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This chapter is from the book

2.2.4 Insecure and Badly Configured Programs (#4)

The number of security bugs and their severity in commonly used Linux programs have been reduced so dramatically that I have dropped the subject of poor physical security from the most deadly sins. In its place is the use of insecure programs (such as rsh, NFS, portmap, and FTP) in other-than-carefully-controlled situations and the failure to properly configure other programs. These "other programs" are capable of good security only if properly configured. Therefore, the Seven Most Deadly Sins has been updated for the second edition to reflect this.

This pushes poor physical security off the list and into the new "must read" section, "Law of the Jungle—Physical Security" on page 121. Additionally, Deadly Sin #5, Insecure CGIs, has been merged with this one. This is because, while locally written or adapted insecure CGI programs continue to be a problem, a substantial percentage of problems are due to SysAdmins making use of known insecure features of programs or failing to take advantage of security features.

Most SysAdmins know that POP and IMAP (unless wrapped in SSL), telnet, and FTP4 send passwords in the clear. They know that NFS and portmap have a history of security problems as well as design defects in their authentication. Many use them anyway and then are surprised when they get broken into. Do not do that! Instead, use spop, simap, SSH, and SSH's scp or sftp. See "Replace These Weak Doors with Brick" on page 94, "New Lamps for Old" on page 103, and "United We Fall, Divided We Stand" on page 115.

Many programs are secure only if properly configured, and it is common for SysAdmins to configure them improperly. Sometimes, it is a lack of training and understanding of the risks, while other times use of an insecure feature is deliberate, because "I just gotta have it." A recent case in point is Apache's PHP capability. It has had a recent history of security problems. PHP's security problems have been well publicized, and still some people cannot seem to use it securely or find an alternative. Security and convenience often are contradictory; frequently, a choice must be made. Chapter 4, "Common Hacking by Subsystem," on page 171 and Chapter 6, "Advanced Security Issues," on page 261 may be helpful.

Before deciding to deploy a service (i.e., changing what capabilities will be used or how the service will be deployed), research its security. Check its security history and understand how it may be deployed securely. If it cannot be deployed securely, what are the secure alternatives? I still encounter people using FTP who don't realize that sftp is an excellent alternative. Putting an insecure service, in this case NFS, behind a firewall was a good solution for one client. For another, putting its insecure Windows networks behind firewalls, with its different offices linked via a VPN between these same Linux firewalls, offered excellent security. Another client had me configure a firewall with separate subnets for its students, its financial administration and human resources, and the rest of its employees, along with internal and external DMZs. The rest of this section will address CGI problems specifically.

A CGI program will allow anyone to access your Web site, good intentions or not. Although other "accepted" servers, such as sendmail and named, will also talk with anyone, the scope of what a client may request is far smaller. While these latter servers have had their share of serious security bugs, those that keep up-to-date (as discussed in "Old Software Versions (#3)" on page 31) have minimal risk. Here are a few hard, fast rules that will help you make your Web site secure. (See "Special Techniques for Web Servers" on page 284 for several ways to increase Web server security.)

  1. Never, ever, run your Web server as a privileged user (such as root).

    Even some documentation for various products tries to seduce you into running Apache as root. You will get scalped if you do.

  2. Know your data (that is supplied by Web clients).

    Establish maximums and minimums for data entry values and lengths of fields.

    Decide what characters are acceptable in each field. Expect the malicious to send you control characters and non-ASCII bytes. Expect that crackers will use the "%" encoding to generate these evil characters. ("%" encoding is a "%" followed by two hexadecimal characters that will be mapped into the equivalent ASCII character.) Unless you stop them, they will use this method to send your CGI programs arbitrary binary bytes. This makes it easy to overflow input buffers and drop machine code (instructions) directly into CGI programs.

    Thus, you need to check for illegal characters both before and after "%" conversion. I have seen this latter attempt used against our sunset CGI program, fortunately without success.

