The Hypertext Transfer Protocol (HTTP) is a stateless application-level request/response protocol that uses extensible semantics and self-descriptive messages for flexible interaction with network-based hypertext information systems. It is typically used for distributed information systems, where the use of response caches can improve performance. This document defines aspects of HTTP related to caching and reusing response messages.
An HTTP
cache
is a local store of response messages and the subsystem that controls storage, retrieval, and deletion of messages in it. A cache stores cacheable responses to reduce the response time and network bandwidth consumption on future equivalent requests. Any client or server
MAY
use a cache, though not when acting as a tunnel (
Section 3.7
of
[HTTP]
).
A
shared cache
is a cache that stores responses for reuse by more than one user; shared caches are usually (but not always) deployed as a part of an intermediary. A
private cache
, in contrast, is dedicated to a single user; often, they are deployed as a component of a user agent.
The goal of HTTP caching is significantly improving performance by reusing a prior response message to satisfy a current request. A cache considers a stored response "fresh", as defined in
Section 4.2
, if it can be reused without "validation" (checking with the origin server to see if the cached response remains valid for this request). A fresh response can therefore reduce both latency and network overhead each time the cache reuses it. When a cached response is not fresh, it might still be reusable if validation can freshen it (
Section 4.3
) or if the origin is unavailable (
Section 4.2.4
).
The key words "
MUST
", "
MUST NOT
", "
REQUIRED
", "
SHALL
", "
SHALL NOT
", "
SHOULD
", "
SHOULD NOT
", "
RECOMMENDED
", "
NOT RECOMMENDED
", "
MAY
", and "
OPTIONAL
" in this document are to be interpreted as described in BCP 14
[RFC2119]
[RFC8174]
when, and only when, they appear in all capitals, as shown here.
Section 2
of
[HTTP]
defines conformance criteria and contains considerations regarding error handling.
This specification uses the Augmented Backus-Naur Form (ABNF) notation of
[RFC5234]
, extended with the notation for case-sensitivity in strings defined in
[RFC7405]
.
It also uses a list extension, defined in
Section 5.6.1
of
[HTTP]
, that allows for compact definition of comma-separated lists using a "#" operator (similar to how the "*" operator indicates repetition).
Appendix A
shows the collected grammar with all list operators expanded to standard ABNF notation.
The following core rule is included by reference, as defined in
[RFC5234]
,
Appendix B.1
: DIGIT (decimal 0-9).
[HTTP]
defines the following rules:
HTTP-date
= <HTTP-date, see
[HTTP]
,
Section 5.6.7
>
OWS
= <OWS, see
[HTTP]
,
Section 5.6.3
>
field-name
= <field-name, see
[HTTP]
,
Section 5.1
>
quoted-string
= <quoted-string, see
[HTTP]
,
Section 5.6.4
>
token
= <token, see
[HTTP]
,
Section 5.6.2
>
The delta-seconds rule specifies a non-negative integer, representing time in seconds.
A recipient parsing a delta-seconds value and converting it to binary form ought to use an arithmetic type of at least 31 bits of non-negative integer range. If a cache receives a delta-seconds value greater than the greatest integer it can represent, or if any of its subsequent calculations overflows, the cache
MUST
consider the value to be 2147483648 (2
31
) or the greatest positive integer it can conveniently represent.
Proper cache operation preserves the semantics of HTTP transfers while reducing the transmission of information already held in the cache. See
Section 3
of
[HTTP]
for the general terminology and core concepts of HTTP.
Although caching is an entirely
OPTIONAL
feature of HTTP, it can be assumed that reusing a cached response is desirable and that such reuse is the default behavior when no requirement or local configuration prevents it. Therefore, HTTP cache requirements are focused on preventing a cache from either storing a non-reusable response or reusing a stored response inappropriately, rather than mandating that caches always store and reuse particular responses.
