Network Working Group R. Fielding Request for Comments: 2616 ...

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designers and implementors be particularly careful in this area.History shows that errors in this area often create serious securityand/or privacy problems and generate highly adverse publicity for theimplementor's company.15.1.1 Abuse of Server Log InformationA server is in the position to save personal data about a user'srequests which might identify their reading patterns or subjects ofinterest. This information is clearly confidential in nature and itshandling can be constrained by law in certain countries. People usingthe HTTP protocol to provide data are responsible for ensuring thatsuch material is not distributed without the permission of anyindividuals that are identifiable by the published results.15.1.2 Transfer of Sensitive InformationLike any generic data transfer protocol, HTTP cannot regulate thecontent of the data that is transferred, nor is there any a priorimethod of determining the sensitivity of any particular piece ofinformation within the context of any given request. Therefore,applications SHOULD supply as much control over this information aspossible to the provider of that information. Four header fields areworth special mention in this context: Server, Via, Referer and From.Revealing the specific software version of the server might allow theserver machine to become more vulnerable to attacks against softwarethat is known to contain security holes. Implementors SHOULD make theServer header field a configurable option.Proxies which serve as a portal through a network firewall SHOULDtake special precautions regarding the transfer of header informationthat identifies the hosts behind the firewall. In particular, theySHOULD remove, or replace with sanitized versions, any Via fieldsgenerated behind the firewall.The Referer header allows reading patterns to be studied and reverselinks drawn. Although it can be very useful, its power can be abusedif user details are not separated from the information contained inFielding, et al. Standards Track [Page 151]RFC 2616 HTTP/1.1 June 1999the Referer. Even when the personal information has been removed, theReferer header might indicate a private document's URI whosepublication would be inappropriate.
The information sent in the From field might conflict with the user'sprivacy interests or their site's security policy, and hence itSHOULD NOT be transmitted without the user being able to disable,enable, and modify the contents of the field. The user MUST be ableto set the contents of this field within a user preference orapplication defaults configuration.We suggest, though do not require, that a convenient toggle interfacebe provided for the user to enable or disable the sending of From andReferer information.The User-Agent (section 14.43) or Server (section 14.38) headerfields can sometimes be used to determine that a specific client orserver have a particular security hole which might be exploited.Unfortunately, this same information is often used for other valuablepurposes for which HTTP currently has no better mechanism.15.1.3 Encoding Sensitive Information in URI'sBecause the source of a link might be private information or mightreveal an otherwise private information source, it is stronglyrecommended that the user be able to select whether or not theReferer field is sent. For example, a browser client could have atoggle switch for browsing openly/anonymously, which wouldrespectively enable/disable the sending of Referer and Frominformation.Clients SHOULD NOT include a Referer header field in a (non-secure)HTTP request if the referring page was transferred with a secureprotocol.Authors of services which use the HTTP protocol SHOULD NOT use GETbased forms for the submission of sensitive data, because this willcause this data to be encoded in the Request-URI. Many existingservers, proxies, and user agents will log the request URI in someplace where it might be visible to third parties. Servers can usePOST-based form submission instead15.1.4 Privacy Issues Connected to Accept HeadersAccept request-headers can reveal information about the user to allservers which are accessed. The Accept-Language header in particularcan reveal information the user would consider to be of a privatenature, because the understanding of particular languages is oftenFielding, et al. Standards Track [Page 152]RFC 2616 HTTP/1.1 June 1999
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eferred to as "HTTP/1.1", and is an
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12.1 Server-driven Negotiation ....
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14.40 Trailer .....................
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1.2 RequirementsThe key words "MUST
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Fielding, et al. Standards Track [P
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equest chain ----------------------
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Elements separated by a bar ("|") a
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HTTP/1.1 header field values can be
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An application that sends a request
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3.2.3 URI ComparisonWhen comparing
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is not desirable and is discouraged
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CRLFchunk = chunk-size [ chunk-exte
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When in canonical form, media subty
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3.9 Quality ValuesHTTP content nego
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RFC 2616 HTTP/1.1 June 1999HTTP/1.1
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message-body = entity-body| Transfe
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in advance.Messages MUST NOT includ
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equested resource, and 501 (Not Imp
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names) MUST use the following rules
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values for the first digit:- 1xx: I
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how the transfer-length of a messag
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connection-token close.If either th
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idempotent (see section 9.1.2). Non
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compatibility with RFC 2068, a serv
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If at any point an error status is
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optional features implemented by th
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- Extending a database through an a
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intervention (or other means) on th
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10.1.2 101 Switching ProtocolsThe s
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esponse MAY include new or updated
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The response MUST include the follo
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If the client is sending data, a se
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If the response could be unacceptab
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10.4.15 414 Request-URI Too LongThe
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10.5.5 504 Gateway TimeoutThe serve
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Caching would be useless if it did
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Warning (but without removing any e
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If the user has overridden the basi
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in the past. This means that the re
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RFC 2616 HTTP/1.1 June 1999the time
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simple:response_is_fresh = (freshne
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match.In HTTP/1.1, a conditional re
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validators or strong validators. Cl
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- MAY send a weak entity tag instea
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Unless specifically constrained by
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warn-code 1xx, which are deleted ev
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for the associated entity, its enti
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- DELETE- POSTIn order to prevent d
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The Accept request-header field can
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image/jpeg = 0.5text/html;level=2 =
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Note: If the request does not inclu
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Page 132 and 133: 14.14 Content-LocationThe Content-L
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Page 154 and 155: Content) instead of 200 (OK).- The
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Page 182 and 183: Notes:1) Additional CRLFs may prece
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Page 190 and 191: CREATE had a race that required an
Page 192 and 193: also needed to be a general header,
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