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CWE-494: Download of Code Without Integrity Check

 
Download of Code Without Integrity Check
Weakness ID: 494 (Weakness Base)Status: Draft
+ Description

Description Summary

The product downloads source code or an executable from a remote location and executes the code without sufficiently verifying the origin and integrity of the code.

Extended Description

An attacker can execute malicious code by compromising the host server, performing DNS spoofing, or modifying the code in transit.

+ Time of Introduction
  • Architecture and Design
  • Implementation
+ Applicable Platforms

Languages

Language-independent

+ Common Consequences
ScopeEffect
Integrity
Availability
Confidentiality
Other

Technical Impact: Execute unauthorized code or commands; Alter execution logic; Other

Executing untrusted code could compromise the control flow of the program. The untrusted code could execute attacker-controlled commands, read or modify sensitive resources, or prevent the software from functioning correctly for legitimate users.

+ Likelihood of Exploit

Medium

+ Detection Methods

Manual Analysis

This weakness can be detected using tools and techniques that require manual (human) analysis, such as penetration testing, threat modeling, and interactive tools that allow the tester to record and modify an active session.

Specifically, manual static analysis is typically required to find the behavior that triggers the download of code, and to determine whether integrity-checking methods are in use.

These may be more effective than strictly automated techniques. This is especially the case with weaknesses that are related to design and business rules.

Black Box

Use monitoring tools that examine the software's process as it interacts with the operating system and the network. This technique is useful in cases when source code is unavailable, if the software was not developed by you, or if you want to verify that the build phase did not introduce any new weaknesses. Examples include debuggers that directly attach to the running process; system-call tracing utilities such as truss (Solaris) and strace (Linux); system activity monitors such as FileMon, RegMon, Process Monitor, and other Sysinternals utilities (Windows); and sniffers and protocol analyzers that monitor network traffic.

Attach the monitor to the process and also sniff the network connection. Trigger features related to product updates or plugin installation, which is likely to force a code download. Monitor when files are downloaded and separately executed, or if they are otherwise read back into the process. Look for evidence of cryptographic library calls that use integrity checking.

+ Demonstrative Examples

Example 1

This example loads an external class from a local subdirectory.

(Bad Code)
Example Language: Java 
URL[] classURLs= new URL[]{
new URL("file:subdir/")
};
URLClassLoader loader = new URLClassLoader(classURLs);
Class loadedClass = Class.forName("loadMe", true, loader);

This code does not ensure that the class loaded is the intended one, for example by verifying the class's checksum. An attacker may be able to modify the class file to execute malicious code.

Example 2

This code includes an external script to get database credentials, then authenticates a user against the database, allowing access to the application.

(Bad Code)
Example Language: PHP 
//assume the password is already encrypted, avoiding CWE-312
function authenticate($username,$password){
include("http://external.example.com/dbInfo.php");
//dbInfo.php makes $dbhost, $dbuser, $dbpass, $dbname available
mysql_connect($dbhost, $dbuser, $dbpass) or die ('Error connecting to mysql');
mysql_select_db($dbname);
$query = 'Select * from users where username='.$username.' And password='.$password;
$result = mysql_query($query);
if(mysql_numrows($result) == 1){
mysql_close();
return true;
}
else{
mysql_close();
return false;
}
}

This code does not verify that the external domain accessed is the intended one. An attacker may somehow cause the external domain name to resolve to an attack server, which would provide the information for a false database. The attacker may then steal the usernames and encrypted passwords from real user login attempts, or simply allow himself to access the application without a real user account.

This example is also vulnerable to a Man in the Middle (CWE-300) attack.

+ Observed Examples
ReferenceDescription
OS does not verify authenticity of its own updates.
online poker client does not verify authenticity of its own updates.
anti-virus product does not verify automatic updates for itself.
VOIP phone downloads applications from web sites without verifying integrity.
+ Potential Mitigations

Phase: Implementation

Perform proper forward and reverse DNS lookups to detect DNS spoofing.

This is only a partial solution since it will not prevent your code from being modified on the hosting site or in transit.

Phases: Architecture and Design; Operation

Encrypt the code with a reliable encryption scheme before transmitting.

This will only be a partial solution, since it will not detect DNS spoofing and it will not prevent your code from being modified on the hosting site.

Phase: Architecture and Design

Strategy: Libraries or Frameworks

Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.

Speficially, it may be helpful to use tools or frameworks to perform integrity checking on the transmitted code.

  • When providing the code that is to be downloaded, such as for automatic updates of the software, then use cryptographic signatures for the code and modify the download clients to verify the signatures. Ensure that the implementation does not contain CWE-295, CWE-320, CWE-347, and related weaknesses.

  • Use code signing technologies such as Authenticode. See references [R.494.1] [R.494.2] [R.494.3].

Phases: Architecture and Design; Operation

Strategy: Environment Hardening

Run your code using the lowest privileges that are required to accomplish the necessary tasks [R.494.7]. If possible, create isolated accounts with limited privileges that are only used for a single task. That way, a successful attack will not immediately give the attacker access to the rest of the software or its environment. For example, database applications rarely need to run as the database administrator, especially in day-to-day operations.

Phases: Architecture and Design; Operation

Strategy: Sandbox or Jail

Run the code in a "jail" or similar sandbox environment that enforces strict boundaries between the process and the operating system. This may effectively restrict which files can be accessed in a particular directory or which commands can be executed by the software.

OS-level examples include the Unix chroot jail, AppArmor, and SELinux. In general, managed code may provide some protection. For example, java.io.FilePermission in the Java SecurityManager allows the software to specify restrictions on file operations.

