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CWE-601: URL Redirection to Untrusted Site ('Open Redirect')

 
URL Redirection to Untrusted Site ('Open Redirect')
Weakness ID: 601 (Weakness Variant)Status: Draft
+ Description

Description Summary

A web application accepts a user-controlled input that specifies a link to an external site, and uses that link in a Redirect. This simplifies phishing attacks.

Extended Description

An http parameter may contain a URL value and could cause the web application to redirect the request to the specified URL. By modifying the URL value to a malicious site, an attacker may successfully launch a phishing scam and steal user credentials. Because the server name in the modified link is identical to the original site, phishing attempts have a more trustworthy appearance.

+ Alternate Terms
Open Redirect
Cross-site Redirect
Cross-domain Redirect
+ Time of Introduction
  • Architecture and Design
  • Implementation
+ Applicable Platforms

Languages

Language-independent

Architectural Paradigms

Web-based

+ Common Consequences
ScopeEffect
Access Control

Technical Impact: Bypass protection mechanism; Gain privileges / assume identity

The user may be redirected to an untrusted page that contains malware which may then compromise the user's machine. This will expose the user to extensive risk and the user's interaction with the web server may also be compromised if the malware conducts keylogging or other attacks that steal credentials, personally identifiable information (PII), or other important data.

Access Control
Confidentiality
Other

Technical Impact: Bypass protection mechanism; Gain privileges / assume identity; Other

The user may be subjected to phishing attacks by being redirected to an untrusted page. The phishing attack may point to an attacker controlled web page that appears to be a trusted web site. The phishers may then steal the user's credentials and then use these credentials to access the legitimate web site.

+ Likelihood of Exploit

Low to Medium

+ Detection Methods

Manual Static Analysis

Since this weakness does not typically appear frequently within a single software package, manual white box techniques may be able to provide sufficient code coverage and reduction of false positives if all potentially-vulnerable operations can be assessed within limited time constraints.

Effectiveness: High

Automated Dynamic Analysis

Automated black box tools that supply URLs to every input may be able to spot Location header modifications, but test case coverage is a factor, and custom redirects may not be detected.

Automated Static Analysis

Automated static analysis tools may not be able to determine whether input influences the beginning of a URL, which is important for reducing false positives.

Other

Whether this issue poses a vulnerability will be subject to the intended behavior of the application. For example, a search engine might intentionally provide redirects to arbitrary URLs.

Automated Static Analysis - Binary / Bytecode

According to SOAR, the following detection techniques may be useful:

Highly cost effective:

  • Bytecode Weakness Analysis - including disassembler + source code weakness analysis

  • Binary Weakness Analysis - including disassembler + source code weakness analysis

Effectiveness: SOAR High

Dynamic Analysis with automated results interpretation

According to SOAR, the following detection techniques may be useful:

Highly cost effective:

  • Web Application Scanner

  • Web Services Scanner

  • Database Scanners

Effectiveness: SOAR High

Dynamic Analysis with manual results interpretation

According to SOAR, the following detection techniques may be useful:

Highly cost effective:

  • Fuzz Tester

  • Framework-based Fuzzer

Effectiveness: SOAR High

Manual Static Analysis - Source Code

According to SOAR, the following detection techniques may be useful:

Highly cost effective:

  • Manual Source Code Review (not inspections)

Effectiveness: SOAR High

Automated Static Analysis - Source Code

According to SOAR, the following detection techniques may be useful:

Highly cost effective:

  • Source code Weakness Analyzer

  • Context-configured Source Code Weakness Analyzer

Effectiveness: SOAR High

Architecture / Design Review

According to SOAR, the following detection techniques may be useful:

Highly cost effective:

  • Formal Methods / Correct-By-Construction

Cost effective for partial coverage:

  • Inspection (IEEE 1028 standard) (can apply to requirements, design, source code, etc.)

Effectiveness: SOAR High

+ Demonstrative Examples

Example 1

The following code obtains a URL from the query string and then redirects the user to that URL.

(Bad Code)
Example Language: PHP 
$redirect_url = $_GET['url'];
header("Location: " . $redirect_url);

The problem with the above code is that an attacker could use this page as part of a phishing scam by redirecting users to a malicious site. For example, assume the above code is in the file example.php. An attacker could supply a user with the following link:

(Attack)
 
http://example.com/example.php?url=http://malicious.example.com

The user sees the link pointing to the original trusted site (example.com) and does not realize the redirection that could take place.