The
cache key
is the information a cache uses to choose a response and is composed from, at a minimum, the request method and target URI used to retrieve the stored response; the method determines under which circumstances that response can be used to satisfy a subsequent request. However, many HTTP caches in common use today only cache GET responses and therefore only use the URI as the cache key.
A cache might store multiple responses for a request target that is subject to content negotiation. Caches differentiate these responses by incorporating some of the original request's header fields into the cache key as well, using information in the
Vary
response header field, as per
Section 4.1
.
Caches might incorporate additional material into the cache key. For example, user agent caches might include the referring site's identity, thereby "double keying" the cache to avoid some privacy risks (see
Section 7.2
).
Most commonly, caches store the successful result of a retrieval request: i.e., a
200 (OK)
response to a GET request, which contains a representation of the target resource (
Section 9.3.1
of
[HTTP]
). However, it is also possible to store redirects, negative results (e.g.,
404 (Not Found)
), incomplete results (e.g.,
206 (Partial Content)
), and responses to methods other than GET if the method's definition allows such caching and defines something suitable for use as a cache key.
A cache is
disconnected
when it cannot contact the origin server or otherwise find a forward path for a request. A disconnected cache can serve stale responses in some circumstances (
Section 4.2.4
).
When presented with a request, a cache
MUST NOT
reuse a stored response unless:
the presented target URI (
Section 7.1
of
[HTTP]
) and that of the stored response match, and
the request method associated with the stored response allows it to be used for the presented request, and
request header fields nominated by the stored response (if any) match those presented (see
Section 4.1
), and
the stored response does not contain the no-cache directive (
Section 5.2.2.4
), unless it is successfully validated (
Section 4.3
), and
the stored response is one of the following:
Note that a cache extension can override any of the requirements listed; see
Section 5.2.3
.
When a stored response is used to satisfy a request without validation, a cache
MUST
generate an
Age
header field (
Section 5.1
), replacing any present in the response with a value equal to the stored response's current_age; see
Section 4.2.3
.
A cache
MUST
write through requests with methods that are unsafe (
Section 9.2.1
of
[HTTP]
) to the origin server; i.e., a cache is not allowed to generate a reply to such a request before having forwarded the request and having received a corresponding response.
Also, note that unsafe requests might invalidate already-stored responses; see
Section 4.4
.
A cache can use a response that is stored or storable to satisfy multiple requests, provided that it is allowed to reuse that response for the requests in question. This enables a cache to
collapse requests
? or combine multiple incoming requests into a single forward request upon a cache miss ? thereby reducing load on the origin server and network. Note, however, that if the cache cannot use the returned response for some or all of the collapsed requests, it will need to forward the requests in order to satisfy them, potentially introducing additional latency.
When more than one suitable response is stored, a cache
MUST
use the most recent one (as determined by the
Date
header field). It can also forward the request with "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to use.
A cache without a clock (
Section 5.6.7
of
[HTTP]
)
MUST
revalidate stored responses upon every use.
When a cache receives a request that can be satisfied by a stored response and that stored response contains a
Vary
header field (
Section 12.5.5
of
[HTTP]
), the cache
MUST NOT
use that stored response without revalidation unless all the presented request header fields nominated by that Vary field value match those fields in the original request (i.e., the request that caused the cached response to be stored).
The header fields from two requests are defined to match if and only if those in the first request can be transformed to those in the second request by applying any of the following:
- adding or removing whitespace, where allowed in the header field's syntax
- combining multiple header field lines with the same field name (see
Section 5.2
of
[HTTP]
)
- normalizing both header field values in a way that is known to have identical semantics, according to the header field's specification (e.g., reordering field values when order is not significant; case-normalization, where values are defined to be case-insensitive)
If (after any normalization that might take place) a header field is absent from a request, it can only match another request if it is also absent there.
A stored response with a
Vary
header field value containing a member "*" always fails to match.
If multiple stored responses match, the cache will need to choose one to use. When a nominated request header field has a known mechanism for ranking preference (e.g., qvalues on
Accept
and similar request header fields), that mechanism
MAY
be used to choose a preferred response. If such a mechanism is not available, or leads to equally preferred responses, the most recent response (as determined by the
Date
header field) is chosen, as per
Section 4
.
