What is the semicolon reserved for in URLs? - url

Question

The RFC 3986 URI: Generic Syntax specification lists a semicolon as a reserved (sub-delim) character:
reserved = gen-delims / sub-delims
gen-delims = ":" / "/" / "?" / "#" / "[" / "]" / "#"
sub-delims = "!" / "$" / "&" / "'" / "(" / ")"
/ "*" / "+" / "," / ";" / "="
What is the reserved purpose of the ";" of the semicolon in URIs? For that matter, what is the purpose of the other sub-delims (I'm only aware of purposes for "&", "+", and "=")?

Answer 1

There is an explanation at the end of section 3.3.
Aside from dot-segments in
hierarchical paths, a path segment is
considered opaque by the generic
syntax. URI producing applications
often use the reserved characters
allowed in a segment to delimit
scheme-specific or
dereference-handler-specific
subcomponents. For example, the
semicolon (";") and equals ("=")
reserved characters are often used
to delimit parameters and parameter
values applicable to that segment.
The comma (",") reserved character is
often used forsimilar purposes.
For example, one URI producer might
use a segment uch as "name;v=1.1"
to indicate a reference to version 1.1
of "name", whereas another might
use a segment such as "name,1.1" to
indicate the same. Parameter types
may be defined by scheme-specific
semantics, but in most cases the
syntax of a parameter is specific to
the implementation of the URI's
dereferencing algorithm.
In other words, it is reserved so that people who want a delimited list of something in the URL can safely use ; as a delimiter even if the parts contain ;, as long as the contents are percent-encoded. In other words, you can do this:
foo;bar;baz%3bqux
and interpret it as three parts: foo, bar, baz;qux. If semicolon were not a reserved character, the ; and %3bwould be equivalent, so the URI would be incorrectly interpreted as four parts: foo, bar, baz, qux.

Answer 2

The intent is clearer if you go back to older versions of the specification:
path_segments = segment *( "/" segment )
segment = *pchar *( ";" param )
Each path segment may include a
sequence of parameters, indicated by the semicolon ";" character.
I believe it has its origins in FTP URIs.

Answer 3

Section 3.3 covers this - it's an opaque delimiter a URI-producing application can use if convenient:
Aside from dot-segments in
hierarchical paths, a path segment is
considered opaque by the generic
syntax. URI producing applications
often use the reserved characters
allowed in a segment to delimit
scheme-specific or
dereference-handler-specific
subcomponents. For example, the
semicolon (";") and equals ("=")
reserved characters are often used to
delimit parameters and parameter
values applicable to that segment. The
comma (",") reserved character is
often used for similar purposes. For
example, one URI producer might use a
segment such as "name;v=1.1" to
indicate a reference to version 1.1 of
"name", whereas another might use a
segment such as "name,1.1" to indicate
the same. Parameter types may be
defined by scheme-specific semantics,
but in most cases the syntax of a
parameter is specific to the
implementation of the URI's
dereferencing algorithm.

Answer 4

There are some conventions around its current usage that are interesting. These speak to when to use a semicolon or comma. From the book "RESTful Web Services":
Use punctuation characters to separate multiple pieces of data at the same level of hierarchy. Use commas when the order of the items matters, ... Use semicolons when the order doesn't matter.

Answer 5

Since 2014, path segments are known to contribute to Reflected File Download attacks. Let's assume we have a vulnerable API that reflects whatever we send to it:
https://google.com/s?q=rfd%22||calc||
{"results":["q", "rfd\"||calc||","I love rfd"]}
Now, this is harmless in a browser as it's JSON, so it's not going to be rendered, but the browser will rather offer to download the response as a file. Now here's the path segments come to help (for the attacker):
https://google.com/s;/setup.bat;?q=rfd%22||calc||
Everything between semicolons (;/setup.bat;) will be not sent to the web service, but instead the browser will interpret it as the file name... to save the API response.
Now, a file called setup.bat will be downloaded and run without asking about dangers of running files downloaded from the Internet (because it contains the word "setup" in its name). The contents will be interpreted as a Windows batch file, and the calc.exe command will be run.
Prevention:
sanitize your API's input (in this case, they should just allow alphanumerics); escaping is not sufficient
add Content-Disposition: attachment; filename="whatever.txt" on APIs that are not going to be rendered; Google was missing the filename part which actually made the attack easier
add X-Content-Type-Options: nosniff header to API responses

Answer 6

I found the following use cases:
It's the final character of an HTML entity:
List of XML and HTML character entity references
To use one of these character entity references in an HTML or XML
document, enter an ampersand followed by the entity name and a
semicolon, e.g., & for the ampersand ("&").
Apache Tomcat 7 (or newer versions?!) us it as path parameter:
Three Semicolon Vulnerabilities
Apache Tomcat is one example of a web server that supports "Path
Parameters". A path parameter is extra content after a file name,
separated by a semicolon. Any arbitrary content after a semicolon does
not affect the landing page of a web browser. This means that
http://example.com/index.jsp;derp will still return index.jsp, and not
some error page.
URI scheme splits by it the MIME and data:
Data URI scheme
It can contain an optional character set parameter, separated from the
preceding part by a semicolon (;) .
<img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUA
AAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO
9TXL0Y4OHwAAAABJRU5ErkJggg==" alt="Red dot" />
And there was a bug in IIS 5 and IIS 6 to bypass file upload restrictions:
Unrestricted File Upload
Blacklisting File Extensions This protection might be bypassed by: ...
by adding a semi-colon character after the forbidden extension and
before the permitted one (e.g. "file.asp;.jpg")
Conclusion:
Do not use semicolons in URLs or they could accidentally produce an HTML entity or URI scheme.