#### Security considerations

The SPARQL 1.1 specifications have a number of informative `Security considerations` sections. This is an informative document describing how those may or may not apply to the implementation of Tracker.

Note that most of these considerations derive from situations where a RDF triple store is exposed through a public endpoint, while Tracker does not do that by default. Users should be careful about creating endpoints. For D-Bus endpoints, access through the portal is encouraged.

# SPARQL specifications

## Queries

(From https://www.w3.org/TR/2013/REC-sparql11-query-20130321/#security)

    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.

Graph URIs are virtual in Tracker and do not cause any access outside of database resources. The only SPARQL syntax capable of dereferencing or accessing external resources are the `SERVICE <uri>` and `LOAD <rdf-file>` features.

(From https://www.w3.org/TR/2013/REC-sparql11-query-20130321/#security)

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

Tracker SPARQL extensions have no special security considerations, other than being code that runs on silicon.

## Federated queries

(From https://www.w3.org/TR/2013/REC-sparql11-federated-query-20130321/#security)

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

(From https://www.w3.org/TR/sparql11-protocol/#policy-security)

    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.

Tracker offers 2 types of endpoint that are susceptible to this vector:

- D-Bus endpoints accessed outside a sandbox.
- HTTP endpoints

Particularly, requests on a D-Bus endpoint happening through the portal from a sandboxed process have all SERVICE access restricted.

Tracker developers encourage that all access to endpoints created on D-Bus happen through the portal, and that all HTTP endpoints validate the provenance of the requests through the `TrackerEndpointHttp::block-remote-address` signal to limit access to resources.

(From https://www.w3.org/TR/sparql11-protocol/#policy-security)

    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.

Tracker does not perform any time or frequency rate limits to queries. HTTP endpoints may perform the latter through the `TrackerEndpointHttp::block-remote-address` signal.

## Updates

(From https://www.w3.org/TR/2013/REC-sparql11-update-20130321/#security)

    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.

(From https://www.w3.org/TR/sparql11-protocol/#policy-security)

    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.

Tracker HTTP endpoints do not implement any update mechanisms. D-Bus endpoints accessed through the portal from inside a sandbox are likewise read-only.

(From https://www.w3.org/TR/2013/REC-sparql11-update-20130321/#security)

    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.

Following the SPARQL 1.1 spec, Tracker implements updates and queries as two different languages with different parser entry points, this separation happens all the way to the public API. As an additional layer of security, readonly queries happen on readonly database connections. It is essentially not possible to perform any data change from the query APIs.

## IRIs

IRIs are a cornerstone of RDF data, since individual RDF resources are typically named through them. Quoting https://www.w3.org/TR/sparql11-protocol/#policy-security:

    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.

The situations where this might be a source of confusion or mischief, or even be possible depends on how those IRIs are created, used, displayed or inserted.

# API user considerations

Users of the Tracker API and SPARQL interface are encouraged to make some considerations and take some precautions:

- Do not expose any endpoints that does not need exposing.
- For local D-Bus endpoints, consider using a graph partitioning scheme that makes it easy to policy the access to the data when accessed through the portal.
- Avoid the possibility of injection attacks. Use `TrackerSparqlStatement` and avoid string-based approaches to build SPARQL queries from user input.
- Consider that IRIs describing RDF resources are susceptible to homograph attacks as described above. Developers are not exempt of validating external input that might end up stored as-is.

# Feature grid

This is a quick reference of the features offered by the different types of endpoint.

Endpoint | Query | Update | Graph Constraints | Service Constraints
--- | --- | --- | --- | ---
D-Bus (portal) | ✓ | ✗ | ✓ | ✓
D-Bus | ✓ | ✓ | ✗ | ✗
HTTP | ✓ | ✗ | ✗ | ✗