    Double-check each entered value. A surprising number of shopping cart packages put the price of items in the form and believe the price in the filled-out form sent by the user. All a user needs to do is to alter this form and give himself a discount. Very little skill is required for a user to use this exploit. Many sites never detect the loss. It has been reported that Cart32 versions 2.5a, 2.6, and 3.0 have this "price in the form" bug and that even though this bug was widely known for four months, the vendor has chosen not to repair this problem.5

    If possible, enumerate the allowed values instead of using ranges.

    Understand, too, that an evil Web client can send any bytes back to your server. The cracker might copy and alter your Web form so that, even if your form pops up a list of the member European Union countries, she could supply

    crash_with_a_long_name_having_control_characters

    Use a secure language. Client-supplied data should never be handed directly to a shell script; there are too many opportunities for a cracker to get herself a shell or to exploit a buffer overflow vulnerability. Do you know how bash or tcsh react to buffer overflows? Neither do I, so you must not trust them in situations where a buffer overflow is possible.

    For many, that secure language will be C, Perl, or Python. If that language offers checking for tainted data, use it. One language does not fit all. Use what is best for each CGI, consistent with programmer skills. Perl has a number of features to enable safer CGI programs.6 These include the "tainted data" feature, the -w flag to warn you about things that you are creating but not using, and the strict capability that is discussed in

    http://www.cpan.org/doc/manual/html/lib/strict.html

    Even more security can be achieved with perlsec, discussed in

    www.perl.com/CPAN-local/doc/manual/html/pod/perlsec.html

  3. Analyze CGIs for vulnerabilities.

    When writing CGI programs, look at them the way a cracker would and try to break them. Check for buffer overflows by using good programming techniques. For example, when using C, make use of the fgets() routine which allows the programmer to specify how large a buffer has been allocated and will not overflow it. An easy way to determine if the line is larger than the buffer is to see that it does not end with a newline character, as this example illustrates.

    #include <stdio.h>
    #include <string.h>
    
    int   c;
    char  buf[200];
    
    if (!fgets(buf, sizeof buf, stdin))
         error();
    else if (!strchr(buf, '\n')) {
              /* Read rest of long line. */
            while ((c = getchar()) != EOF
             && c != '\n')
                 ;
            overflow();
    }

    Do not use the gets() routine because it does not do any checking for buffer overflows; use fgets() instead. Many of the other popular C string functions have similar weaknesses. The strcpy() function, for example, "lets" you copy a large buffer into a small buffer, overwriting unrelated memory. The strncpy() function is a good replacement for it.

    A safe way to copy strings is:

strncpy(dest_buf, source_buf, sizeof dest_buf);
dest_buf[sizeof dest_buf - 1] = '\0';
 

To detect a problem, one possibility is:

if (strlen(source_buf) >= sizeof dest_buf)
    error();
else
    strcpy(dest_buf, source_buf);

Check for escape sequences, the possibility of a client issuing Linux commands (by inserting spaces, quotes, or semicolons), binary data, calls to other programs, etc. Often it is safer to have a list of allowed characters rather than trying to determine each unsafe character.

The following C code may be used to process a field that the client should supply her name in. In this example, the calling process supplies a NUL-terminated string and this routine returns 0 if the string is a legal name and –1 otherwise. The second argument specifies the maximum legal string allowed, including the terminating NUL byte. Note that the calling routine must be careful to ensure that its buffer does not overflow. I chose clear code over slightly more efficient code.

#include <string.h>

char  okname[] = " .'abcdefghijklmnopqrstuvwxyz"
          "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
/* Return 0 on legal names, -1 otherwise. */
legal(char *name, int maxlen)
{
     if (!name || !*name
      || strlen(name) >= maxlen)
         return -1;
     while (*name)
         if (!strchr(okname, *name++))
              return -1;
     return 0;
}

Many of the system break-ins relating to Linux Web servers happen via insecure CGIs. All it takes is one buggy CGI and most systems will break.

  • Implement "Rings of Security" in CGIs.

    Try to design your application so that even if a CGI vulnerability is found, the system is protected from major damage. One solution is to have CGIs be just front ends for a solidly written server running on a different machine. The more hurdles a cracker must jump to reach the goal, the more likely it is that he will stumble.

