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Protection Against Exploits
Spring Security provides protection against common exploits. Whenever possible, the protection is enabled by default. Below you will find high level description of the various exploits that Spring Security protects against.
Cross Site Request Forgery (CSRF)
Spring provides comprehensive support for protecting against Cross Site Request Forgery (CSRF) attacks. In the following sections we will explore:
What is a CSRF Attack?
The best way to understand a CSRF attack is by taking a look at a concrete example.
Assume that your bank’s website provides a form that allows transferring money from the currently logged in user to another bank account. For example, the transfer form might look like:
<form method="post"
action="/transfer">
<input type="text"
name="amount"/>
<input type="text"
name="routingNumber"/>
<input type="text"
name="account"/>
<input type="submit"
value="Transfer"/>
</form>The corresponding HTTP request might look like:
POST /transfer HTTP/1.1
Host: bank.example.com
Cookie: JSESSIONID=randomid
Content-Type: application/x-www-form-urlencoded
amount=100.00&routingNumber=1234&account=9876Now pretend you authenticate to your bank’s website and then, without logging out, visit an evil website. The evil website contains an HTML page with the following form:
<form method="post"
action="https://bank.example.com/transfer">
<input type="hidden"
name="amount"
value="100.00"/>
<input type="hidden"
name="routingNumber"
value="evilsRoutingNumber"/>
<input type="hidden"
name="account"
value="evilsAccountNumber"/>
<input type="submit"
value="Win Money!"/>
</form>You like to win money, so you click on the submit button. In the process, you have unintentionally transferred $100 to a malicious user. This happens because, while the evil website cannot see your cookies, the cookies associated with your bank are still sent along with the request.
Worst yet, this whole process could have been automated using JavaScript. This means you didn’t even need to click on the button. Furthermore, it could just as easily happen when visiting an honest site that is a victim of a XSS attack. So how do we protect our users from such attacks?
Protecting Against CSRF Attacks
The reason that a CSRF attack is possible is that the HTTP request from the victim’s website and the request from the attacker’s website are exactly the same. This means there is no way to reject requests coming from the evil website and allow requests coming from the bank’s website. To protect against CSRF attacks we need to ensure there is something in the request that the evil site is unable to provide so we can differentiate the two requests.
Spring provides two mechanisms to protect against CSRF attacks:
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Specifying the SameSite Attribute on your session cookie
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Both protections require that Safe Methods Must be Idempotent |
Safe Methods Must be Idempotent
In order for either protection against CSRF to work, the application must ensure that "safe" HTTP methods are idempotent.
This means that requests with the HTTP method GET, HEAD, OPTIONS, and TRACE should not change the state of the application.
Synchronizer Token Pattern
The predominant and most comprehensive way to protect against CSRF attacks is to use the Synchronizer Token Pattern. This solution is to ensure that each HTTP request requires, in addition to our session cookie, a secure random generated value called a CSRF token must be present in the HTTP request.
When an HTTP request is submitted, the server must look up the expected CSRF token and compare it against the actual CSRF token in the HTTP request. If the values do not match, the HTTP request should be rejected.
The key to this working is that the actual CSRF token should be in a part of the HTTP request that is not automatically included by the browser. For example, requiring the actual CSRF token in an HTTP parameter or an HTTP header will protect against CSRF attacks. Requiring the actual CSRF token in a cookie does not work because cookies are automatically included in the HTTP request by the browser.
We can relax the expectations to only require the actual CSRF token for each HTTP request that updates state of the application. For that to work, our application must ensure that safe HTTP methods are idempotent. This improves usability since we want to allow linking to our website using links from external sites. Additionally, we do not want to include the random token in HTTP GET as this can cause the tokens to be leaked.
Let’s take a look at how our example would change when using the Synchronizer Token Pattern.
Assume the actual CSRF token is required to be in an HTTP parameter named _csrf.
