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preventing the use of HTTP compression</span> </div> </a> <ul id="toc-Problems_preventing_the_use_of_HTTP_compression-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Security_implications" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Security_implications"> <div class="vector-toc-text"> <span class="vector-toc-numb">5</span> <span>Security implications</span> </div> </a> <ul id="toc-Security_implications-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-References" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#References"> <div class="vector-toc-text"> <span class="vector-toc-numb">6</span> <span>References</span> </div> </a> <ul id="toc-References-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-External_links" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" 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src="//upload.wikimedia.org/wikipedia/commons/thumb/5/5b/HTTP_logo.svg/180px-HTTP_logo.svg.png" decoding="async" width="180" height="96" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/5/5b/HTTP_logo.svg/270px-HTTP_logo.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/5/5b/HTTP_logo.svg/360px-HTTP_logo.svg.png 2x" data-file-width="512" data-file-height="274" /></a></span></td></tr><tr><td class="sidebar-content"> <ul><li><a href="/wiki/HTTP_persistent_connection" title="HTTP persistent connection">Persistence</a></li> <li><a class="mw-selflink selflink">Compression</a></li> <li><a href="/wiki/HTTPS" title="HTTPS">HTTPS</a></li> <li><a href="/wiki/QUIC" title="QUIC">QUIC</a></li></ul></td> </tr><tr><th class="sidebar-heading"> <a href="/wiki/HTTP#Request_methods" title="HTTP">Request methods</a></th></tr><tr><td class="sidebar-content"> <ul><li><a href="/wiki/HTTP#Request_methods" title="HTTP">OPTIONS</a></li> <li><a href="/wiki/HTTP#Request_methods" title="HTTP">GET</a></li> <li><a href="/wiki/HTTP#Request_methods" title="HTTP">HEAD</a></li> <li><a href="/wiki/POST_(HTTP)" title="POST (HTTP)">POST</a></li> <li><a href="/wiki/HTTP#Request_methods" title="HTTP">PUT</a></li> <li><a href="/wiki/HTTP#Request_methods" title="HTTP">DELETE</a></li> <li><a href="/wiki/HTTP#Request_methods" title="HTTP">TRACE</a></li> <li><a href="/wiki/HTTP#Request_methods" title="HTTP">CONNECT</a></li> <li><a href="/wiki/PATCH_(HTTP)" title="PATCH (HTTP)">PATCH</a></li></ul></td> </tr><tr><th class="sidebar-heading"> <a href="/wiki/List_of_HTTP_header_fields" title="List of HTTP header fields">Header fields</a></th></tr><tr><td class="sidebar-content"> <ul><li><a href="/wiki/HTTP_cookie" title="HTTP cookie">Cookie</a></li> <li><a href="/wiki/HTTP_ETag" title="HTTP ETag">ETag</a></li> <li><a href="/wiki/HTTP_location" title="HTTP location">Location</a></li> <li><a href="/wiki/HTTP_referer" title="HTTP referer">HTTP referer</a></li> <li><a href="/wiki/Do_Not_Track" title="Do Not Track">DNT</a></li> <li><a href="/wiki/X-Forwarded-For" title="X-Forwarded-For">X-Forwarded-For</a></li></ul></td> </tr><tr><th class="sidebar-heading"> <a href="/wiki/List_of_HTTP_status_codes" title="List of HTTP status codes">Response status codes</a></th></tr><tr><td class="sidebar-content"> <ul><li><a href="/wiki/HTTP_301" title="HTTP 301">301 Moved Permanently</a></li> <li><a href="/wiki/HTTP_302" title="HTTP 302">302 Found</a></li> <li><a href="/wiki/HTTP_303" title="HTTP 303">303 See Other</a></li> <li><a href="/wiki/HTTP_403" title="HTTP 403">403 Forbidden</a></li> <li><a href="/wiki/HTTP_404" title="HTTP 404">404 Not Found</a></li> <li><a href="/wiki/HTTP_451" title="HTTP 451">451 Unavailable for Legal Reasons</a></li></ul></td> </tr><tr><th class="sidebar-heading"> Security access control methods</th></tr><tr><td class="sidebar-content"> <ul><li><a href="/wiki/Basic_access_authentication" title="Basic access authentication">Basic access authentication</a></li> <li><a href="/wiki/Digest_access_authentication" title="Digest access authentication">Digest access authentication</a></li></ul></td> </tr><tr><th class="sidebar-heading"> Security vulnerabilities</th></tr><tr><td class="sidebar-content"> <ul><li><a href="/wiki/HTTP_header_injection" title="HTTP header injection">HTTP header injection</a></li> <li><a