Some resources mistakenly omit the Vary header field from their default response (i.e., the one sent when the request does not express any preferences), with the effect of choosing it for subsequent requests to that resource even when more preferable responses are available. When a cache has multiple stored responses for a target URI and one or more omits the Vary header field, the cache
SHOULD
choose the most recent (see
Section 4.2.3
) stored response with a valid Vary field value.
If no stored response matches, the cache cannot satisfy the presented request. Typically, the request is forwarded to the origin server, potentially with preconditions added to describe what responses the cache has already stored (
Section 4.3
).
A
fresh
response is one whose age has not yet exceeded its freshness lifetime. Conversely, a
stale
response is one where it has.
A response's
freshness lifetime
is the length of time between its generation by the origin server and its expiration time. An
explicit expiration time
is the time at which the origin server intends that a stored response can no longer be used by a cache without further validation, whereas a
heuristic expiration time
is assigned by a cache when no explicit expiration time is available.
A response's
age
is the time that has passed since it was generated by, or successfully validated with, the origin server.
When a response is fresh, it can be used to satisfy subsequent requests without contacting the origin server, thereby improving efficiency.
The primary mechanism for determining freshness is for an origin server to provide an explicit expiration time in the future, using either the
Expires
header field (
Section 5.3
) or the max-age response directive (
Section 5.2.2.1
). Generally, origin servers will assign future explicit expiration times to responses in the belief that the representation is not likely to change in a semantically significant way before the expiration time is reached.
If an origin server wishes to force a cache to validate every request, it can assign an explicit expiration time in the past to indicate that the response is already stale. Compliant caches will normally validate a stale cached response before reusing it for subsequent requests (see
Section 4.2.4
).
Since origin servers do not always provide explicit expiration times, caches are also allowed to use a heuristic to determine an expiration time under certain circumstances (see
Section 4.2.2
).
The calculation to determine if a response is fresh is:
response_is_fresh = (freshness_lifetime > current_age)
Clients can send the max-age or min-fresh request directives (
Section 5.2.1
) to suggest limits on the freshness calculations for the corresponding response. However, caches are not required to honor them.
When calculating freshness, to avoid common problems in date parsing:
- Although all date formats are specified to be case-sensitive, a cache recipient
SHOULD
match the field value case-insensitively.
- If a cache recipient's internal implementation of time has less resolution than the value of an HTTP-date, the recipient
MUST
internally represent a parsed
Expires
date as the nearest time equal to or earlier than the received value.
- A cache recipient
MUST NOT
allow local time zones to influence the calculation or comparison of an age or expiration time.
- A cache recipient
SHOULD
consider a date with a zone abbreviation other than "GMT" to be invalid for calculating expiration.
Note that freshness applies only to cache operation; it cannot be used to force a user agent to refresh its display or reload a resource. See
Section 6
for an explanation of the difference between caches and history mechanisms.
A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a response by evaluating the following rules and using the first match:
- If the cache is shared and the s-maxage response directive (
Section 5.2.2.10
) is present, use its value, or
- If the max-age response directive (
Section 5.2.2.1
) is present, use its value, or
- If the
Expires
response header field (
Section 5.3
) is present, use its value minus the value of the
Date
response header field (using the time the message was received if it is not present, as per
Section 6.6.1
of
[HTTP]
), or
- Otherwise, no explicit expiration time is present in the response. A heuristic freshness lifetime might be applicable; see
Section 4.2.2
.
Note that this calculation is intended to reduce clock skew by using the clock information provided by the origin server whenever possible.
When there is more than one value present for a given directive (e.g., two
Expires
header field lines or multiple Cache-Control: max-age directives), either the first occurrence should be used or the response should be considered stale. If directives conflict (e.g., both max-age and no-cache are present), the most restrictive directive should be honored. Caches are encouraged to consider responses that have invalid freshness information (e.g., a max-age directive with non-integer content) to be stale.