Our application’s transfer form would look like:
<form method="post"
action="/transfer">
<input type="hidden"
name="_csrf"
value="4bfd1575-3ad1-4d21-96c7-4ef2d9f86721"/>
<input type="text"
name="amount"/>
<input type="text"
name="routingNumber"/>
<input type="hidden"
name="account"/>
<input type="submit"
value="Transfer"/>
</form>The form now contains a hidden input with the value of the CSRF token. External sites cannot read the CSRF token since the same origin policy ensures the evil site cannot read the response.
The corresponding HTTP request to transfer money would look like this:
POST /transfer HTTP/1.1
Host: bank.example.com
Cookie: JSESSIONID=randomid
Content-Type: application/x-www-form-urlencoded
amount=100.00&routingNumber=1234&account=9876&_csrf=4bfd1575-3ad1-4d21-96c7-4ef2d9f86721You will notice that the HTTP request now contains the _csrf parameter with a secure random value.
The evil website will not be able to provide the correct value for the _csrf parameter (which must be explicitly provided on the evil website) and the transfer will fail when the server compares the actual CSRF token to the expected CSRF token.
SameSite Attribute
An emerging way to protect against CSRF Attacks is to specify the SameSite Attribute on cookies.
A server can specify the SameSite attribute when setting a cookie to indicate that the cookie should not be sent when coming from external sites.
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Spring Security does not directly control the creation of the session cookie, so it does not provide support for the SameSite attribute.
Spring Session provides support for the |
An example, HTTP response header with the SameSite attribute might look like:
Set-Cookie: JSESSIONID=randomid; Domain=bank.example.com; Secure; HttpOnly; SameSite=LaxValid values for the SameSite attribute are:
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Strict- when specified any request coming from the same-site will include the cookie. Otherwise, the cookie will not be included in the HTTP request. -
Lax- when specified cookies will be sent when coming from the same-site or when the request comes from top-level navigations and the method is idempotent. Otherwise, the cookie will not be included in the HTTP request.
Let’s take a look at how our example could be protected using the SameSite attribute.
The bank application can protect against CSRF by specifying the SameSite attribute on the session cookie.
With the SameSite attribute set on our session cookie, the browser will continue to send the JSESSIONID cookie with requests coming from the banking website.
However, the browser will no longer send the JSESSIONID cookie with a transfer request coming from the evil website.
Since the session is no longer present in the transfer request coming from the evil website, the application is protected from the CSRF attack.
There are some important considerations that one should be aware about when using SameSite attribute to protect against CSRF attacks.
Setting the SameSite attribute to Strict provides a stronger defense but can confuse users.
Consider a user that stays logged into a social media site hosted at https://social.example.com.
The user receives an email at https://email.example.org that includes a link to the social media site.
If the user clicks on the link, they would rightfully expect to be authenticated to the social media site.
However, if the SameSite attribute is Strict the cookie would not be sent and so the user would not be authenticated.
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We could improve the protection and usability of |
Another obvious consideration is that in order for the SameSite attribute to protect users, the browser must support the SameSite attribute.
Most modern browsers do support the SameSite attribute.
However, older browsers that are still in use may not.
For this reason, it is generally recommended to use the SameSite attribute as a defense in depth rather than the sole protection against CSRF attacks.
When to use CSRF protection
When should you use CSRF protection? Our recommendation is to use CSRF protection for any request that could be processed by a browser by normal users. If you are only creating a service that is used by non-browser clients, you will likely want to disable CSRF protection.
CSRF protection and JSON
A common question is "do I need to protect JSON requests made by javascript?" The short answer is, it depends. However, you must be very careful as there are CSRF exploits that can impact JSON requests. For example, a malicious user can create a CSRF with JSON using the following form:
<form action="https://bank.example.com/transfer" method="post" enctype="text/plain">
<input name='{"amount":100,"routingNumber":"evilsRoutingNumber","account":"evilsAccountNumber", "ignore_me":"' value='test"}' type='hidden'>
<input type="submit"
value="Win Money!"/>
</form>This will produce the following JSON structure
{ "amount": 100,
"routingNumber": "evilsRoutingNumber",
"account": "evilsAccountNumber",
"ignore_me": "=test"
}If an application were not validating the Content-Type, then it would be exposed to this exploit.