href="/wiki/HTTP_request_smuggling" title="HTTP request smuggling">HTTP request smuggling</a></li> <li><a href="/wiki/HTTP_response_splitting" title="HTTP response splitting">HTTP response splitting</a></li> <li><a href="/wiki/HTTP_parameter_pollution" title="HTTP parameter pollution">HTTP parameter pollution</a></li></ul></td> </tr><tr><td class="sidebar-navbar"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1129693374"><style data-mw-deduplicate="TemplateStyles:r1239400231">.mw-parser-output .navbar{display:inline;font-size:88%;font-weight:normal}.mw-parser-output .navbar-collapse{float:left;text-align:left}.mw-parser-output .navbar-boxtext{word-spacing:0}.mw-parser-output .navbar ul{display:inline-block;white-space:nowrap;line-height:inherit}.mw-parser-output .navbar-brackets::before{margin-right:-0.125em;content:"[ "}.mw-parser-output .navbar-brackets::after{margin-left:-0.125em;content:" ]"}.mw-parser-output .navbar li{word-spacing:-0.125em}.mw-parser-output .navbar a>span,.mw-parser-output .navbar a>abbr{text-decoration:inherit}.mw-parser-output .navbar-mini abbr{font-variant:small-caps;border-bottom:none;text-decoration:none;cursor:inherit}.mw-parser-output .navbar-ct-full{font-size:114%;margin:0 7em}.mw-parser-output .navbar-ct-mini{font-size:114%;margin:0 4em}html.skin-theme-clientpref-night .mw-parser-output .navbar li a abbr{color:var(--color-base)!important}@media(prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .navbar li a abbr{color:var(--color-base)!important}}@media print{.mw-parser-output .navbar{display:none!important}}</style><div class="navbar plainlinks hlist navbar-mini"><ul><li class="nv-view"><a href="/wiki/Template:HTTP" title="Template:HTTP"><abbr title="View this template">v</abbr></a></li><li class="nv-talk"><a href="/wiki/Template_talk:HTTP" title="Template talk:HTTP"><abbr title="Discuss this template">t</abbr></a></li><li class="nv-edit"><a href="/wiki/Special:EditPage/Template:HTTP" title="Special:EditPage/Template:HTTP"><abbr title="Edit this template">e</abbr></a></li></ul></div></td></tr></tbody></table> <p><b>HTTP compression</b> is a capability that can be built into <a href="/wiki/Web_server" title="Web server">web servers</a> and <a href="/wiki/Web_client" class="mw-redirect" title="Web client">web clients</a> to improve transfer speed and bandwidth utilization.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> </p><p>HTTP data is <a href="/wiki/Data_compression" title="Data compression">compressed</a> before it is sent from the server: compliant browsers will announce what methods are supported to the server before downloading the correct format; browsers that do not support compliant compression method will download uncompressed data. The most common compression schemes include <a href="/wiki/Gzip" title="Gzip">gzip</a> and <a href="/wiki/Brotli" title="Brotli">Brotli</a>; a full list of available schemes is maintained by the <a href="/wiki/Internet_Assigned_Numbers_Authority" title="Internet Assigned Numbers Authority">IANA</a>.<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> </p><p>There are two different ways compression can be done in HTTP. At a lower level, a Transfer-Encoding header field may indicate the payload of an HTTP message is compressed. At a higher level, a Content-Encoding header field may indicate that a resource being transferred, <a href="/wiki/Web_cache" title="Web cache">cached</a>, or otherwise referenced is compressed. Compression using Content-Encoding is more widely supported than Transfer-Encoding, and some browsers do not advertise support for Transfer-Encoding compression to avoid triggering bugs in servers.