    Consider using the SubDomain kernel module from WireX. It allows implementing kernel-based access control lists on a per-program basis to add additional limits to what any given program may do. It can prevent many of the recent security breaches, such as the named bug and the Dansie "fiasco," both discussed elsewhere in the book. Its object is only about 24 KB and is easy to install; WireX is generous with its allowance for free use. Its developer previously created StackGuard. It is available at:

    http://www.wirex.com/subdomain.html

  • Watch for bug reports in third-party CGIs and inspect their code.

    If using third-party-supplied CGI scripts (such as shopping carts), subscribe to their mailing lists and watch for security bulletins. If possible, get the source code and review it. If you do not know the language, get people who do and have them review it.

    In the spring of 2000 Joe Harris made a posting to Bugtraq, one of the security mailing lists, claiming that a popular Perl-based shopping cart program, Dansie, has a back door. Harris claimed that this back door lets anyone who knows the secret form name (URL on the server's system) to execute any arbitrary Linux command as the user running the shopping cart software. Dansie denied the existence of the back door but someone else also claimed to have seen this problem and it was mentioned in InternetNews on April 13, 2000.

    No customers have disputed the claim of a back door and Dansie's phone was disconnected shortly after this matter became public. I am not convinced that Dansie's denial is true because a second alleged user confirmed the problem and because there were no reports of Dansie denying the back door; Dansie gave a vague response to my e-mail sent to the company following the news reports: Inspect source code. A source quoted in an article on the Dansie problem said that this destroyed his confidence in the product.

    But Dansie ain't got (sic) nothin' on Red Hat. On April 25, 2000, it was reported that Red Hat left an undocumented back door in its Piranha product that allows anyone who knows it to execute arbitrary commands on the server as whatever user is running the product. This unintentional bug was in version 0.4.12 of piranha-gui. Patches are available.

    Many shopping cart packages and CGIs, both commercial and open source, have severe security holes that are well-known to the cracker community. We see our sites probed for these vulnerabilities periodically and we do not even handle money online.

  • Avoid creating and using set-UID and set-GID programs to the maximum extent possible (and try real hard).

    Many of these programs run as root. Where root is found on a system, a cracker is not far behind, probing for weaknesses. Many programs that run set-UID to root (or which are invoked by root) do not need to be.

    Frequently all these programs need to be set-UID for is to run as some user to gain access to data that should not be world-accessible. The solution is to create a new user that has no other purpose or access. Then make such programs set-UID to that new user. Different programs might need to be set-UID to different users to protect them from each other.

  • Do not keep clients' confidential data on the Web server.

    Avoid storing users' privileged data (credit card numbers, financial details, mailing addresses, and phone numbers, etc.) on the same machine as the Web server. This separation will force a cracker to have to crack two systems instead of just one to get this data. (See "One-Way Credit Card Data Path for Top Security" on page 302 for an innovative solution to this problem.)

  • Do not include users' confidential data (credit card numbers, financial details, mailing addresses, phone numbers, and so forth) in a URL or cookie.7

    Frequently this is done as arguments to a CGI program, for example:

    http://www.abroker.com/
    cgi-bin/address_change?account=666?
    passwd=secret&addr=1+Maple+St.&phone=555-1212

    The problem is that this information will show as the "referring URL" if the user then clicks on a link shown on the results page. Thus, this privileged information will be given to complete strangers (the Web site that the link points to). Several well-known companies were scandalized by this in early 2000 and some were investigated by the authorities.

    Some browsers might store this URL (containing confidential data) in a history file. If someone is browsing from a public terminal, such as a school or library, you could be liable for careless handling of that person's data. Similar issues are present for cookies.

  • Be very sure that the privileged data that a user supplies on a form does not show up as the default data for the next person to "pull down" that form and see.

    Yes, this actually has happened.

  • Always protect the user who types in his password.

    Take him to a secured area prior to this information being entered. This will decrease the likelihood that the password will be stolen. Commonly this protected area will be https (SSL-wrapped http) to encrypt his password to guard against network sniffing.

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