Depending on the setup, a Spring MVC application that validates the Content-Type could still be exploited by updating the URL suffix to end with .json as shown below:
<form action="https://bank.example.com/transfer.json" method="post" enctype="text/plain">
<input name='{"amount":100,"routingNumber":"evilsRoutingNumber","account":"evilsAccountNumber", "ignore_me":"' value='test"}' type='hidden'>
<input type="submit"
value="Win Money!"/>
</form>CSRF and Stateless Browser Applications
What if my application is stateless? That doesn’t necessarily mean you are protected. In fact, if a user does not need to perform any actions in the web browser for a given request, they are likely still vulnerable to CSRF attacks.
For example, consider an application that uses a custom cookie that contains all the state within it for authentication instead of the JSESSIONID. When the CSRF attack is made the custom cookie will be sent with the request in the same manner that the JSESSIONID cookie was sent in our previous example. This application will be vulnerable to CSRF attacks.
Applications that use basic authentication are also vulnerable to CSRF attacks. The application is vulnerable since the browser will automatically include the username and password in any requests in the same manner that the JSESSIONID cookie was sent in our previous example.
CSRF Considerations
There are a few special considerations to consider when implementing protection against CSRF attacks.
Logging In
In order to protect against forging log in requests the log in HTTP request should be protected against CSRF attacks. Protecting against forging log in requests is necessary so that a malicious user cannot read a victim’s sensitive information. The attack is performed as follows:
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A malicious user performs a CSRF log in using the malicious user’s credentials. The victim is now authenticated as the malicious user.
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The malicious user then tricks the victim to visit the compromised website and enter sensitive information
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The information is associated to the malicious user’s account so the malicious user can log in with their own credentials and view the vicitim’s sensitive information
A possible complication to ensuring log in HTTP requests are protected against CSRF attacks is that the user might experience a session timeout that causes the request to be rejected. A session timeout is surprising to users who do not expect to need to have a session in order to log in. For more information refer to CSRF and Session Timeouts.
Logging Out
In order to protect against forging log out requests, the log out HTTP request should be protected against CSRF attacks. Protecting against forging log out requests is necessary so a malicious user cannot read a victim’s sensitive information. For details on the attack refer to this blog post.
A possible complication to ensuring log out HTTP requests are protected against CSRF attacks is that the user might experience a session timeout that causes the request to be rejected. A session timeout is surprising to users who do not expect to need to have a session in order to log out. For more information refer to CSRF and Session Timeouts.
CSRF and Session Timeouts
More often than not, the expected CSRF token is stored in the session. This means that as soon as the session expires the server will not find an expected CSRF token and reject the HTTP request. There are a number of options to solve timeouts each of which come with trade offs.
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The best way to mitigate the timeout is by using JavaScript to request a CSRF token on form submission. The form is then updated with the CSRF token and submitted.
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Another option is to have some JavaScript that lets the user know their session is about to expire. The user can click a button to continue and refresh the session.
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Finally, the expected CSRF token could be stored in a cookie. This allows the expected CSRF token to outlive the session.
One might ask why the expected CSRF token isn’t stored in a cookie by default. This is because there are known exploits in which headers (for example, to specify the cookies) can be set by another domain. This is the same reason Ruby on Rails no longer skips CSRF checks when the header X-Requested-With is present. See this webappsec.org thread for details on how to perform the exploit. Another disadvantage is that by removing the state (that is, the timeout), you lose the ability to forcibly invalidate the token if it is compromised.
Multipart (file upload)
Protecting multipart requests (file uploads) from CSRF attacks causes a chicken and the egg problem. In order to prevent a CSRF attack from occurring, the body of the HTTP request must be read to obtain actual CSRF token. However, reading the body means that the file will be uploaded which means an external site can upload a file.
There are two options to using CSRF protection with multipart/form-data. Each option has its trade-offs.