<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> </p> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="Compression_scheme_negotiation">Compression scheme negotiation</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=HTTP_compression&action=edit&section=1" title="Edit section: Compression scheme negotiation"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The negotiation is done in two steps, described in RFC 2616 and RFC 9110: </p><p>1. The <a href="/wiki/Web_client" class="mw-redirect" title="Web client">web client</a> advertises which compression schemes it supports by including a list of tokens in the <a href="/wiki/HTTP_request" class="mw-redirect" title="HTTP request">HTTP request</a>. For <i>Content-Encoding</i>, the list is in a field called <i>Accept-Encoding</i>; for <i>Transfer-Encoding</i>, the field is called <i>TE</i>. </p> <div class="mw-highlight mw-highlight-lang-http mw-content-ltr" dir="ltr"><pre><span></span><span class="nf">GET</span> <span class="nn">/encrypted-area</span> <span class="kr">HTTP</span><span class="o">/</span><span class="m">1.1</span> <span class="na">Host</span><span class="o">:</span> <span class="l">www.example.com</span> <span class="hll"><span class="na">Accept-Encoding</span><span class="o">:</span> <span class="l">gzip, deflate</span> </span></pre></div> <p>2. If the server supports one or more compression schemes, the outgoing data may be compressed by one or more methods supported by both parties. If this is the case, the server will add a <i>Content-Encoding</i> or <i>Transfer-Encoding</i> field in the HTTP response with the used schemes, separated by commas. </p> <div class="mw-highlight mw-highlight-lang-http mw-content-ltr" dir="ltr"><pre><span></span><span class="kr">HTTP</span><span class="o">/</span><span class="m">1.1</span> <span class="m">200</span> <span class="ne">OK</span> <span class="na">Date</span><span class="o">:</span> <span class="l">mon, 26 June 2016 22:38:34 GMT</span> <span class="na">Server</span><span class="o">:</span> <span class="l">Apache/1.3.3.7 (Unix) (Red-Hat/Linux)</span> <span class="na">Last-Modified</span><span class="o">:</span> <span class="l">Wed, 08 Jan 2003 23:11:55 GMT</span> <span class="na">Accept-Ranges</span><span class="o">:</span> <span class="l">bytes</span> <span class="na">Content-Length</span><span class="o">:</span> <span class="l">438</span> <span class="na">Connection</span><span class="o">:</span> <span class="l">close</span> <span class="na">Content-Type</span><span class="o">:</span> <span class="l">text/html; charset=UTF-8</span> <span class="hll"><span class="na">Content-Encoding</span><span class="o">:</span> <span class="l">gzip</span> </span></pre></div> <p>The <a href="/wiki/Web_server" title="Web server">web server</a> is by no means obligated to use any compression method – this depends on the internal settings of the web server and also may depend on the internal architecture of the website in question. </p> <div class="mw-heading mw-heading2"><h2 id="Content-Encoding_tokens">Content-Encoding tokens</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=HTTP_compression&action=edit&section=2" title="Edit section: Content-Encoding tokens"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The official list of tokens available to servers and client is maintained by IANA,<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> and it includes: </p> <ul><li>br – <a href="/wiki/Brotli" title="Brotli">Brotli</a>, a compression algorithm specifically designed for HTTP content encoding, defined in <style data-mw-deduplicate="TemplateStyles:r1238218222">.mw-parser-output cite.citation{font-style:inherit;word-wrap:break-word}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .citation:target{background-color:rgba(0,127,255,0.133)}.mw-parser-output .id-lock-free.id-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited.id-lock-limited a,.mw-parser-output .id-lock-registration.id-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription.id-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-free a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-limited a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-registration a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-subscription a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .cs1-ws-icon a{background-size:contain;padding:0 1em 0 0}.mw-parser-output .cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;color:var(--color-error,#d33)}.mw-parser-output .cs1-visible-error{color:var(--color-error,#d33)}.mw-parser-output .cs1-maint{display:none;color:#085;margin-left:0.3em}.mw-parser-output .cs1-kern-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}@media screen{.mw-parser-output .cs1-format{font-size:95%}html.skin-theme-clientpref-night .mw-parser-output .cs1-maint{color:#18911f}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .cs1-maint{color:#18911f}}</style>RFC <a rel="nofollow" class="external text" href="https://datatracker.ietf.org/doc/html/rfc7932">7932</a> and implemented in all modern major browsers.