The Future of Zero Trust and SASE is Now! Watch on-demand

  • Why Netskope chevron

    Changing the way networking and security work together.

  • Our Customers chevron

    Netskope serves more than 3,000 customers worldwide including more than 25 of the Fortune 100

  • Our Partners chevron

    We partner with security leaders to help you secure your journey to the cloud.

Highest in Execution. Furthest in Vision.

Netskope recognized as a Leader in the 2023 Gartner® Magic Quadrant™ for Security Service Edge.

Get the report
Netskope recognized as a Leader in the 2023 Gartner® Magic Quadrant™ for Security Service Edge.
We help our customers to be Ready for Anything

See our customers
Woman smiling with glasses looking out window
Netskope’s partner-centric go-to-market strategy enables our partners to maximize their growth and profitability while transforming enterprise security.

Learn about Netskope Partners
Group of diverse young professionals smiling
Your Network of Tomorrow

Plan your path toward a faster, more secure, and more resilient network designed for the applications and users that you support.

Get the white paper
Your Network of Tomorrow
Introducing the Netskope One Platform

Netskope One is a cloud-native platform that offers converged security and networking services to enable your SASE and zero trust transformation.

Learn about Netskope One
Abstract with blue lighting
Embrace a Secure Access Service Edge (SASE) architecture

Netskope NewEdge is the world’s largest, highest-performing security private cloud and provides customers with unparalleled service coverage, performance and resilience.

Learn about NewEdge
Netskope Cloud Exchange

The Netskope Cloud Exchange (CE) provides customers with powerful integration tools to leverage investments across their security posture.

Learn about Cloud Exchange
Netskope video
The platform of the future is Netskope

Intelligent Security Service Edge (SSE), Cloud Access Security Broker (CASB), Cloud Firewall, Next Generation Secure Web Gateway (SWG), and Private Access for ZTNA built natively into a single solution to help every business on its journey to Secure Access Service Edge (SASE) architecture.

Go to Products Overview
Netskope video
Next Gen SASE Branch is hybrid — connected, secured, and automated

Netskope Next Gen SASE Branch converges Context-Aware SASE Fabric, Zero-Trust Hybrid Security, and SkopeAI-powered Cloud Orchestrator into a unified cloud offering, ushering in a fully modernized branch experience for the borderless enterprise.

Learn about Next Gen SASE Branch
People at the open space office
Designing a SASE Architecture For Dummies

Get your complimentary copy of the only guide to SASE design you’ll ever need.

Get the eBook
Make the move to market-leading cloud security services with minimal latency and high reliability.

Learn about NewEdge
Lighted highway through mountainside switchbacks
Safely enable the use of generative AI applications with application access control, real-time user coaching, and best-in-class data protection.

Learn how we secure generative AI use
Safely Enable ChatGPT and Generative AI
Zero trust solutions for SSE and SASE deployments

Learn about Zero Trust
Boat driving through open sea
Netskope achieves FedRAMP High Authorization

Choose Netskope GovCloud to accelerate your agency’s transformation.

Learn about Netskope GovCloud
Netskope GovCloud
  • Resources chevron

    Learn more about how Netskope can help you secure your journey to the cloud.

  • Blog chevron

    Learn how Netskope enables security and networking transformation through security service edge (SSE)

  • Events and Workshops chevron

    Stay ahead of the latest security trends and connect with your peers.

  • Security Defined chevron

    Everything you need to know in our cybersecurity encyclopedia.

Security Visionaries Podcast

How to Use a Magic Quadrant and Other Industry Research
In this episode Max Havey, Steve Riley and Mona Faulkner dissect the intricate process of creating a Magic Quadrant and why it's much more than just a chart.

Play the podcast
How to Use a Magic Quadrant and Other Industry Research podcast
Latest Blogs

Read how Netskope can enable the Zero Trust and SASE journey through security service edge (SSE) capabilities.

Read the blog
Sunrise and cloudy sky
SASE Week 2023: Your SASE journey starts now!

Replay sessions from the fourth annual SASE Week.

Explore sessions
SASE Week 2023
What is Security Service Edge?

