SPARQL queries using FROM, FROM NAMED, or GRAPH may cause the specified URI to
be dereferenced. This may cause additional use of network, disk or CPU resources
along with associated secondary issues such as denial of service. The security
issues of Uniform Resource Identifier (URI): Generic Syntax [RFC3986] Section 7
should be considered. In addition, the contents of file: URIs can in some cases
be accessed, processed and returned as results, providing unintended access to
local resources.

SPARQL requests may cause additional requests to be issued from the SPARQL
endpoint, such as FROM NAMED. The endpoint is potentially within an
organisations firewall or DMZ, and so such queries may be a source of
indirection attacks.

The SPARQL language permits extensions, which will have their own security
implications.

SPARQL queries using SERVICE imply that a URI will be dereferenced, and that the
result will be incorporated into a working data set.

Since a SPARQL protocol service may make HTTP requests of other origin servers
on behalf of its clients, it may be used as a vector of attacks against other
sites or services. Thus, SPARQL protocol services may effectively act as proxies
for third-party clients. Such services may place restrictions on the resources
that they retrieve or on the rate at which external resources can be retrieved.
SPARQL protocol services may log client requests in such a way as to facilitate
tracing them with regard to third-party origin servers or services.

SPARQL protocol services may choose to detect these and other costly, or
otherwise unsafe, queries, impose time or memory limits on queries, or impose
other restrictions to reduce the service's (and other service's) vulnerability
to denial-of-service attacks. They also may refuse to process such query
requests.

There are at least two possible sources of denial-of-service attacks against
SPARQL protocol services. First, under-constrained queries can result in very
large numbers of results, which may require large expenditures of computing
resources to process, assemble, or return. Another possible source are queries
containing very complex — either because of resource size, the number of
resources to be retrieved, or a combination of size and number — RDF Dataset
descriptions, which the service may be unable to assemble without significant
expenditure of resources, including bandwidth, CPU, or secondary storage. In
some cases such expenditures may effectively constitute a denial-of-service
attack. A SPARQL protocol service may place restrictions on the resources that
it retrieves or on the rate at which external resources are retrieved. There
may be other sources of denial-of-service attacks against SPARQL query
processing services.

Write access to data makes it inherently vulnerable to malicious access.
Standard access and authentication techniques should be used in any networked
environment. In particular, HTTPS should be used, especially when implementing
the SPARQL HTTP-based protocols. (i.e., encryption with challenge/response based
password presentation, encrypted session tokens, etc). Some of the weak points
addressed by HTTPS are: authentication, active session integrity between client
and server, preventing replays, preventing continuation of defunct sessions.

SPARQL protocol services may remove, insert, and change underlying data via the
update operation. To protect against malicious or destructive updates,
implementations may choose not to implement the update operation. Alternatively,
implementations may choose to use HTTP authentication mechanisms or other
implementation-defined mechanisms to prevent unauthorized invocations of the
update operation.

Systems that provide both read-only and writable interfaces can be subject to
injection attacks in the read-only interface. In particular, a SPARQL endpoint
with a Query service should be careful of injection attacks aimed at interacting
with an Update service on the same SPARQL endpoint. Like any client code,
interaction between the query service and the update service should ensure
correct escaping of strings provided by the user.

While SPARQL Update and SPARQL Query are separate languages, some
implementations may choose to offer both at the same SPARQL endpoint. In this
case, it is important to consider that an Update operation may be obscured to
masquerade as a query. For instance, a string of unicode escapes in a PREFIX
clause could be used to hide an Update Operation. Therefore, simple syntactic
tests are inadequate to determine if a string describes a query or an update.

Different IRIs may have the same appearance. Characters in different scripts
may look similar (a Cyrillic "о" may appear similar to a Latin "o"). A
character followed by combining characters may have the same visual
representation as another character (LATIN SMALL LETTER E followed by
COMBINING ACUTE ACCENT has the same visual representation as LATIN SMALL
LETTER E WITH ACUTE). Users of SPARQL must take care to construct queries
with IRIs that match the IRIs in the data. Further information about matching
of similar characters can be found in Unicode Security Considerations
[UNISEC] and Internationalized Resource Identifiers (IRIs) [RFC3987]
Section 8.