Vulnerability Note VU#610291

Microsoft Internet Information Server (IIS) 4.0 and 5.0 buffer overflow in chunked encoding transfer mechanism for ASP

Buffers used to store chunks are allocated on the heap, and therefore this vulnerability can be called a heap-based buffer overflow. Exploiting a heap-based buffer overflow to gain control of a system can sometimes be more difficult than exploiting other kinds of buffer overflows to gain control. However, the failure is more conducive to gaining control of the system than other typical heap-based buffer overflows. Quoting from Microsoft Security Bulletin MS02-018:

[...] in addition to causing the wrong-sized buffer to be allocated, the arithmetic error also prevents IIS 4.0 and 5.0 from placing any real limits on the size of a chunk. As a result, it would be possible for a client to send a chunk that would overwrite most or all of the memory on the system.

This is a critical point, because it goes to the heart of why this vulnerability poses such a serious threat to servers. This vulnerability is an example of so-called heap overrun; these are frequently difficult or impossible to exploit, because of the dynamic nature of system memory. Data on the server can change locations from one moment to the next, impeding the attacker’s ability to overwrite selected programs or data. However, in this case, the attacker wouldn’t need to know where programs were located, but could instead simply overwrite large portions of system memory indiscriminately.

This vulnerability was discovered by eEye Digital Security. For more information, see

http://www.eeye.com/html/Research/Advisories/AD20020410.html

Until a patch can be applied, you may wish to disable the ASP ISAPI extension by using the IIS Lockdown tool, available at http://www.microsoft.com/technet/security/tools/locktool.asp. In addition, you can use the URLScan tool to block URLs that contain non-ASCII data. This may be useful in limiting the damage an intruder could do through this vulnerability.

Systems Affected

http://www.kb.cert.org/vuls/id/25716
http://www.kb.cert.org/vuls/id/313819
http://www.kb.cert.org/vuls/id/669779
http://www.microsoft.com/technet/security/bulletin/MS02-018.asp
http://www.eeye.com/html/Research/Advisories/AD20020410.html
http://www.eeye.com/html/Research/Advisories/AD20020410.html
http://www.microsoft.com/technet/security/URLScan.asp
http://www.securityfocus.com/bid/4485

Credit

Our thanks to Microsoft Corporation, upon whose advisory this document is based, and eEye Digital Security who discovered the vulnerability and whose advisory provides additional technical detail.

This document was written by Shawn V. Hernan.

Other Information

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