent intent = new Intent(); intent.setAction("com.example.service.UserExists"); intent.putExtra("Username", uname_string); sendStickyBroadcast(intent);
(attack code)
Example Language: Java
IntentFilter filter = new IntentFilter("com.example.service.UserExists"); MyReceiver receiver = new MyReceiver(); registerReceiver(receiver, filter);
Sticky broadcasts can be read by any application at any time, and so should never contain sensitive information such as a username.
Example 3
This application is sending an ordered broadcast, asking other applications to open a URL:
(bad code)
Example Language: Java
Intent intent = new Intent(); intent.setAction("com.example.OpenURL"); intent.putExtra("URL_TO_OPEN", url_string); sendOrderedBroadcastAsUser(intent);
Any application in the broadcast chain may alter the data within the intent. This malicious application is altering the URL to point to an attack site:
(attack code)
Example Language: Java
public class CallReceiver extends BroadcastReceiver {
@Override public void onReceive(Context context, Intent intent) {
The final receiving application will then open the attack URL. Where possible, send intents to specific trusted applications instead of using a broadcast chain.
Example 4
This application sends a special intent with a flag that allows the receiving application to read a data file for backup purposes.
(bad code)
Example Language: Java
Intent intent = new Intent(); intent.setAction("com.example.BackupUserData"); intent.setData(file_uri); intent.addFlags(FLAG_GRANT_READ_URI_PERMISSION); sendBroadcast(intent);
(attack code)
Example Language: Java
public class CallReceiver extends BroadcastReceiver {
@Override public void onReceive(Context context, Intent intent) {
Uri userData = intent.getData(); stealUserData(userData);
}
}
Any malicious application can register to receive this intent. Because of the FLAG_GRANT_READ_URI_PERMISSION included with the intent, the malicious receiver code can read the user's data.
An Android application does not use FLAG_IMMUTABLE when creating a PendingIntent.
Potential Mitigations
Phase: Implementation
If the application only requires communication with its own components, then the destination is always known, and an explicit intent could be used.
Detection Methods
Automated Static Analysis
Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)
Effectiveness: High
Memberships
This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.
Nature
Type
ID
Name
MemberOf
Category - a CWE entry that contains a set of other entries that share a common characteristic.
(this CWE ID could be used to map to real-world vulnerabilities)
Reason: Acceptable-Use
Rationale:
This CWE entry is at the Variant level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities.
Comments:
Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction.
Description
This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.
