MS RFC 131: Allow file paths in MapCache second level dimension values¶

Date: 2020-05-20
Author: Jérome Boué
Contact: jbo-ads@mailo.com
Status: Adopted
Last update: 2020-08-06
Version: MapCache 1.12

Overview¶

Dimension values in MapCache are mainly used as a means to identify caches where to store or fetch tiles, by using a templating mechanism: “{dim}” or “{dim:dimname}” (see Storing Dimensions and Using Multiple SQLite Database Files). In this pattern, dimension values are portions of file names or directory names.

Currently, file paths (portions of directory hierarchy, i.e. containing / or . characters) are forbidden as dimension values in MapCache. This prevents a malicious client from potentially exploring server’s filesystem.

However, being able to organize caches in an arbitrary filesystem hierarchy would be a great feature from a geographic imagery provider viewpoint.

In order to increase flexibility on server side, this RFC proposes to allow a limited use of file paths in dimension values without compromising server’s security.

Incidentally, the need at origin of this RFC arose on SQLite caches. Therefore, this feature is proposed on that cache type only.

Proposed solution¶

From preliminary discussions on [mapserver-dev] mailing list, two main principles have emerged:

Don’t allow users to directly supply a path; instead provide a keyword-to-path mapping.
Allow only paths relative to a root-dimension-path, so that only a controlled subset of the filesystem can be accessed; forbid “../” in dimension paths.

First principle: keyword-to-path mapping¶

Indeed this feature is already provided in MapCache: this is the so-called Second Level Dimensions mechanism.

The proposed solution should therefore allow paths only with second level dimensions. This shall be implemented by a specific check on dimension type when fetching dimension value.

On dimensions that don’t allow paths in dimension values, the current behaviour applies: replace ‘.’ and ‘/’ characters by ‘#’ character.

Second principle: restrict dimension paths to a subset of filesystem¶

The root-dimension-path mechanism is already provided in MapCache: templates apply on a hard-coded path specified in cache configuration.

Moreover, in order to make clear that a specific cache supports paths as dimension values, an attribute is introduced at cache configuration level.

<cache type="sqlite"
       allow_path_in_dim="yes"
       name="foo">
    <!-- ... -->
    <dbfile>/path/to/{grid}/{dim}/{tileset}/{z}/{x}-{y}.sqlite3</dbfile>
 </cache>

In addition, the proposed solution shall implement a “../” sanitization by replacing that string by a safe character such as ‘#’.

On caches that don’t explicitely allow paths in dimension values, the current behaviour applies: replace ‘.’ and ‘/’ characters by ‘#’ character.

Documentation update¶

Subsection of Using Multiple SQLite Database Files shall be modified.

Implementation details¶

Only lib/cache_sqlite.c is impacted.

Backward compatibility¶

As this is a new feature, no backward compatibility is expected.

Voting History¶

Adopted with +1 from JeffM, SethG, TomK, MikeS, EvenR, DanielM, SteveL, JeromeB

DOCS PREVIEW