Explore the security side of SASE, the future of network and protection in the cloud.

Learn about Security Service Edge
Four-way roundabout
  • Company chevron

    We help you stay ahead of cloud, data, and network security challenges.

  • Leadership chevron

    Our leadership team is fiercely committed to doing everything it takes to make our customers successful.

  • Customer Solutions chevron

    We are here for you and with you every step of the way, ensuring your success with Netskope.

  • Training and Certification chevron

    Netskope training will help you become a cloud security expert.

Supporting sustainability through data security

Netskope is proud to participate in Vision 2045: an initiative aimed to raise awareness on private industry’s role in sustainability.

Find out more
Supporting Sustainability Through Data Security
Thinkers, builders, dreamers, innovators. Together, we deliver cutting-edge cloud security solutions to help our customers protect their data and people.

Meet our team
Group of hikers scaling a snowy mountain
Netskope’s talented and experienced Professional Services team provides a prescriptive approach to your successful implementation.

Learn about Professional Services
Netskope Professional Services
Secure your digital transformation journey and make the most of your cloud, web, and private applications with Netskope training.

Learn about Training and Certifications
Group of young professionals working

CORS Exploitation in the Cloud

Jan 09 2020

Cross-Origin Resource Sharing (CORS) is a mechanism which uses HTTP headers to tell a browser that a web application running at one origin has permission to access selected resources from a server at a different origin. This functionality exists for cases where an application developer would want to deliberately ignore a same origin policy (SOP) which mitigates many common attacks against browsers (notably cross-site scripting). This makes it a powerful tool for applications deployed on our increasingly distributed Internet but also a potential source of vulnerability. 

Background: What is the Same-Origin Policy?

The Same-Origin Policy is a mitigating control which restricts how scripts or other resources from one origin interact with resources from a third party. 

Some examples of cross-origin requests are:

  • A different domain ( to
  • A different subdomain ( to
  • A different port ( to
  • A different protocol ( to

The Same-Origin policy mitigates some common web application attacks and is a critical tool for protecting users and applications on the modern web.

More details are available at the Mozilla Developer Network.

How Does CORS Work?

There are two headers which primarily govern CORS: Access-Control-Allow-Origin, and Access-Control-Allow-Credentials. When a script from wants to make a request to, the browser sends a pre-flight request to with in the Origin header. This is how the browser asks for permission for the resource to interact with the requesting site. The service on will then return an Access-Control-Allow-Origin response header if the domain matches an allowed origin. Only if the domain matches the one hosting the script does the browser send the HTTP request.

The Access Control Allow Origin header accepts various origins:

  • Domains and subdomains (,
  • Wildcards (*)
  • ‘Null’

The case of the ‘Null’ origin is interesting and can result in some misconfigurations which we do not explore in this article. ‘Null’ is an origin assigned by default to many documents, sandboxed code, and redirects. While this is useful, it  can potentially open up your application to attack. The Mozilla Developer Network and w3c specify that the null origin should not be used. More details about that vulnerability are available in Portswigger’s blog on the subject.

Access-Control-Allow-Credentials is a boolean – that is, it can be only True or False. If our application requires a user to be authenticated to use it, and that application makes CORS requests on behalf of that logged in user, we require Access-Control-Allow-Credentials to be true. If the resources we are accessing do not absolutely require that the user’s credentials be passed to the cross-domain resource, we should set this to False. 

In some cases, you may want to bypass the same-origin policy to share content from a CDN, an S3 bucket, or an on-premises server.

  • If you host your website in S3, but want to use JavaScript to be able to make authenticated GET and PUT requests against the same bucket by using the Amazon S3 API endpoint for the bucket.
  • Loading a web font.
  • Loading dynamic content from another webpage.
  • Making an authenticated GET request to an on-prem server.

If you want to allow any domain to make a cross-origin request, you can certainly use the setting Access-Control-Allow-Origin: *

Unfortunately, per the CORS specification – if you have wildcards in your ACAO header, then Access-Control-Allow-Credentials cannot be true. While you might think that maintaining a list of allowed origins makes good sense, it turns out that CORS policy cannot accept a list either! So in order to allow credentials to be passed over a CORS request, you must process the origin and ensure that it’s valid. As we’ll explain, this is a recipe for disaster when developers are not careful.