Since origin servers do not always provide explicit expiration times, a cache
MAY
assign a heuristic expiration time when an explicit time is not specified, employing algorithms that use other field values (such as the
Last-Modified
time) to estimate a plausible expiration time. This specification does not provide specific algorithms, but it does impose worst-case constraints on their results.
A cache
MUST NOT
use heuristics to determine freshness when an explicit expiration time is present in the stored response. Because of the requirements in
Section 3
, heuristics can only be used on responses without explicit freshness whose status codes are defined as
heuristically cacheable
(e.g., see
Section 15.1
of
[HTTP]
) and on responses without explicit freshness that have been marked as explicitly cacheable (e.g., with a public response directive).
Note that in previous specifications, heuristically cacheable response status codes were called "cacheable by default".
If the response has a
Last-Modified
header field (
Section 8.8.2
of
[HTTP]
), caches are encouraged to use a heuristic expiration value that is no more than some fraction of the interval since that time. A typical setting of this fraction might be 10%.
The
Age
header field is used to convey an estimated age of the response message when obtained from a cache. The Age field value is the cache's estimate of the number of seconds since the origin server generated or validated the response. The Age value is therefore the sum of the time that the response has been resident in each of the caches along the path from the origin server, plus the time it has been in transit along network paths.
Age calculation uses the following data:
- age_value
- The term "age_value" denotes the value of the
Age
header field (
Section 5.1
), in a form appropriate for arithmetic operation; or 0, if not available.
- date_value
- The term "date_value" denotes the value of the Date header field, in a form appropriate for arithmetic operations. See
Section 6.6.1
of
[HTTP]
for the definition of the Date header field and for requirements regarding responses without it.
- now
- The term "now" means the current value of this implementation's clock (
Section 5.6.7
of
[HTTP]
).
- request_time
- The value of the clock at the time of the request that resulted in the stored response.
- response_time
- The value of the clock at the time the response was received.
A response's age can be calculated in two entirely independent ways:
- the "apparent_age": response_time minus date_value, if the implementation's clock is reasonably well synchronized to the origin server's clock. If the result is negative, the result is replaced by zero.
- the "corrected_age_value", if all of the caches along the response path implement HTTP/1.1 or greater. A cache
MUST
interpret this value relative to the time the request was initiated, not the time that the response was received.
apparent_age = max(0, response_time - date_value);
response_delay = response_time - request_time;
corrected_age_value = age_value + response_delay;
The corrected_age_value
MAY
be used as the corrected_initial_age. In circumstances where very old cache implementations that might not correctly insert
Age
are present, corrected_initial_age can be calculated more conservatively as
corrected_initial_age = max(apparent_age, corrected_age_value);
The current_age of a stored response can then be calculated by adding the time (in seconds) since the stored response was last validated by the origin server to the corrected_initial_age.
resident_time = now - response_time;
current_age = corrected_initial_age + resident_time;
A "stale" response is one that either has explicit expiry information or is allowed to have heuristic expiry calculated, but is not fresh according to the calculations in
Section 4.2
.
A cache
MUST NOT
generate a stale response if it is prohibited by an explicit in-protocol directive (e.g., by a no-cache response directive, a must-revalidate response directive, or an applicable s-maxage or proxy-revalidate response directive; see
Section 5.2.2
).
A cache
MUST NOT
generate a stale response unless it is disconnected or doing so is explicitly permitted by the client or origin server (e.g., by the max-stale request directive in
Section 5.2.1
, extension directives such as those defined in
[RFC5861]
, or configuration in accordance with an out-of-band contract).
When a cache has one or more stored responses for a requested URI, but cannot serve any of them (e.g., because they are not fresh, or one cannot be chosen; see
Section 4.1
), it can use the conditional request mechanism (
Section 13
of
[HTTP]
) in the forwarded request to give the next inbound server an opportunity to choose a valid stored response to use, updating the stored metadata in the process, or to replace the stored response(s) with a new response. This process is known as
validating
or
revalidating
the stored response.
