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Web applciations will often server static content from the file system provided by the operating system running underneatth the JVM. However because file systems often implement multiple aliased names for the same file, then security constraints and other servlet URI space mappings my inadvertantly be bypassed by aliases.
I key example of this is case insensitivety and 8.3 names implemented
by the Windows File system. If a file within a webapplication called
/mysecretfile.txt is protected by a security constraint on the
/mysecretfile.txt, then a request to
/MySecretFile.TXT will not match the URI constraint because
URIs are case sensitive, but the windows file system will report that a file
does exist at that name and it will be served despite the security
constraint. Less well known than case insensitivity is that windows files
systems also support 8.3 filenames for
compatibility with legacy programs. Thus a request to a URI like
/MYSECR~1.TXT will again not match the security constraint, but
will be reported as an existing file by the file system and served.
There are many examples of aliases, not just on windows:
NTFS Alternate stream names like c:\test\file.txt::$DATA:name
OpenVMS support file versionig so that
/mysecret.txt;N refers to version N of
/mysecret.txt and is essentially an alias.
The clearcase software configuration management system provides a file system where @@ in a file name is an alias to a specific version.
The unix files system supports
/./foo.txt as and
Many JVM implementations incorrectly assume the null character is
a string terminator, so that a file name resulting from
/foobar.txt%00 is an alias for
Unix symbolic links and hard links are a form of aliases that allow the same file or directory to have multiple names.
In addition, it is not just URI security constraints that can be
bypassed. For example the mapping of the URI pattern
the JSP Servlet may be bypassed by an a request to an alias like
/foobar.jsp%00, thus rather than execute the JSP, the source code of
the JSP is returned by the file system.
Part of the problem with aliases is that the standard web application security model is to allow all requests except the ones that are specifically denied by security constraints. A best practise for security is to deny all requests and to permit only those that are specifically identified as allowable. While it is possible to design web application security constraints in this style, it can be difficult in all circumstances and it is not the default. Thus it is important for Jetty to be able to detect and deny requests to aliased static content.
It is impossible for Jetty to know of all the aliases that may be
implemented by the file system running beneath it, thus it does not
attempt to make any specific checks for any know aliases. Instead jetty
detects aliases by using the canonical path of a file. If a file resource
handled by jetty has a canonical name that differs from the name used to
request the resource, then Jetty determines that the resource is an
aliased request and it will not be returned by the
ServletContext.getResource(String) method (or similar) and
thus will not be served as static content nor used as the basis of a
This if Jetty is running on a windows operation system, then a file
/MySecret.TXT will have a canonical name that exactly
matches that case. So while a request to
/MYSECR~1.TXT will result in a File Resource that matches
the file, the different canonical name will indicate that those requests
are aliases and they will not be served as static content and instead a
404 response returned.
Unfortunately this approach denies all aliases, including symbolic links, which can be useful in assembling complex web applications.
Not all aliases are bad nor should be seen as attempts to subvert security constraints. Specifically symbolic links can be very useful when assembling complex web applications, yet by default Jetty will not serve them. Thus Jetty contexts support an extensible AliasCheck mechanism to allow aliases resources to be inspected an conditionally served. In this way, "good" aliases can be detected and served. Jetty provides several utility implementations of the AliasCheck interface as nested classes with ContextHandler:
Approve all aliases (USE WITH CAUTION!).
Approve Aliases using the java-7 Files.readSymbolicLink(path) and Path.toRealPath(...) APIs to check that alias are valid symbolic links.
An application is free to implement its own Alias checking. Alias
Checkers can be installed in a context via the following XML used in a
context deployer file or