CORS in the Cloud

In addition to the fact that any VM you deploy in your IaaS environment which supports HTTP can enable CORS, many individual services support CORS, including: 

  • Azure Functions
  • Azure Logic Apps
  • Azure Blob Storage
  • Google Cloud Functions
  • Google Cloud Storage
  • Google Cloud Endpoints
  • Amazon Lambda
  • Amazon S3
  • Amazon API Gateway

Clearly, this is a functionality that is widespread, and often used by web developers. As your infrastructure grows and matures, CORS is very likely to see use across your cloud infrastructure.

In the case of Lambda and API Gateway, Amazon provides the following guidance:

“To enable cross-origin resource sharing (CORS) for a proxy integration, you must add Access-Control-Allow-Origin: <domain_name> to the output headers. <domain_name> can be * for any domain name.”

As mentioned above, this works wonderfully, as long as you don’t need the Access-Controls-Allow-Credentials field to be true. If you need credentials, you’ll need to process the user-supplied origin. If you have never deployed CORS before and turn to the Internet for a template, you may end up using one of the many code examples which only reflect the origin and do not provide specific instructions on ensuring CORS is deployed securely.

Realistically, the most secure configuration is a hard-coded allow list that is maintained by the developer. However, as discussed below, even this does not stop CORS from opening up an exploitable hole.

Exploiting Misconfigurations

One of the most common misconfigurations, as mentioned above, is reflecting the user-supplied origin header in the server response, effectively allowing requests from any origin. As an example, consider an AWS Lambda and API Gateway serverless architecture where the Lambda accesses data stored in a DynamoDB instance. If you want to include more than a single domain in the ACAO header (e.g., you have more than one subdomain that will need to access the API), then you’ll need to write a Lambda to handle the headers. Note that while we use AWS and Amazon services here as examples, these same sorts of vulnerabilities can also occur with the same architecture in both Azure and GCP. In the Azure case, this architecture would be comprised of Azure Application Gateway, Azure Functions, and CosmosDB. In GCP, the corresponding services would be Cloud Endpoints, Google Cloud Functions, and BigTable.

Figure 1: Simple CORS-enabled API Gateway Architecture

Now in this case, we can see that a request made to the API Gateway will trigger the lambda which generates the CORS headers. As we mentioned with this misconfiguration – the lambda is simply reflecting the origin provided by the user, and that request is then handed off to the second lambda to make database queries by way of the API gateway. This database can contain whatever sort of information you might access via API but don’t want disclosed to the world (e.g. customer records, API keys, etc.). 

This means that all an attacker needs to do is get an unwitting but authorized user to make the request on their behalf from their domain. This can be attained by any number of methods: script injection, phishing, or any other way that a user might inadvertently navigate to an attacker-controlled resource. When the victim navigates to the attacker controlled resource, the sensitive request is made from the attacker’s origin with the authorized user’s credentials, and CORS allows this to occur, providing the sensitive data back to the attacker.

Impacts of CORS Misconfigurations

Examples of CORS misconfigurations being exploited:

  • A US Department of Defense Website had an improper access control in CORS which allowed an attacker to steal user sessions.
  • A bitcoin exchange had a vulnerability which could steal users’ private API key, allowing all of their BTC to be transferred to an arbitrary address.

These vulnerabilities and others like them underscore the need to verify that your CORS configuration is correct. This means ensuring that you are not simply reflecting the origin that you are provided by the browser, but maintaining an accurate, up-to-date allow list. 

The Cream in Your XSS Coffee

(AUTHOR NOTE: It doesn’t matter if you personally like cream in your coffee. This is just a euphemism. I personally favor a venti Americano with a splash of cream and 3 splenda.)

Using CORS trust relationships, we can actually use even properly configured environments to make a cross-site scripting vulnerability on one site far more damaging. Given an XSS vulnerability on a page which is trusted via CORS, an attacker can retrieve sensitive information from the site which trusts the vulnerable page. 