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Before you integrate Spring Security’s CSRF protection with multipart file upload, ensure that you can upload without the CSRF protection first. More information about using multipart forms with Spring can be found within the 1.1.11. Multipart Resolver section of the Spring reference and the MultipartFilter javadoc. |
Place CSRF Token in the Body
The first option is to include the actual CSRF token in the body of the request. By placing the CSRF token in the body, the body will be read before authorization is performed. This means that anyone can place temporary files on your server. However, only authorized users will be able to submit a File that is processed by your application. In general, this is the recommended approach because the temporary file upload should have a negligible impact on most servers.
Include CSRF Token in URL
If allowing unauthorized users to upload temporary files is not acceptable, an alternative is to include the expected CSRF token as a query parameter in the action attribute of the form. The disadvantage to this approach is that query parameters can be leaked. More generally, it is considered best practice to place sensitive data within the body or headers to ensure it is not leaked. Additional information can be found in RFC 2616 Section 15.1.3 Encoding Sensitive Information in URI’s.
HiddenHttpMethodFilter
In some applications a form parameter can be used to override the HTTP method.
For example, the form below could be used to treat the HTTP method as a delete rather than a post.
<form action="/process"
method="post">
<!-- ... -->
<input type="hidden"
name="_method"
value="delete"/>
</form>Overriding the HTTP method occurs in a filter.
That filter must be placed before Spring Security’s support.
Note that overriding only happens on a post, so this is actually unlikely to cause any real problems.
However, it is still best practice to ensure it is placed before Spring Security’s filters.
Security HTTP Response Headers
There are many HTTP response headers that can be used to increase the security of web applications. This section is dedicated to the various HTTP response headers that Spring Security provides explicit support for. If necessary, Spring Security can also be configured to provide custom headers.
Default Security Headers
Spring Security provides a default set of security related HTTP response headers to provide secure defaults.
The default for Spring Security is to include the following headers:
Cache-Control: no-cache, no-store, max-age=0, must-revalidate
Pragma: no-cache
Expires: 0
X-Content-Type-Options: nosniff
Strict-Transport-Security: max-age=31536000 ; includeSubDomains
X-Frame-Options: DENY
X-XSS-Protection: 1; mode=block| Strict-Transport-Security is only added on HTTPS requests |
If the defaults do not meet your needs, you can easily remove, modify, or add headers from these defaults. For additional details on each of these headers, refer to the corresponding sections:
Cache Control
Spring Security’s default is to disable caching to protect user’s content.
If a user authenticates to view sensitive information and then logs out, we don’t want a malicious user to be able to click the back button to view the sensitive information. The cache control headers that are sent by default are:
Cache-Control: no-cache, no-store, max-age=0, must-revalidate
Pragma: no-cache
Expires: 0In order to be secure by default, Spring Security adds these headers by default. However, if your application provides its own cache control headers Spring Security will back out of the way. This allows for applications to ensure that static resources like CSS and JavaScript can be cached.
Content Type Options
Historically browsers, including Internet Explorer, would try to guess the content type of a request using content sniffing. This allowed browsers to improve the user experience by guessing the content type on resources that had not specified the content type. For example, if a browser encountered a JavaScript file that did not have the content type specified, it would be able to guess the content type and then run it.
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There are many additional things one should do (i.e. only display the document in a distinct domain, ensure Content-Type header is set, sanitize the document, etc) when allowing content to be uploaded. However, these measures are out of the scope of what Spring Security provides. It is also important to point out when disabling content sniffing, you must specify the content type in order for things to work properly. |
The problem with content sniffing is that this allowed malicious users to use polyglots (i.e. a file that is valid as multiple content types) to perform XSS attacks. For example, some sites may allow users to submit a valid postscript document to a website and view it. A malicious user might create a postscript document that is also a valid JavaScript file and perform a XSS attack with it.
Spring Security disables content sniffing by default by adding the following header to HTTP responses:
X-Content-Type-Options: nosniffHTTP Strict Transport Security (HSTS)
When you type in your bank’s website, do you enter mybank.example.com or do you enter https://mybank.example.com? If you omit the https protocol, you are potentially vulnerable to Man in the Middle attacks. Even if the website performs a redirect to https://mybank.example.com a malicious user could intercept the initial HTTP request and manipulate the response (e.g. redirect to https://mibank.example.com and steal their credentials).
