CAPEC

Common Attack Pattern Enumeration and Classification
Common Attack Pattern Enumeration and Classification

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Individual CAPEC Dictionary Definition (Release 1.1)
Individual CAPEC Dictionary Definition (Release 1.1)

Embedding Scripts within Scripts
Attack Pattern ID
Pattern Abstraction: Standard

19

Typical Severity

High

Description

Summary


An attack of this type exploits a programs' vulnerabilities that are brought on by allowing remote hosts to execute scripts. The attacker leverages this capability to execute scripts to execute his/her own script by embedding it within other scripts that the target software is likely to execute. The attacker must have the ability to inject script into script that is likely to be executed. If this is done, then the attacker can potentially launch a variety of probes and attacks against the web server's local environment, in many cases the so-called DMZ, back end resources the web server can communicate with, and other hosts.

With the proliferation of intermediaries, such as Web App Firewalls, network devices, and even printers having JVMs and Web servers, there are many locales where an attacker can inject malicious scripts. Since this attack pattern defines scripts within scripts, there are likely privileges to execute said attack on the host.

Of course, these attacks are not solely limited to the server side, client side scripts like Ajax and client side Javascript can contain malicious scripts as well. In general all that is required is for there to be sufficient privileges to execute a script, but not protected against writing.

Attack Prerequisites

Target software must be able to execute scripts, and also allow attacker to write/upload script

Typical Likelihood of Exploit

High

Methods of Attack
  • Injection
  • API Abuse
Examples-Instances

Description


Ajax applications enable rich functionality for browser based web applications. Applications like Google Maps deliver unprecedented ability to zoom in and out, scroll graphics, and change grapic presentation through Ajax. The security issues that an attacker may exploit in this instance are the relative lack of security features in Javascript and the various browser's implementation of Javascript, these security gaps are what XSS and a host of other client side vulnerabilities are based on. While Ajax may not open up new security holes, per se, due to the conversational aspects between client and server of Ajax communication, attacks can be optimized. A single zoom in or zoom out on a graphic in an Ajax application may round trip to the server dozens of times. One of the first steps many attackers take is frequently footprinting an environment, this can include scanning local addresses like 192.*.*.* IP addresses, checking local directories, files, and settings for known vulnerabilities, and so on.

<IMG SRC=javascript:alert('XSS')>

The XSS script that is embeded in a given IMG tag can be manipulated to probe a different address on every click of the mouse or other motions that the Ajax application is aware of.

In addition the enumerations allow for the attacker to nest sequential logic in the attacks. While Ajax applications do not open up brand new attack vectors, the existing attack vectors are more than adequate to execute attacks, and now these attacks can be optimized to sequentially execute and enumerate host environments.

Attacker Skill or Knowledge Required

Low → To load malicious script into open, e.g. world writable directory Medium → Executing remote scripts on host and collecting output

Resources Required

Ability to deploy a custom script on host

Solutions and Mitigations

Design: Use browser technologies that do not allow client side scripting.

Design: Utilize strict type, character, and encoding enforcement

Design: Server side developers should not proxy content via XHR or other means, if a http proxy for remote content is setup on the server side, the client's browser has no way of discerning where the data is originating from.

Implementation: Ensure all content that is delivered to client is sanitized against an acceptable content specification.

Implementation: Perform input validation for all remote content.

Implementation: Perform output validation for all remote content.

Implementation: Disable scripting languages such as Javascript in browser

Implementation: Session tokens for specific host

Implementation: Patching software. There are many attack vectors for XSS on the client side and the server side. Many vulnerabilities are fixed in service packs for browser, web servers, and plug in technologies, staying current on patch release that deal with XSS countermeasures mitigates this.

Implementation: Privileges are constrained, if a script is loaded, ensure system runs in chroot jail or other limited authority mode

Attack Motivation-Consequences
  • Run Arbitrary Code
  • Privilege Escalation
Context Description


“Attack Pattern: Embedding Scripts within Scripts
The technology that runs the Internet is diverse and complex. There are hundreds of development languages, compilers, and interpreters that can build and execute code. Every developer has a sense for only part of the overall technology. Investments in time and money are made into each particular technology. As these systems evolve, the need to maintain backward compatibility becomes paramount. In management speak, this is the need to capitalize on an existing software investment. This is one reason that some newer scripting languages have backward support for older scripting languages.

As a result of this rapid and barely controlled evolution, much of the technology found in the wild can embed or otherwise access other languages and technologies in some form. This adds multiple layers of complexity and makes keeping track of all the disparate (yet available) functionality difficult at best. Filtering rules and security assumptions get swamped by the flow of new stuff. Looking for unanticipated functionality forgotten in the nooks and crannies of a system is an excellent technique."
[Hoglund and McGraw 04]

Injection Vector

Malicious input delivered through standard script page, e.g. ASP web page

Payload

Varies with instantiation of attack pattern. MAy contain network probe or attacks that run against or on host using host account permissions

Activation Zone

Web server scripting host

Payload Activation Impact

Enables attacker to execute scripts on remote host

Related Weaknesses
CWE-IDWeakness NameWeakness Relationship Type
79Failure to Sanitize Directives in a Web Page (aka 'Cross-site scripting' (XSS))Targeted
276Insecure Default PermissionsTargeted
279Insecure Execution-assigned PermissionsSecondary
284Access Control IssuesSecondary
Related Attack Patterns
IDNameRelationship TypeRelationship Description
18Embedding Scripts in Nonscript ElementsSimilar
Purpose

Penetration

CIA Impact
Confidentiality ImpactIntegrity ImpactAvailability Impact
MediumHighLow
Technical Context
Architectural ParadigmFrameworkPlatformLanguage
AllAllAllAll
References

G. Hoglund and G. McGraw. Exploiting Software: How to Break Code. Addison-Wesley, February 2004.

Source
Submission(s)
SubmitterOrganizationDateComment
G. Hoglund and G. McGraw. Exploiting Software: How to Break Code. Addison-Wesley, February 2004.Cigital, Inc2007-01-01
Modification(s)
ModifierOrganizationDateComment
Gunnar PetersonCigital, Inc2007-02-28Fleshed out content to CAPEC schema from the original descriptions in "Exploiting Software"
Sean BarnumCigital, Inc2007-03-09Review and revise
Richard StruseVOXEM, Inc2007-03-26Review and feedback leading to changes in ADescription and Related Attack Patterns
Sean BarnumCigital, Inc2007-04-13Modified pattern content according to review and feedback
 
Page Last Updated: April 18, 2008