This may not be a feasible solution, and it only limits the impact to the operating system; the rest of the application may still be subject to compromise.

Be careful to avoid CWE-243 and other weaknesses related to jails.

Effectiveness: Limited

The effectiveness of this mitigation depends on the prevention capabilities of the specific sandbox or jail being used and might only help to reduce the scope of an attack, such as restricting the attacker to certain system calls or limiting the portion of the file system that can be accessed.

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfCategoryCategory490Mobile Code Issues
Development Concepts (primary)699
ChildOfWeakness ClassWeakness Class669Incorrect Resource Transfer Between Spheres
Research Concepts (primary)1000
ChildOfCategoryCategory7522009 Top 25 - Risky Resource Management
Weaknesses in the 2009 CWE/SANS Top 25 Most Dangerous Programming Errors (primary)750
ChildOfCategoryCategory8022010 Top 25 - Risky Resource Management
Weaknesses in the 2010 CWE/SANS Top 25 Most Dangerous Programming Errors (primary)800
ChildOfCategoryCategory859CERT Java Secure Coding Section 14 - Platform Security (SEC)
Weaknesses Addressed by the CERT Java Secure Coding Standard (primary)844
ChildOfCategoryCategory8652011 Top 25 - Risky Resource Management
Weaknesses in the 2011 CWE/SANS Top 25 Most Dangerous Software Errors (primary)900
ChildOfCategoryCategory991SFP Secondary Cluster: Tainted Input to Environment
Software Fault Pattern (SFP) Clusters (primary)888
PeerOfWeakness BaseWeakness Base79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Research Concepts1000
MemberOfViewView884CWE Cross-section
CWE Cross-section (primary)884
CanFollowWeakness BaseWeakness Base79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Research Concepts1000
+ Research Gaps

This is critical for mobile code, but it is likely to become more and more common as developers continue to adopt automated, network-based product distributions and upgrades. Software-as-a-Service (SaaS) might introduce additional subtleties. Common exploitation scenarios may include ad server compromises and bad upgrades.

+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
CLASPInvoking untrusted mobile code
CERT Java Secure CodingSEC06-JDo not rely on the default automatic signature verification provided by URLClassLoader and java.util.jar
Software Fault PatternsSFP27Tainted input to environment
+ References
[R.494.1] Microsoft. "Introduction to Code Signing". <http://msdn.microsoft.com/en-us/library/ms537361(VS.85).aspx>.
[R.494.3] Apple. "Code Signing Guide". Apple Developer Connection. 2008-11-19. <http://developer.apple.com/documentation/Security/Conceptual/CodeSigningGuide/Introduction/chapter_1_section_1.html>.
[R.494.4] Anthony Bellissimo, John Burgess and Kevin Fu. "Secure Software Updates: Disappointments and New Challenges". <http://prisms.cs.umass.edu/~kevinfu/papers/secureupdates-hotsec06.pdf>.
[R.494.5] [REF-17] Michael Howard, David LeBlanc and John Viega. "24 Deadly Sins of Software Security". "Sin 18: The Sins of Mobile Code." Page 267. McGraw-Hill. 2010.
[R.494.6] Johannes Ullrich. "Top 25 Series - Rank 20 - Download of Code Without Integrity Check". SANS Software Security Institute. 2010-04-05. <http://blogs.sans.org/appsecstreetfighter/2010/04/05/top-25-series-rank-20-download-code-integrity-check/>.
[R.494.7] [REF-31] Sean Barnum and Michael Gegick. "Least Privilege". 2005-09-14. <https://buildsecurityin.us-cert.gov/daisy/bsi/articles/knowledge/principles/351.html>.
+ Content History
Submissions
Submission DateSubmitterOrganizationSource
CLASPExternally Mined
Modifications
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigitalExternal
updated Time_of_Introduction
2008-09-08CWE Content TeamMITREInternal
updated Relationships, Other_Notes, Taxonomy_Mappings
2009-01-12CWE Content TeamMITREInternal
updated Applicable_Platforms, Common_Consequences, Description, Name, Other_Notes, Potential_Mitigations, References, Relationships, Research_Gaps, Type
2009-03-10CWE Content TeamMITREInternal
updated Potential_Mitigations
2009-07-27CWE Content TeamMITREInternal
updated Description, Observed_Examples, Related_Attack_Patterns
2010-02-16CWE Content TeamMITREInternal
updated Detection_Factors, References, Relationships
2010-04-05CWE Content TeamMITREInternal
updated Applicable_Platforms
2010-06-21CWE Content TeamMITREInternal
updated Common_Consequences, Detection_Factors, Potential_Mitigations, References
2010-09-27CWE Content TeamMITREInternal
updated Potential_Mitigations, References
2010-12-13CWE Content TeamMITREInternal
updated Potential_Mitigations
2011-03-29CWE Content TeamMITREInternal
updated Demonstrative_Examples
2011-06-01CWE Content TeamMITREInternal
updated Common_Consequences, Relationships, Taxonomy_Mappings
2011-06-27CWE Content TeamMITREInternal
updated Relationships
2011-09-13CWE Content TeamMITREInternal
updated Potential_Mitigations, References
2012-05-11CWE Content TeamMITREInternal
updated References, Relationships, Taxonomy_Mappings
2012-10-30CWE Content TeamMITREInternal
updated Potential_Mitigations
2014-07-30CWE Content TeamMITREInternal
updated Relationships, Taxonomy_Mappings
Previous Entry Names
Change DatePrevious Entry Name
2008-04-11Mobile Code: Invoking Untrusted Mobile Code
2009-01-12Download of Untrusted Mobile Code Without Integrity Check
Page Last Updated: July 30, 2014