In this scenario, we’ll use the same API Gateway configuration as above, but assume that it actually does the CORS Origin checks correctly – that is, it looks for a valid subdomain and returns only if the user is authenticated and the request comes from a subdomain which is on an accurate allow list. 

Figure 2: Simple CORS-enabled S3 Front-End for API Gateway Architecture

In this scenario, there is no CORS misconfiguration, but instead the page on the S3 Bucket is vulnerable to Cross-Site Scripting. In this scenario, the attacker exploits that cross-site scripting vulnerability and instead injects JavaScript which retrieves information from the database. All of this happens because the API gateway, and therefore the database, trust the website hosted in the bucket. 

These attacks and more have been detailed in a non-cloud context by the folks over at Portswigger, the makers of Burp Suite. 

Even though these are hypothetical examples, knowing about CORS security is key because cross-domain requests are even more common in the cloud than they are in on-premises systems and so there are more opportunities for these sorts of vulnerabilities to exist and be exploited. So how do we deal with attacks that target our CORS-enabled cloud applications?

Thinking About CORS Under ATT&CK

The MITRE ATT&CK framework provides a useful way of looking at where CORS, whether configured correction and used to exploit trust relationships or misconfigured and exploited, can impact your enterprise and the ripple effects of ensuring that your web applications are safe from cross-site scripting. In this section, we summarize the associated tactics and techniques.

Enticing a user to access a page with a malicious script

Tactics: User Execution
Techniques: Drive-by Compromise, Spearphishing Link
Mitigations: Restrict Web-Based Content

Blocking JavaScript is a powerful tool for preventing this entire exploit chain in the first place. Of course, this can be difficult to do generally due to the ubiquity of JavaScript on the modern web. Having an allow list for running Javascript from trusted sources is crucial.
This mitigation is, due to the nature of the attack, the only one in this list designed to protect users on the endpoint rather than protect the application from exploitation.

Using the CORS misconfiguration to gain privileged access via origin reflection

Tactics: Initial Access
Techniques: Attacker in the Browser
Mitigations (proposed): Application Developer Guidance, Vulnerability Scanning

Although MITRE does not currently link Application Developer Guidance to Attacker in the Browser, ensuring CORS is properly configured is an application developer responsibility. Some vulnerability scanners will detect CORS misconfigurations.

 Exploiting cross-site scripting on a vulnerable page

Tactics: Initial Access
Techniques: Exploit Public-Facing Application
Mitigations: Vulnerability Scanning

Cross-site scripting can be challenging to notice in code review, but there are lots of good tools for automatically finding cross-site scripting. In particular, input validation is one of the most powerful tools for mitigating XSS.

Exploiting the trust relationship between the page and the API gateway

Tactics: Privilege Escalation
Techniques: Process Injection
Mitigations: Vulnerability Scanning

Although conventionally, process injection refers to applications running on the endpoint, here we’re still making the same connection – an application is issuing commands on behalf of an attacker via injected code. By expanding our definition of process injection to include web applications, we cover the case of an application with a cross-site scripting vulnerability making a malicious call on behalf of an attacker.  Unfortunately, the only mitigation is to ensure that applications which are CORS-enabled do not have latent vulnerabilities.

Returning sensitive data from the database

Tactics: Collection
Techniques: Data from Information Repositories
Mitigations: Audit

More than auditing the database itself for sensitive data or for access from unauthorized places, it is crucial to audit the applications which access and allow access to sensitive data. 


If you’re running cloud infrastructure, especially APIs in the cloud, you almost certainly have CORS deployed somewhere. Verifying that you do not have CORS misconfigurations in your cloud apps is a critical step in securing your cloud infrastructure. However, even if you have CORS properly configured, it can bite you if the apps which leverage CORS connections are not themselves secure since CORS can act as an amplifier for any latent vulnerability. By using the Mitre ATT&CK framework, we can create tabletop scenarios which allow us to profile and emulate attacks before they happen and give us a holistic view of our defense posture against certain types of exploits.

Stay informed!

Subscribe for the latest from the Netskope Blog