When generating a conditional request for validation, a cache either starts with a request it is attempting to satisfy or ? if it is initiating the request independently ? synthesizes a request using a stored response by copying the method, target URI, and request header fields identified by the Vary header field (
Section 4.1
).
It then updates that request with one or more precondition header fields. These contain validator metadata sourced from a stored response(s) that has the same URI. Typically, this will include only the stored response(s) that has the same cache key, although a cache is allowed to validate a response that it cannot choose with the request header fields it is sending (see
Section 4.1
).
The precondition header fields are then compared by recipients to determine whether any stored response is equivalent to a current representation of the resource.
Another validator is the entity tag given in an
ETag
field (
Section 8.8.3
of
[HTTP]
). One or more entity tags, indicating one or more stored responses, can be used in an
If-None-Match
header field for response validation, or in an
If-Match
or
If-Range
header field for representation selection (i.e., the client is referring specifically to one or more previously obtained representations with the listed entity tags).
When generating a conditional request for validation, a cache:
- MUST
send the relevant entity tags (using
If-Match
,
If-None-Match
, or
If-Range
) if the entity tags were provided in the stored response(s) being validated.
- SHOULD
send the
Last-Modified
value (using
If-Modified-Since
) if the request is not for a subrange, a single stored response is being validated, and that response contains a Last-Modified value.
- MAY
send the
Last-Modified
value (using
If-Unmodified-Since
or
If-Range
) if the request is for a subrange, a single stored response is being validated, and that response contains only a Last-Modified value (not an entity tag).
In most cases, both validators are generated in cache validation requests, even when entity tags are clearly superior, to allow old intermediaries that do not understand entity tag preconditions to respond appropriately.
Each client in the request chain may have its own cache, so it is common for a cache at an intermediary to receive conditional requests from other (outbound) caches. Likewise, some user agents make use of conditional requests to limit data transfers to recently modified representations or to complete the transfer of a partially retrieved representation.
If a cache receives a request that can be satisfied by reusing a stored
200 (OK)
or
206 (Partial Content)
response, as per
Section 4
, the cache
SHOULD
evaluate any applicable conditional header field preconditions received in that request with respect to the corresponding validators contained within the stored response.
A cache
MUST NOT
evaluate conditional header fields that only apply to an origin server, occur in a request with semantics that cannot be satisfied with a cached response, or occur in a request with a target resource for which it has no stored responses; such preconditions are likely intended for some other (inbound) server.
A request containing an
If-None-Match
header field (
Section 13.1.2
of
[HTTP]
) indicates that the client wants to validate one or more of its own stored responses in comparison to the stored response chosen by the cache (as per
Section 4
).
If a request contains an
If-Modified-Since
header field and the
Last-Modified
header field is not present in a stored response, a cache
SHOULD
use the stored response's
Date
field value (or, if no Date field is present, the time that the stored response was received) to evaluate the conditional.
When a cache decides to forward a request to revalidate its own stored responses for a request that contains an
If-None-Match
list of entity tags, the cache
MAY
combine the received list with a list of entity tags from its own stored set of responses (fresh or stale) and send the union of the two lists as a replacement
If-None-Match
header field value in the forwarded request. If a stored response contains only partial content, the cache
MUST NOT
include its entity tag in the union unless the request is for a range that would be fully satisfied by that partial stored response. If the response to the forwarded request is
304 (Not Modified)
and has an ETag field value with an entity tag that is not in the client's list, the cache
MUST
generate a
200 (OK)
response for the client by reusing its corresponding stored response, as updated by the 304 response metadata (
Section 4.3.4
).
Cache handling of a response to a conditional request depends upon its status code:
- A
304 (Not Modified)
response status code indicates that the stored response can be updated and reused; see
Section 4.3.4
.