</li> <li><a href="/wiki/Compress" class="mw-redirect" title="Compress">compress</a> – UNIX "compress" program method (historic; deprecated in most applications and replaced by gzip or deflate)</li> <li>deflate – compression based on the <a href="/wiki/DEFLATE" class="mw-redirect" title="DEFLATE">deflate</a> algorithm (described in <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222">RFC <a rel="nofollow" class="external text" href="https://datatracker.ietf.org/doc/html/rfc1951">1951</a>), a combination of the <a href="/wiki/LZ77_and_LZ78#LZ77" title="LZ77 and LZ78">LZ77</a> algorithm and Huffman coding, wrapped inside the <a href="/wiki/Zlib" title="Zlib">zlib</a> data format (<link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222">RFC <a rel="nofollow" class="external text" href="https://datatracker.ietf.org/doc/html/rfc1950">1950</a>);</li> <li>exi – W3C <a href="/wiki/Efficient_XML_Interchange" title="Efficient XML Interchange">Efficient XML Interchange</a></li> <li><a href="/wiki/Gzip" title="Gzip">gzip</a> – GNU zip format (described in <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222">RFC <a rel="nofollow" class="external text" href="https://datatracker.ietf.org/doc/html/rfc1952">1952</a>). Uses the <a href="/wiki/DEFLATE" class="mw-redirect" title="DEFLATE">deflate</a> algorithm for compression, but the data format and the checksum algorithm differ from the "deflate" content-encoding. This method is the most broadly supported as of March 2011.<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup></li> <li><a href="/wiki/Identity_function" title="Identity function">identity</a> – No transformation is used. This is the default value for content coding.</li> <li><a href="/wiki/Pack200" title="Pack200">pack200-gzip</a> – Network Transfer Format for Java Archives<sup id="cite_ref-6" class="reference"><a href="#cite_note-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup></li> <li><a href="/wiki/Zstd" title="Zstd">zstd</a> – Zstandard compression, defined in <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222">RFC <a rel="nofollow" class="external text" href="https://datatracker.ietf.org/doc/html/rfc8478">8478</a></li></ul> <p>In addition to these, a number of unofficial or non-standardized tokens are used in the wild by either servers or clients: </p> <ul><li><a href="/wiki/Bzip2" title="Bzip2">bzip2</a> – compression based on the free bzip2 format, supported by <a href="/wiki/Lighttpd" title="Lighttpd">lighttpd</a><sup id="cite_ref-7" class="reference"><a href="#cite_note-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup></li> <li><a href="/wiki/Lempel%E2%80%93Ziv%E2%80%93Markov_chain_algorithm" title="Lempel–Ziv–Markov chain algorithm">lzma</a> – compression based on (raw) LZMA is available in Opera 20, and in elinks via a compile-time option<sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup></li> <li>peerdist<sup id="cite_ref-9" class="reference"><a href="#cite_note-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup> – Microsoft Peer Content Caching and Retrieval</li> <li><a href="/wiki/Rsync" title="Rsync">rsync</a><sup id="cite_ref-10" class="reference"><a href="#cite_note-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup> – <a href="/wiki/Delta_encoding#Delta_encoding_in_HTTP" title="Delta encoding">delta encoding in HTTP</a>, implemented by a pair of <i>rproxy</i> proxies.</li> <li>xpress – Microsoft compression protocol used by Windows 8 and later for Windows Store application updates. <a href="/wiki/LZ77_and_LZ78#LZ77" title="LZ77 and LZ78">LZ77</a>-based compression optionally using a Huffman encoding.<sup id="cite_ref-11" class="reference"><a href="#cite_note-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup></li> <li><a href="/wiki/XZ_Utils" title="XZ Utils">xz</a> – LZMA2-based content compression, supported by a non-official Firefox patch;<sup id="cite_ref-12" class="reference"><a href="#cite_note-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup> and fully implemented in mget since 2013-12-31.