Essentially any API which make data and metadata easily confused should be viewed with suspicion.Dan Bernstein says something similar in his explanation of the design of qmail:
Looked at from Dan's point of view, the second argument of two-argument open is part of the user interface, a piece of unstructured data (a string) which Perl must parse into structured data (a record that lists the filename and the open mode). A program that wants to use open to open a certain filename with a certain mode must quote this information, turning it into a string, which is then given to open, which then parses it back into a name and a mode. As Dan predicts, the quoting and parsing processes don't always interpret the interface in the same way. Three-argument open allows the structured data to remain more structured throughout, because the mode and filename never need to be combined into a single unstructured string.
I have discovered that there are two types of command interfaces in the world of computing: good interfaces and user interfaces.
The essence of user interfaces is parsing: converting an unstructured sequence of commands, in a format usually determined more by psychology than by solid engineering, into structured data.
When another programmer wants to talk to a user interface, he has to quote: convert his structured data into an unstructured sequence of commands that the parser will, he hopes, convert back into the original structured data.
This situation is a recipe for disaster. The parser often has bugs: it fails to handle some inputs according to the documented interface. The quoter often has bugs: it produces outputs that do not have the right meaning. Only on rare joyous occasions does it happen that the parser and the quoter both misinterpret the interface in the same way.
When the original data is controlled by a malicious user, many of these bugs translate into security holes. Some examples: the Linux login -froot security hole; the classic find | xargs rm security hole; the Majordomo injection security hole. Even a simple parser like getopt is complicated enough for people to screw up the quoting.
In qmail, all the internal file structures are incredibly simple: text0 lines beginning with single-character commands. (text0 format means that lines are separated by a 0 byte instead of line feed.) The program-level interfaces don't take options.
All the complexity of parsing RFC 822 address lists and rewriting headers is in the qmail-inject program, which runs without privileges and is essentially part of the UA.