Example 2

The following code is a Java servlet that will receive a GET request with a url parameter in the request to redirect the browser to the address specified in the url parameter. The servlet will retrieve the url parameter value from the request and send a response to redirect the browser to the url address.

(Bad Code)
Example Language: Java 
public class RedirectServlet extends HttpServlet {

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String query = request.getQueryString();
if (query.contains("url")) {
String url = request.getParameter("url");
response.sendRedirect(url);
}
}
}

The problem with this Java servlet code is that an attacker could use the RedirectServlet as part of a e-mail phishing scam to redirect users to a malicious site. An attacker could send an HTML formatted e-mail directing the user to log into their account by including in the e-mail the following link:

(Attack)
Example Language: HTML 
<a href="http://bank.example.com/redirect?url=http://attacker.example.net">Click here to log in</a>

The user may assume that the link is safe since the URL starts with their trusted bank, bank.example.com. However, the user will then be redirected to the attacker's web site (attacker.example.net) which the attacker may have made to appear very similar to bank.example.com. The user may then unwittingly enter credentials into the attacker's web page and compromise their bank account. A Java servlet should never redirect a user to a URL without verifying that the redirect address is a trusted site.

+ Observed Examples
ReferenceDescription
URL parameter loads the URL into a frame and causes it to appear to be part of a valid page.
An open redirect vulnerability in the search script in the software allows remote attackers to redirect users to arbitrary web sites and conduct phishing attacks via a URL as a parameter to the proper function.
Open redirect vulnerability in the software allows remote attackers to redirect users to arbitrary web sites and conduct phishing attacks via a URL in the proper parameter.
+ Potential Mitigations

Phase: Implementation

Strategy: Input Validation

Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a whitelist of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.

When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue."

Do not rely exclusively on looking for malicious or malformed inputs (i.e., do not rely on a blacklist). A blacklist is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, blacklists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.

Use a whitelist of approved URLs or domains to be used for redirection.

Phase: Architecture and Design

Use an intermediate disclaimer page that provides the user with a clear warning that they are leaving the current site. Implement a long timeout before the redirect occurs, or force the user to click on the link. Be careful to avoid XSS problems (CWE-79) when generating the disclaimer page.

Phase: Architecture and Design

Strategy: Enforcement by Conversion

When the set of acceptable objects, such as filenames or URLs, is limited or known, create a mapping from a set of fixed input values (such as numeric IDs) to the actual filenames or URLs, and reject all other inputs.

For example, ID 1 could map to "/login.asp" and ID 2 could map to "http://www.example.com/". Features such as the ESAPI AccessReferenceMap [R.601.4] provide this capability.

Phase: Architecture and Design

Ensure that no externally-supplied requests are honored by requiring that all redirect requests include a unique nonce generated by the application [R.601.1]. Be sure that the nonce is not predictable (CWE-330).

Note that this can be bypassed using XSS (CWE-79).

Phases: Architecture and Design; Implementation

Strategy: Identify and Reduce Attack Surface

Understand all the potential areas where untrusted inputs can enter your software: parameters or arguments, cookies, anything read from the network, environment variables, reverse DNS lookups, query results, request headers, URL components, e-mail, files, filenames, databases, and any external systems that provide data to the application. Remember that such inputs may be obtained indirectly through API calls.

Many open redirect problems occur because the programmer assumed that certain inputs could not be modified, such as cookies and hidden form fields.

Phase: Operation

Strategy: Firewall

Use an application firewall that can detect attacks against this weakness. It can be beneficial in cases in which the code cannot be fixed (because it is controlled by a third party), as an emergency prevention measure while more comprehensive software assurance measures are applied, or to provide defense in depth.

Effectiveness: Moderate

An application firewall might not cover all possible input vectors. In addition, attack techniques might be available to bypass the protection mechanism, such as using malformed inputs that can still be processed by the component that receives those inputs. Depending on functionality, an application firewall might inadvertently reject or modify legitimate requests. Finally, some manual effort may be required for customization.

+ Background Details

Phishing is a general term for deceptive attempts to coerce private information from users that will be used for identity theft.

