What is the http-header X-XSS-Protection

Question

So I ve been toying around with HTTP for fun in telnet now  i e  just typing in telnet google com 80 and putting in random GETs and POSTs with different headers and the like  but I ve come across something that google com transmits in it s headers that I don t know   I ve been looking through http   www w3 org Protocols rfc2616 rfc2616 html and have found no definition for this particular http-header that google seems to be spouting out   GET   HTTP 1 1  HTTP 1 1 200 OK Date  Wed  01 Feb 2012 03 42 24 GMT Expires  -1 Cache-Control  private  max-age 0 Content-Type  text html  charset ISO-8859-1 Set-Cookie  PREF ID 6ddbc0a0342e7e63 FF 0 TM 1328067744 LM 1328067744 S 4d4farvCGl5Ww0C3  expires Fri  31-Jan-2014 03 42 24 GMT  path    domain  google com Set-Cookie  NID 56 PgRwCKa8EltKnHS5clbFuhwyWsd3cPXiV1-iXzgyKsiy5RKXEKbg89gWWpjzYZjLPWTKrCWhOUhdInOlYU56LOb2W7XpC7uBnKAjMbxQSBw1UIprzw2BFK5dnaY7PRji  expires Thu  02-Aug-2012 03 42 24 GMT  path    domain  google com  HttpOnly P3P  CP  This is not a P3P policy  See http   www google com support accounts bin answer py hl en amp answer 151657 for more info   Server  gws X-XSS-Protection  1  mode block X-Frame-Options  SAMEORIGIN Transfer-Encoding  chunked  1000   Anyone know what X-XSS-Protection is

User · Answer

This header is getting somehow deprecated. You can read more about it here - X-XSS-Protection

Chrome has removed their XSS Auditor

Firefox has not, and will not implement X-XSS-Protection

Edge has retired their XSS filter

This means that if you do not need to support legacy browsers, it is recommended that you use Content-Security-Policy without allowing unsafe-inline scripts instead.

User · Answer

X-XSS-Protection  1   Force XSS protection  useful if XSS protection was disabled by the user  X-XSS-Protection  0   Disable XSS protection The token mode block will prevent browser  IE8  and Webkit browsers  to render pages  instead of sanitizing  if a potential XSS reflection    non-persistent  attack is detected        Warning  mode block creates a vulnerability in IE8  more info    More informations   http   blogs msdn com b ie archive 2008 07 02 ie8-security-part-iv-the-xss-filter aspx and http   blog veracode com 2014 03 guidelines-for-setting-security-headers

User · Answer

TL DR  All well written web sites   apps  must emit the header X-XSS-Protection  0 and just forget about this feature  If you want to have extra security that better user agents can provide  use a strict Content-Security-Policy header  Long answer  HTTP header X-XSS-Protection is one of those things that Microsoft introduced in Internet Explorer 8 0  MSIE 8  that was supposed to improve security of incorrectly written web sites  The idea is to apply some kind of heuristics to try to detect reflection XSS attack and automatically neuter the attack  The problematic part of this is  quot heuristics quot  and  quot neutering quot   The heuristics causes false positives and neutering cannot be safely done because it causes side-effects that can be used to implement XSS attacks and DoS attacks on perfectly safe web sites  The bad part is that if a web site does not emit the header X-XSS-Protection then the browser will behave as if the header X-XSS-Protection  1 had been emitted  The worst part is that this value is the least-safe value of all possible values for this header  For a given secure web site  that is  the site does not have reflected XSS vulnerabilities  this  quot XSS protection quot  feature allows following attacks  X-XSS-Protection  1 allows attacker to selectively block parts of JavaScript and keep rest of the scripts running  This is possible because the heuristics of this feature are simply  quot if value of any GET parameter is found in the scripting part of the page source  the script will be automatically modified in user agent dependant way quot   In practice  the attacker can e g  add parameter disablexss  lt script src  quot framebuster js quot  and the browser will automatically remove the string  lt script src  quot framebuster js quot  from the actual page source  Note that the rest of the page continues run and the attacker just removed this part of page security  In practice  any JS in the page source can be modified  For some cases  a page without XSS vulnerability having reflected content can be used to run selected JavaScript on page due the neutering incorrectly turning plain text data into executable JavaScript code   That is  turn textual data within a normal DOM text node into content of  lt script gt  tag and execute it   X-XSS-Protection  1  mode block allows attacker to leak data from the page source by using the behavior of the page as side-channel  For example  if the page contains JavaScript code along the lines of var csrf secret  quot 521231347843 quot   the attacker simply adds an extra parameter e g  leak var 20csrf secret  quot 3 and if the page is NOT blocked  the 3 was incorrect first digit  The attacker tries again  this time leak var 20csrf secret  quot 5 and the page loading will be aborted  This allows the attacker to know that the first digit of the secret is 5  The attacker then continues to guess the next digit  This allows easily brute-forcing of CSRF secrets or any other secret value in the  lt script gt  source  In the end  if your site is full of XSS reflection attacks  using the default value of 1 will reduce the attack surface a little bit  However  if your site is secure and you don t emit X-XSS-Protection  0  your site will be vulnerable with any browser that supports this feature  If you want defense in depth support from browsers against yet-unknown XSS vulnerabilities on your site  use a strict Content-Security-Policy header and keep sending 0 for this mis-feature  That doesn t open your site to any known vulnerabilities  Currently this feature is enabled by default in MSIE  Safari and Google Chrome  This used to be enabled in Edge but Microsoft already removed this mis-feature from Edge  Mozilla Firefox never implemented this  See also  https   homakov blogspot com 2013 02 hacking-facebook-with-oauth2-and-chrome html https   blog innerht ml the-misunderstood-x-xss-protection  http   p42 us ie8xss Abusing IE8s XSS Filters pdf https   www slideshare net masatokinugawa xxn-en https   bugs chromium org p chromium issues detail id 396544 https   bugs chromium org p chromium issues detail id 498982

