Product: Numerous CGI's
Vendor: Numerous Vendors
Severity: Remote; Severity varies, but can often be used to attain CGI administrator status, which can result in read/write/execute privileges.
Cause: Failure to validate input
In Short: Numerous CGI's store data, including passwords, in a flat file database, using special characters as field and row delimiters. An attacker may be able to manipulate these databases. While many types of CGI's may be vulnerable, CGI's which allow multiple users to log on, and grant certain users privileged or administrator status, are most likely to be exploitable.
Note: A flat file database is a standard text file used to store database style (i.e., fields and rows) information. Fields are delimited by a special character, such as a pipe symbol ( | ) or a colon ( : ). Rows are usually delimited by a newline. A common example is the Unix /etc/passwd file.
Unfortunately, data stored in this format is often susceptible to manipulation by an attacker. When the database is used to store both user supplied data (such as e-mail address), as well as system data (such as user privileges), an attacker may be able to manipulate the system data. By inserting a row or field delimiting character into the user supplied data, the attacker can fool the database into thinking that the user supplied data is actually the system data of a different row or field.
This is best illustrated by an example:
A particular CGI allows multiple users to log on to a web site. It allows anyone to log on, but provides additional privileges to paying customers. Furthermore, the webmaster may log on to modify the CGI settings. The CGI stores the user data in a flat file database, using the pipe symbol ( | ) as a field delimiter, and a newline as a row delimiter. The database stores the following fields: password, logon name, privilege level, first name, last name, and e-mail address. Here is a sample file:
By registering with a last name containing url-encoded newlines and pipes, an attacker can imbed a second line into his last name, which will be recorded as an entirely new line in the password file, containing whatever information the attacker wants. For instance, an attacker may register as follows:
When url encoded and submitted properly, this will add two lines to the database.
The example database will now look like this:
CGI developers should ensure that their CGI's remove delimiter characters from user supplied data. A redundancy of checking for delimiters before writing to the database is also advisable.
SQL databases, with stored, parameterized queries will generally be more secure.
qDefense originally discovered this vulnerability class when auditing D.C. Forum, and issued an advisory, DCForum Password File Manipulation Vulnerability (qDefense Advisory Number QDAV-5-2000-2). However, further research has shown that this class of vulnerability is prevalent among CGI's, particularly those which allow users to log on using passwords. As this form of attack represents a new method which has not (to qDefense's knowledge) been publicized as of yet, qDefense has decided to issue a general advisory, instead of issuing specific advisories for all of the CGI's that we have found vulnerable.
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