<sup id="cite_ref-13" class="reference"><a href="#cite_note-13"><span class="cite-bracket">[</span>13<span class="cite-bracket">]</span></a></sup></li></ul> <div class="mw-heading mw-heading2"><h2 id="Servers_that_support_HTTP_compression">Servers that support HTTP compression</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=HTTP_compression&action=edit&section=3" title="Edit section: Servers that support HTTP compression"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a href="/wiki/SAP_NetWeaver" title="SAP NetWeaver">SAP NetWeaver</a></li> <li><a href="/wiki/Internet_Information_Services" title="Internet Information Services">Microsoft IIS</a>: built-in or using third-party module</li> <li><a href="/wiki/Apache_HTTP_Server" title="Apache HTTP Server">Apache HTTP Server</a>, via <b><a rel="nofollow" class="external text" href="https://httpd.apache.org/docs/current/mod/mod_deflate.html">mod_deflate</a></b> (despite its name, only supporting gzip<sup id="cite_ref-14" class="reference"><a href="#cite_note-14"><span class="cite-bracket">[</span>14<span class="cite-bracket">]</span></a></sup>), and <b><a rel="nofollow" class="external text" href="https://httpd.apache.org/docs/current/mod/mod_brotli.html">mod_brotli</a></b></li> <li><a href="/wiki/Hiawatha_(web_server)" title="Hiawatha (web server)">Hiawatha HTTP server</a>: serves pre-compressed files<sup id="cite_ref-15" class="reference"><a href="#cite_note-15"><span class="cite-bracket">[</span>15<span class="cite-bracket">]</span></a></sup></li> <li><a href="/wiki/Cherokee_(Webserver)" class="mw-redirect" title="Cherokee (Webserver)">Cherokee HTTP server</a>, On the fly gzip and deflate compressions</li> <li><a href="/wiki/Oracle_iPlanet_Web_Server" title="Oracle iPlanet Web Server">Oracle iPlanet Web Server</a></li> <li><a href="/wiki/Zeus_Web_Server" title="Zeus Web Server">Zeus Web Server</a></li> <li><a href="/wiki/Lighttpd" title="Lighttpd">lighttpd</a></li> <li><a href="/wiki/Nginx" title="Nginx">nginx</a> – built-in</li> <li>Applications based on <a href="/wiki/Tornado_(web_server)" title="Tornado (web server)">Tornado</a>, if "compress_response" is set to True in the application settings (for versions prior to 4.0, set "gzip" to True)</li> <li><a href="/wiki/Jetty_(Web_server)" class="mw-redirect" title="Jetty (Web server)">Jetty Server</a> – built-into default static content serving and available via servlet filter configurations</li> <li><a href="/wiki/GeoServer" title="GeoServer">GeoServer</a></li> <li><a href="/wiki/Apache_Tomcat" title="Apache Tomcat">Apache Tomcat</a></li> <li><a href="/wiki/IBM_Websphere" class="mw-redirect" title="IBM Websphere">IBM Websphere</a></li> <li><a href="/wiki/AOLserver" title="AOLserver">AOLserver</a></li> <li><a href="/wiki/Ruby_(programming_language)" title="Ruby (programming language)">Ruby</a> <a href="/wiki/Rack_(web_server_interface)" title="Rack (web server interface)">Rack</a>, via the <b>Rack::Deflater</b> middleware</li> <li><a href="/wiki/HAProxy" title="HAProxy">HAProxy</a></li> <li><a href="/wiki/Varnish_(software)" title="Varnish (software)">Varnish</a> – built-in. Works also with <a href="/wiki/Edge_Side_Includes" title="Edge Side Includes">ESI</a></li> <li><a rel="nofollow" class="external text" href="https://line.github.io/armeria/">Armeria</a> – Serving pre-compressed files<sup id="cite_ref-16" class="reference"><a href="#cite_note-16"><span class="cite-bracket">[</span>16<span class="cite-bracket">]</span></a></sup></li> <li><a href="/wiki/NaviServer" title="NaviServer">NaviServer</a> – built-in, dynamic and static compression</li> <li><a href="/wiki/Caddy_(web_server)" title="Caddy (web server)">Caddy</a> – built-in via <a rel="nofollow" class="external text" href="https://caddyserver.com/docs/caddyfile/directives/encode">encode</a></li></ul> <p>Many <a href="/wiki/Content_delivery_network" title="Content delivery network">content delivery networks</a> also implement HTTP compression to improve speedy delivery of resources to end users. </p><p>The compression in HTTP can also be achieved by using the functionality of <a href="/wiki/Server-side_scripting" title="Server-side scripting">server-side scripting</a> languages like <a href="/wiki/PHP" title="PHP">PHP</a>, or programming languages like <a href="/wiki/Java_(programming_language)" title="Java (programming language)">Java</a>. </p><p>Various online tools exist to verify a working implementation of HTTP compression. These online tools usually request multiple variants of a URL, each with different request headers (with varying Accept-Encoding content). HTTP compression is considered to be implemented correctly when the server returns a document in a compressed format.