Many users omit the https protocol and this is why HTTP Strict Transport Security (HSTS) was created. Once mybank.example.com is added as a HSTS host, a browser can know ahead of time that any request to mybank.example.com should be interpreted as https://mybank.example.com. This greatly reduces the possibility of a Man in the Middle attack occurring.
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In accordance with RFC6797, the HSTS header is only injected into HTTPS responses. In order for the browser to acknowledge the header, the browser must first trust the CA that signed the SSL certificate used to make the connection (not just the SSL certificate). |
One way for a site to be marked as a HSTS host is to have the host preloaded into the browser.
Another is to add the Strict-Transport-Security header to the response.
For example, Spring Security’s default behavior is to add the following header which instructs the browser to treat the domain as an HSTS host for a year (there are approximately 31536000 seconds in a year):
Strict-Transport-Security: max-age=31536000 ; includeSubDomains ; preloadThe optional includeSubDomains directive instructs the browser that subdomains (e.g. secure.mybank.example.com) should also be treated as an HSTS domain.
The optional preload directive instructs the browser that domain should be preloaded in browser as HSTS domain.
For more details on HSTS preload please see https://hstspreload.org.
HTTP Public Key Pinning (HPKP)
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In order to remain passive Spring Security still provides support for HPKP in servlet environments, but for the reasons listed above HPKP is no longer recommended by the security team. |
HTTP Public Key Pinning (HPKP) specifies to a web client which public key to use with certain web server to prevent Man in the Middle (MITM) attacks with forged certificates. When used correctly, HPKP could add additional layers of protection against compromised certificates. However, due to the complexity of HPKP many experts no longer recommend using it and Chrome has even removed support for it.
For additional details around why HPKP is no longer recommended read Is HTTP Public Key Pinning Dead? and I’m giving up on HPKP.
X-Frame-Options
Allowing your website to be added to a frame can be a security issue. For example, using clever CSS styling users could be tricked into clicking on something that they were not intending. For example, a user that is logged into their bank might click a button that grants access to other users. This sort of attack is known as Clickjacking.
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Another modern approach to dealing with clickjacking is to use Content Security Policy (CSP). |
There are a number ways to mitigate clickjacking attacks. For example, to protect legacy browsers from clickjacking attacks you can use frame breaking code. While not perfect, the frame breaking code is the best you can do for the legacy browsers.
A more modern approach to address clickjacking is to use X-Frame-Options header. By default Spring Security disables rendering pages within an iframe using with the following header:
X-Frame-Options: DENYX-XSS-Protection
Some browsers have built in support for filtering out reflected XSS attacks. This is by no means foolproof, but does assist in XSS protection.
The filtering is typically enabled by default, so adding the header typically just ensures it is enabled and instructs the browser what to do when a XSS attack is detected. For example, the filter might try to change the content in the least invasive way to still render everything. At times, this type of replacement can become a XSS vulnerability in itself. Instead, it is best to block the content rather than attempt to fix it. By default Spring Security blocks the content using the following header:
X-XSS-Protection: 1; mode=blockContent Security Policy (CSP)
Content Security Policy (CSP) is a mechanism that web applications can leverage to mitigate content injection vulnerabilities, such as cross-site scripting (XSS). CSP is a declarative policy that provides a facility for web application authors to declare and ultimately inform the client (user-agent) about the sources from which the web application expects to load resources.
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Content Security Policy is not intended to solve all content injection vulnerabilities. Instead, CSP can be leveraged to help reduce the harm caused by content injection attacks. As a first line of defense, web application authors should validate their input and encode their output. |
A web application may employ the use of CSP by including one of the following HTTP headers in the response:
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Content-Security-Policy -
Content-Security-Policy-Report-Only
Each of these headers are used as a mechanism to deliver a security policy to the client. A security policy contains a set of security policy directives, each responsible for declaring the restrictions for a particular resource representation.