User · Answer

X-XSS-Protection is a HTTP header understood by Internet Explorer 8  and newer versions   This header lets domains toggle on and off the  XSS Filter  of IE8  which prevents some categories of XSS attacks  IE8 has the filter activated by default  but servers can switch if off by setting     X-XSS-Protection  0   See also http   blogs msdn com b ieinternals archive 2011 01 31 controlling-the-internet-explorer-xss-filter-with-the-x-xss-protection-http-header aspx

User · Answer

You can see in this List of useful HTTP headers      X-XSS-Protection  This header enables the Cross-site scripting  XSS  filter built into most recent web browsers  It s usually enabled by default anyway  so the role of this header is to re-enable the filter for this particular website if it was disabled by the user  This header is supported in IE 8   and in Chrome  not sure which versions   The anti-XSS filter was added in Chrome 4  Its unknown if that version honored this header

User · Answer

This response header can be used to configure a user-agent s built in reflective XSS protection  Currently  only Microsoft s Internet Explorer  Google Chrome and Safari  WebKit  support this header   Internet Explorer 8 included a new feature to help prevent reflected cross-site scripting attacks  known as the XSS Filter  This filter runs by default in the Internet  Trusted  and Restricted security zones  Local Intranet zone pages may opt-in to the protection using the same header   About the header that you posted in your question   The header X-XSS-Protection  1  mode block enables the XSS Filter  Rather than sanitize the page  when a XSS attack is detected  the browser will prevent rendering of the page      In March of 2010  we added to IE8 support for a new token in the   X-XSS-Protection header  mode block    X-XSS-Protection  1  mode block      When this token is present  if a potential XSS Reflection attack is   detected  Internet Explorer will prevent rendering of the page    Instead of attempting to sanitize the page to surgically remove the   XSS attack  IE will render only                Internet Explorer recognizes a possible cross-site scripting attack    It logs the event and displays an appropriate message to the user  The   MSDN article describes how this header works    How this filter works in IE   More on this article  https   blogs msdn microsoft com ie 2008 07 02 ie8-security-part-iv-the-xss-filter      The XSS Filter operates as an IE8 component with visibility into all   requests   responses flowing through the browser  When the filter   discovers likely XSS in a cross-site request  it identifies and   neuters the attack if it is replayed in the server   s response  Users   are not presented with questions they are unable to answer     IE simply   blocks the malicious script from executing       With the new XSS Filter  IE8 Beta 2 users encountering a Type-1 XSS   attack will see a notification like the following       IE8 XSS Attack Notification      The page has been modified and the XSS attack is blocked       In this case  the XSS Filter has identified a cross-site scripting   attack in the URL   It has neutered this attack as the identified   script was replayed back into the response page   In this way  the   filter is effective without modifying an initial request to the server   or blocking an entire response       The Cross-Site Scripting Filter event is logged when Windows Internet   Explorer 8 detects and mitigates a cross-site scripting  XSS  attack    Cross-site scripting attacks occur when one website  generally   malicious  injects  adds  JavaScript code into otherwise legitimate   requests to another website  The original request is generally   innocent  such as a link to another page or a Common Gateway Interface    CGI  script providing a common service  such as a guestbook   The   injected script generally attempts to access privileged information or   services that the second website does not intend to allow  The   response or the request generally reflects results back to the   malicious website  The XSS Filter  a feature new to Internet Explorer   8  detects JavaScript in URL and HTTP POST requests  If JavaScript is   detected  the XSS Filter searches evidence of reflection  information   that would be returned to the attacking website if the attacking   request were submitted unchanged  If reflection is detected  the XSS   Filter sanitizes the original request so that the additional   JavaScript cannot be executed  The XSS Filter then logs that action as   a Cross-Site Script Filter event  The following image shows an example   of a site that is modified to prevent a cross-site scripting attack    Source  https   msdn microsoft com en-us library dd565647 v vs 85  aspx  Web developers may wish to disable the filter for their content  They can do so by setting an HTTP header    X-XSS-Protection  0   More on security headers in    Guidelines for Setting Security Headers Security HTTP Headers - X-XSS-PROTECTION MDN Docs X-XSS-Protection

[http] What is the http-header "X-XSS-Protection"?

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