<sup id="cite_ref-17" class="reference"><a href="#cite_note-17"><span class="cite-bracket">[</span>17<span class="cite-bracket">]</span></a></sup> By comparing the sizes of the returned documents, the effective compression ratio can be calculated (even between different compression algorithms). </p> <div class="mw-heading mw-heading2"><h2 id="Problems_preventing_the_use_of_HTTP_compression">Problems preventing the use of HTTP compression</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=HTTP_compression&action=edit&section=4" title="Edit section: Problems preventing the use of HTTP compression"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>A 2009 article by Google engineers Arvind Jain and Jason Glasgow states that more than 99 person-years are wasted<sup id="cite_ref-google-use-compression_18-0" class="reference"><a href="#cite_note-google-use-compression-18"><span class="cite-bracket">[</span>18<span class="cite-bracket">]</span></a></sup> daily due to increase in page load time when users do not receive compressed content. This occurs when anti-virus software interferes with connections to force them to be uncompressed, where proxies are used (with overcautious web browsers), where servers are misconfigured, and where browser bugs stop compression being used. Internet Explorer 6, which drops to HTTP 1.0 (without features like compression or pipelining) when behind a proxy – a common configuration in corporate environments – was the mainstream browser most prone to failing back to uncompressed HTTP.<sup id="cite_ref-google-use-compression_18-1" class="reference"><a href="#cite_note-google-use-compression-18"><span class="cite-bracket">[</span>18<span class="cite-bracket">]</span></a></sup> </p><p>Another problem found while deploying HTTP compression on large scale is due to the <b>deflate</b> encoding definition: while HTTP 1.1 defines the <b>deflate</b> encoding as data compressed with deflate (RFC 1951) inside a <a href="/wiki/Zlib" title="Zlib">zlib</a> formatted stream (RFC 1950), Microsoft server and client products historically implemented it as a "raw" deflated stream,<sup id="cite_ref-19" class="reference"><a href="#cite_note-19"><span class="cite-bracket">[</span>19<span class="cite-bracket">]</span></a></sup> making its deployment unreliable.<sup id="cite_ref-20" class="reference"><a href="#cite_note-20"><span class="cite-bracket">[</span>20<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-21" class="reference"><a href="#cite_note-21"><span class="cite-bracket">[</span>21<span class="cite-bracket">]</span></a></sup> For this reason, some software, including the Apache HTTP Server, only implement <b>gzip</b> encoding. </p> <div class="mw-heading mw-heading2"><h2 id="Security_implications">Security implications</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=HTTP_compression&action=edit&section=5" title="Edit section: Security implications"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1236090951">.mw-parser-output .hatnote{font-style:italic}.mw-parser-output div.hatnote{padding-left:1.6em;margin-bottom:0.5em}.mw-parser-output .hatnote i{font-style:normal}.mw-parser-output .hatnote+link+.hatnote{margin-top:-0.5em}@media print{body.ns-0 .mw-parser-output .hatnote{display:none!important}}</style><div role="note" class="hatnote navigation-not-searchable">Main articles: <a href="/wiki/CRIME" title="CRIME">CRIME</a> and <a href="/wiki/BREACH" title="BREACH">BREACH</a></div> <p>Compression allows a form of <a href="/wiki/Chosen_plaintext" class="mw-redirect" title="Chosen plaintext">chosen plaintext</a> attack to be performed: if an attacker can inject any chosen content into the page, they can know whether the page contains their given content by observing the size increase of