- A full response (i.e., one containing content) indicates that none of the stored responses nominated in the conditional request are suitable. Instead, the cache
MUST
use the full response to satisfy the request. The cache
MAY
store such a full response, subject to its constraints (see
Section 3
).
- However, if a cache receives a
5xx (Server Error)
response while attempting to validate a response, it can either forward this response to the requesting client or act as if the server failed to respond. In the latter case, the cache can send a previously stored response, subject to its constraints on doing so (see
Section 4.2.4
), or retry the validation request.
When a cache receives a
304 (Not Modified)
response, it needs to identify stored responses that are suitable for updating with the new information provided, and then do so.
The initial set of stored responses to update are those that could have been chosen for that request ? i.e., those that meet the requirements in
Section 4
, except the last requirement to be fresh, able to be served stale, or just validated.
Then, that initial set of stored responses is further filtered by the first match of:
- If the new response contains one or more
strong validators
(see
Section 8.8.1
of
[HTTP]
), then each of those strong validators identifies a selected representation for update. All the stored responses in the initial set with one of those same strong validators are identified for update. If none of the initial set contains at least one of the same strong validators, then the cache
MUST NOT
use the new response to update any stored responses.
- If the new response contains no strong validators but does contain one or more
weak validators
, and those validators correspond to one of the initial set's stored responses, then the most recent of those matching stored responses is identified for update.
- If the new response does not include any form of validator (such as where a client generates an
If-Modified-Since
request from a source other than the
Last-Modified
response header field), and there is only one stored response in the initial set, and that stored response also lacks a validator, then that stored response is identified for update.
For each stored response identified, the cache
MUST
update its header fields with the header fields provided in the
304 (Not
Modified)
response, as per
Section 3.2
.
A response to the HEAD method is identical to what an equivalent request made with a GET would have been, without sending the content. This property of HEAD responses can be used to invalidate or update a cached GET response if the more efficient conditional GET request mechanism is not available (due to no validators being present in the stored response) or if transmission of the content is not desired even if it has changed.
When a cache makes an inbound HEAD request for a target URI and receives a
200 (OK)
response, the cache
SHOULD
update or invalidate each of its stored GET responses that could have been chosen for that request (see
Section 4.1
).
For each of the stored responses that could have been chosen, if the stored response and HEAD response have matching values for any received validator fields (
ETag
and
Last-Modified
) and, if the HEAD response has a
Content-Length
header field, the value of
Content-Length
matches that of the stored response, the cache
SHOULD
update the stored response as described below; otherwise, the cache
SHOULD
consider the stored response to be stale.
If a cache updates a stored response with the metadata provided in a HEAD response, the cache
MUST
use the header fields provided in the HEAD response to update the stored response (see
Section 3.2
).
Because unsafe request methods (
Section 9.2.1
of
[HTTP]
) such as PUT, POST, or DELETE have the potential for changing state on the origin server, intervening caches are required to invalidate stored responses to keep their contents up to date.
A cache
MUST
invalidate the target URI (
Section 7.1
of
[HTTP]
) when it receives a non-error status code in response to an unsafe request method (including methods whose safety is unknown).
A cache
MAY
invalidate other URIs when it receives a non-error status code in response to an unsafe request method (including methods whose safety is unknown). In particular, the URI(s) in the
Location
and
Content-Location
response header fields (if present) are candidates for invalidation; other URIs might be discovered through mechanisms not specified in this document. However, a cache
MUST NOT
trigger an invalidation under these conditions if the origin (
Section 4.3.1
of
[HTTP]
) of the URI to be invalidated differs from that of the target URI (
Section 7.1
of
[HTTP]
). This helps prevent denial-of-service attacks.
Invalidate
means that the cache will either remove all stored responses whose target URI matches the given URI or mark them as "invalid" and in need of a mandatory validation before they can be sent in response to a subsequent request.
Note that this does not guarantee that all appropriate responses are invalidated globally; a state-changing request would only invalidate responses in the caches it travels through.