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfWeakness ClassWeakness Class20Improper Input Validation
Development Concepts (primary)699
ChildOfCategoryCategory442Web Problems
Development Concepts699
ChildOfWeakness ClassWeakness Class610Externally Controlled Reference to a Resource in Another Sphere
Research Concepts (primary)1000
ChildOfCategoryCategory722OWASP Top Ten 2004 Category A1 - Unvalidated Input
Weaknesses in OWASP Top Ten (2004) (primary)711
ChildOfCategoryCategory8012010 Top 25 - Insecure Interaction Between Components
Weaknesses in the 2010 CWE/SANS Top 25 Most Dangerous Programming Errors (primary)800
ChildOfCategoryCategory819OWASP Top Ten 2010 Category A10 - Unvalidated Redirects and Forwards
Weaknesses in OWASP Top Ten (2010) (primary)809
ChildOfCategoryCategory8642011 Top 25 - Insecure Interaction Between Components
Weaknesses in the 2011 CWE/SANS Top 25 Most Dangerous Software Errors (primary)900
ChildOfCategoryCategory938OWASP Top Ten 2013 Category A10 - Unvalidated Redirects and Forwards
Weaknesses in OWASP Top Ten (2013) (primary)928
ChildOfCategoryCategory990SFP Secondary Cluster: Tainted Input to Command
Software Fault Pattern (SFP) Clusters (primary)888
MemberOfViewView884CWE Cross-section
CWE Cross-section (primary)884
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
Anonymous Tool Vendor (under NDA)
WASC38URl Redirector Abuse
Software Fault PatternsSFP24Tainted input to command
+ References
[R.601.1] Craig A. Shue, Andrew J. Kalafut and Minaxi Gupta. "Exploitable Redirects on the Web: Identification, Prevalence, and Defense". <http://www.cs.indiana.edu/cgi-pub/cshue/research/woot08.pdf>.
[R.601.2] Russ McRee. "Open redirect vulnerabilities: definition and prevention". Page 43. Issue 17. (IN)SECURE. July 2008. <http://www.net-security.org/dl/insecure/INSECURE-Mag-17.pdf>.
[R.601.3] Jason Lam. "Top 25 Series - Rank 23 - Open Redirect". SANS Software Security Institute. 2010-03-25. <http://software-security.sans.org/blog/2010/03/25/top-25-series-rank-23-open-redirect>.
[R.601.4] [REF-21] OWASP. "OWASP Enterprise Security API (ESAPI) Project". <http://www.owasp.org/index.php/ESAPI>.
+ Content History
Submissions
Submission DateSubmitterOrganizationSource
Anonymous Tool Vendor (under NDA)Externally Mined
Modifications
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigitalExternal
updated Potential_Mitigations, Time_of_Introduction
2008-09-08CWE Content TeamMITREInternal
updated Alternate_Terms, Background_Details, Description, Detection_Factors, Likelihood_of_Exploit, Name, Relationships, Observed_Example, Taxonomy_Mappings
2008-10-03CWE Content TeamMITREInternal
updated References and Observed_Examples
2008-10-14CWE Content TeamMITREInternal
updated Alternate_Terms, Observed_Examples, References
2009-03-10CWE Content TeamMITREInternal
updated Relationships
2009-05-27CWE Content TeamMITREInternal
updated Name
2009-12-28CWE Content TeamMITREInternal
updated Demonstrative_Examples, Detection_Factors, Likelihood_of_Exploit, Potential_Mitigations
2010-02-16CWE Content TeamMITREInternal
updated Applicable_Platforms, Common_Consequences, Detection_Factors, Potential_Mitigations, Related_Attack_Patterns, Relationships, Taxonomy_Mappings
2010-04-05CWE Content TeamMITREInternal
updated Demonstrative_Examples
2010-06-21CWE Content TeamMITREInternal
updated Common_Consequences, Potential_Mitigations, References, Relationships
2010-09-27CWE Content TeamMITREInternal
updated Potential_Mitigations
2011-06-01CWE Content TeamMITREInternal
updated Common_Consequences
2011-06-27CWE Content TeamMITREInternal
updated Relationships
2011-09-13CWE Content TeamMITREInternal
updated Potential_Mitigations, References
2012-05-11CWE Content TeamMITREInternal
updated Relationships
2012-10-30CWE Content TeamMITREInternal
updated Potential_Mitigations
2013-07-17CWE Content TeamMITREInternal
updated References, Relationships
2014-07-30CWE Content TeamMITREInternal
updated Detection_Factors, Relationships, Taxonomy_Mappings
Previous Entry Names
Change DatePrevious Entry Name
2008-04-11Unsafe URL Redirection
2008-09-09URL Redirection to Untrusted Site
2009-05-27URL Redirection to Untrusted Site (aka 'Open Redirect')
Page Last Updated: July 30, 2014