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Local buffer overflow attacks are extremely popular As discussed in the Remote Access section earlier, buffer overflow vulnerabilities allow attackers to execute arbitrary code or commands on a target system Most times, buffer overflow conditions are used to exploit SUID root files, enabling the attackers to execute commands with root privileges We already covered how buffer overflow conditions allow arbitrary command execution (See Buffer Overflow Attacks, earlier in the chapter) In this section, we discuss and give examples of how a local buffer overflow attack works In May 1999, Shadow Penguin Security released an advisory related to a buffer overflow condition in libc relating to the environmental variable LC_MESSAGES Any SUID program that is dynamically linked to libc and that honors the LC_MESSAGES environmental variable is subject to a buffer overflow attack This buffer overflow condition affects many different programs because it is a buffer overflow in the system libraries (libc) rather than in one specific program, as discussed earlier This is an important point and one of the reasons we chose this example It is possible for a buffer overflow condition to affect many different programs if the overflow condition exists in libc Let s discuss how this vulnerability is exploited First, we need to compile the actual exploit Your mileage will vary greatly because exploit code is very persnickety Often, you will have to tinker with the code to get it to compile because it is platform dependent This particular exploit is written for Solaris 26 and 7 To compile the code, we used gcc, or the GNU compiler Solaris does not come with a compiler, unless purchased separately, but gcc may be downloaded for free at http://wwwsunfreewarecom The source code is designated by *c The executable will be saved as ex_lobc by using the o option:

Next, we execute ex_lobc, which will exploit the overflow condition in libc via an SUID program such as /bin/passwd:

creating documents in which each part or chapter starts on a new page (as in this book) or for using a different paper size (for example, an envelope)

[itchy]$ /ex_lobc jumping address : efffe7a8 #

.

Continuous The new section starts at the next paragraph on the same page A nextpage section is good for creating layouts that use different numbers of newspaper-style columns on the same page 11 shows you how to create such columns

The exploit then jumps to a specific address in memory, and /bin/sh is run with root privileges This results in the unmistakable # sign, indicating that we have gained root

To insert a new section, follow these steps: 1 Position the insertion point at the beginning of the paragraph with which you want the new section to start 2 Choose Page Layout | Page Setup | Breaks, and then click Next Page, Continuous, Even Page, or Odd Page on the panel Word inserts the section break and rearranges the pages as necessary Like paragraph marks and spaces, section breaks are normally hidden To see your section breaks, press CTRL-SHIFT-8 or choose Home | Paragraph | Show/Hide The next illustration shows a continuous section break and a next-page section break

access This exercise was quite simple and can make anyone look like a security expert In reality, the Shadow Penguin Security group performed the hard work by discovering and exploiting this vulnerability As you can imagine, the ease of obtaining root access is a major attraction to most attackers when using local buffer overflow exploits

Once you ve inserted one or more section breaks in a document, you may find it helpful to add the Section readout to the beginning of the status bar so that you can see which section the insertion point is currently in To do so, right-click the status bar, select the Section check box on the Customize Status Bar menu, and then click anywhere in the document to hide the menu again

The best buffer overflow countermeasure is secure coding practices combined with a nonexecutable stack If the stack had been nonexecutable, we would have had a much harder time trying to exploit this vulnerability See the Buffer Overflow Attacks section, earlier in the chapter, for a complete listing of countermeasures Evaluate and remove the SUID bit on any file that does not absolutely require SUID permissions