For example, a web application can declare that it expects to load scripts from specific, trusted sources, by including the following header in the response:
Content-Security-Policy: script-src https://trustedscripts.example.comAn attempt to load a script from another source other than what is declared in the script-src directive will be blocked by the user-agent.
Additionally, if the report-uri directive is declared in the security policy, then the violation will be reported by the user-agent to the declared URL.
For example, if a web application violates the declared security policy, the following response header will instruct the user-agent to send violation reports to the URL specified in the policy’s report-uri directive.
Content-Security-Policy: script-src https://trustedscripts.example.com; report-uri /csp-report-endpoint/Violation reports are standard JSON structures that can be captured either by the web application’s own API or by a publicly hosted CSP violation reporting service, such as, https://report-uri.com/.
The Content-Security-Policy-Report-Only header provides the capability for web application authors and administrators to monitor security policies, rather than enforce them.
This header is typically used when experimenting and/or developing security policies for a site.
When a policy is deemed effective, it can be enforced by using the Content-Security-Policy header field instead.
Given the following response header, the policy declares that scripts may be loaded from one of two possible sources.
Content-Security-Policy-Report-Only: script-src 'self' https://trustedscripts.example.com; report-uri /csp-report-endpoint/If the site violates this policy, by attempting to load a script from evil.com, the user-agent will send a violation report to the declared URL specified by the report-uri directive, but still allow the violating resource to load nevertheless.
Applying Content Security Policy to a web application is often a non-trivial undertaking. The following resources may provide further assistance in developing effective security policies for your site.
Referrer Policy
Referrer Policy is a mechanism that web applications can leverage to manage the referrer field, which contains the last page the user was on.
Spring Security’s approach is to use Referrer Policy header, which provides different policies:
Referrer-Policy: same-originThe Referrer-Policy response header instructs the browser to let the destination knows the source where the user was previously.
Feature Policy
Feature Policy is a mechanism that allows web developers to selectively enable, disable, and modify the behavior of certain APIs and web features in the browser.
Feature-Policy: geolocation 'self'With Feature Policy, developers can opt-in to a set of "policies" for the browser to enforce on specific features used throughout your site. These policies restrict what APIs the site can access or modify the browser’s default behavior for certain features.
Permissions Policy
Permissions Policy is a mechanism that allows web developers to selectively enable, disable, and modify the behavior of certain APIs and web features in the browser.
Permissions-Policy: geolocation=(self)With Permissions Policy, developers can opt-in to a set of "policies" for the browser to enforce on specific features used throughout your site. These policies restrict what APIs the site can access or modify the browser’s default behavior for certain features.
Clear Site Data
Clear Site Data is a mechanism by which any browser-side data - cookies, local storage, and the like - can be removed when an HTTP response contains this header:
Clear-Site-Data: "cache", "cookies", "storage", "executionContexts"This is a nice clean-up action to perform on logout.
Custom Headers
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Refer to the relevant sections to see how to configure both servlet based applications. |
Spring Security has mechanisms to make it convenient to add the more common security headers to your application. However, it also provides hooks to enable adding custom headers.
HTTP
All HTTP based communication, including static resources, should be protected using TLS.
As a framework, Spring Security does not handle HTTP connections and thus does not provide support for HTTPS directly. However, it does provide a number of features that help with HTTPS usage.
Strict Transport Security
Spring Security provides support for Strict Transport Security and enables it by default.
Proxy Server Configuration
When using a proxy server it is important to ensure that you have configured your application properly. For example, many applications will have a load balancer that responds to request for https://example.com/ by forwarding the request to an application server at https://192.168.1:8080 Without proper configuration, the application server will not know that the load balancer exists and treat the request as though https://192.168.1:8080 was requested by the client.
To fix this you can use RFC 7239 to specify that a load balancer is being used. To make the application aware of this, you need to either configure your application server aware of the X-Forwarded headers. For example Tomcat uses the RemoteIpValve and Jetty uses ForwardedRequestCustomizer. Alternatively, Spring users can leverage ForwardedHeaderFilter.
Spring Boot users may use the server.use-forward-headers property to configure the application.
See the Spring Boot documentation for further details.