Applications using HTTP often specify additional forms of caching. For example, Web browsers often have history mechanisms such as "Back" buttons that can be used to redisplay a representation retrieved earlier in a session.
Likewise, some Web browsers implement caching of images and other assets within a page view; they may or may not honor HTTP caching semantics.
The requirements in this specification do not necessarily apply to how applications use data after it is retrieved from an HTTP cache. For example, a history mechanism can display a previous representation even if it has expired, and an application can use cached data in other ways beyond its freshness lifetime.
This specification does not prohibit the application from taking HTTP caching into account; for example, a history mechanism might tell the user that a view is stale, or it might honor cache directives (e.g., Cache-Control: no-store).
However, when an application caches data and does not make this apparent to or easily controllable by the user, it is strongly encouraged to define its operation with respect to HTTP cache directives so as not to surprise authors who expect caching semantics to be honored. For example, while it might be reasonable to define an application cache "above" HTTP that allows a response containing Cache-Control: no-store to be reused for requests that are directly related to the request that fetched it (such as those created during the same page load), it would likely be surprising and confusing to users and authors if it were allowed to be reused for requests unrelated in any way to the one from which it was obtained.
This section is meant to inform developers, information providers, and users of known security concerns specific to HTTP caching. More general security considerations are addressed in "HTTP/1.1" (
Section 11
of
[HTTP/1.1]
) and "HTTP Semantics" (
Section 17
of
[HTTP]
).
Caches expose an additional attack surface because the contents of the cache represent an attractive target for malicious exploitation. Since cache contents persist after an HTTP request is complete, an attack on the cache can reveal information long after a user believes that the information has been removed from the network. Therefore, cache contents need to be protected as sensitive information.
In particular, because private caches are restricted to a single user, they can be used to reconstruct a user's activity. As a result, it is important for user agents to allow end users to control them, for example, by allowing stored responses to be removed for some or all origin servers.
Storing malicious content in a cache can extend the reach of an attacker to affect multiple users. Such "cache poisoning" attacks happen when an attacker uses implementation flaws, elevated privileges, or other techniques to insert a response into a cache. This is especially effective when shared caches are used to distribute malicious content to many clients.
One common attack vector for cache poisoning is to exploit differences in message parsing on proxies and in user agents; see
Section 6.3
of
[HTTP/1.1]
for the relevant requirements regarding HTTP/1.1.
Because one of the primary uses of a cache is to optimize performance, its use can "leak" information about which resources have been previously requested.
For example, if a user visits a site and their browser caches some of its responses and then navigates to a second site, that site can attempt to load responses it knows exist on the first site. If they load quickly, it can be assumed that the user has visited that site, or even a specific page on it.
Such "timing attacks" can be mitigated by adding more information to the cache key, such as the identity of the referring site (to prevent the attack described above). This is sometimes called "double keying".
Handling of duplicate and conflicting cache directives has been clarified. (
Section 4.2.1
)
Cache invalidation of the URIs in the Location and Content-Location header fields is no longer required but is still allowed. (
Section 4.4
)
Cache invalidation of the URIs in the Location and Content-Location header fields is disallowed when the origin is different; previously, it was the host. (
Section 4.4
)
Handling invalid and multiple Age header field values has been clarified. (
Section 5.1
)
Some cache directives defined by this specification now have stronger prohibitions against generating the quoted form of their values, since this has been found to create interoperability problems. Consumers of extension cache directives are no longer required to accept both token and quoted-string forms, but they still need to parse them properly for unknown extensions. (
Section 5.2
)
The public and private cache directives were clarified, so that they do not make responses reusable under any condition. (
Section 5.2.2
)
The must-understand cache directive was introduced; caches are no longer required to understand the semantics of new response status codes unless it is present. (
Section 5.2.2.3
)
The Warning response header was obsoleted. Much of the information supported by Warning could be gleaned by examining the response, and the remaining information ? although potentially useful ? was entirely advisory. In practice, Warning was not added by caches or intermediaries. (
Section 5.5
)