the encrypted stream. If the increase is smaller than expected for random injections, it means that the compressor has found a repeat in the text, i.e. the injected content overlaps the secret information. This is the idea behind CRIME. </p><p>In 2012, a general attack against the use of data compression, called <a href="/wiki/CRIME" title="CRIME">CRIME</a>, was announced. While the CRIME attack could work effectively against a large number of protocols, including but not limited to TLS, and application-layer protocols such as SPDY or HTTP, only exploits against TLS and SPDY were demonstrated and largely mitigated in browsers and servers. The CRIME exploit against HTTP compression has not been mitigated at all, even though the authors of CRIME have warned that this vulnerability might be even more widespread than SPDY and TLS compression combined. </p><p>In 2013, a new instance of the CRIME attack against HTTP compression, dubbed BREACH, was published. A BREACH attack can extract login tokens, email addresses or other sensitive information from TLS encrypted web traffic in as little as 30 seconds (depending on the number of bytes to be extracted), provided the attacker tricks the victim into visiting a malicious web link.<sup id="cite_ref-Gooin20130801_22-0" class="reference"><a href="#cite_note-Gooin20130801-22"><span class="cite-bracket">[</span>22<span class="cite-bracket">]</span></a></sup> All versions of TLS and SSL are at risk from BREACH regardless of the encryption algorithm or cipher used.<sup id="cite_ref-23" class="reference"><a href="#cite_note-23"><span class="cite-bracket">[</span>23<span class="cite-bracket">]</span></a></sup> Unlike previous instances of <a href="/wiki/CRIME_(security_exploit)" class="mw-redirect" title="CRIME (security exploit)">CRIME</a>, which can be successfully defended against by turning off TLS compression or SPDY header compression, BREACH exploits HTTP compression which cannot realistically be turned off, as virtually all web servers rely upon it to improve data transmission speeds for users.<sup id="cite_ref-Gooin20130801_22-1" class="reference"><a href="#cite_note-Gooin20130801-22"><span class="cite-bracket">[</span>22<span class="cite-bracket">]</span></a></sup> </p><p>As of 2016, the TIME attack and the HEIST attack are now public knowledge.<sup id="cite_ref-24" class="reference"><a href="#cite_note-24"><span class="cite-bracket">[</span>24<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-25" class="reference"><a href="#cite_note-25"><span class="cite-bracket">[</span>25<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-26" class="reference"><a href="#cite_note-26"><span class="cite-bracket">[</span>26<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-27" class="reference"><a href="#cite_note-27"><span class="cite-bracket">[</span>27<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="References">References</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=HTTP_compression&action=edit&section=6" title="Edit section: References"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1239543626">.mw-parser-output .reflist{margin-bottom:0.5em;list-style-type:decimal}@media screen{.mw-parser-output .reflist{font-size:90%}}.mw-parser-output .reflist .references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output .reflist-columns-3{column-width:25em}.mw-parser-output .reflist-columns{margin-top:0.3em}.mw-parser-output .reflist-columns ol{margin-top:0}.mw-parser-output .reflist-columns li{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .reflist-upper-alpha{list-style-type:upper-alpha}.mw-parser-output .reflist-upper-roman{list-style-type:upper-roman}.mw-parser-output .reflist-lower-alpha{list-style-type:lower-alpha}.mw-parser-output .reflist-lower-greek{list-style-type:lower-greek}.mw-parser-output .reflist-lower-roman{list-style-type:lower-roman}</style><div class="reflist"> <div class="mw-references-wrap mw-references-columns"><ol class="references"> <li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://www.microsoft.com/technet/prodtechnol/WindowsServer2003/Library/IIS/d52ff289-94d3-4085-bc4e-24eb4f312e0e.mspx?mfr=true">"Using HTTP Compression (IIS 6.0)"</a>. 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