NSFOCUS Security Advisory(SA2000-08) Topic: Microsoft IIS for Far East Editions File Disclosure Vulnerability Release Date£ºDec 13rd, 2000 CVE Candidate Numbers: CAN-2000-1090 Affected system: ================== - Microsoft IIS 4.0 for Far East editions ( < SP6 ) - Microsoft IIS 5.0 for Far East editions (Chinese (Traditional and Simplified), Japanese, and Korean (Hangeul)) Non-affected system: ================== - Microsoft IIS 4.0 for Far East editions( SP6/SP6a ) - Microsoft IIS 5.0/4.0 other editions Impact: ========= NSFOCUS security team has found a security flaw in Microsoft IIS 4.0/5.0 (Far East editions) when responding to a HTTP request containing incomplete double-byte characters(DBCS). It could lead to exposure of files under Web directory to a remote attacker. Description£º ============ Microsoft IIS for Far East editions include Chinese (Traditional and Simplified), Japanese, and Korean (Hangeul), all of which use double-byte character set(DBCS). When IIS receives a HTTP request with non-ASCII character in the file name, it will check if it is a lead-byte(Lead-byte ranges: 0x81 - 0x9F, 0xE0 - 0xFC). If it is, then IIS go on checking for a trail-byte. And if a trail-byte is not available, IIS will simply drop the lead-byte. All this will result in an opening of a different file. Submitting a malformed URL, attacker can entrap IIS to call some certain ISAPI DLL, open some kinds of file that it can not interpret. attacker may obtain the content of file like plain text file(.asp, .ini, .asa etc.)or binary system file(.exe) under Web or virtual directory. This problem has been solved in IIS 4.0 with SP6 before it comes forth again in IIS 5.0. Other IIS 4.0/5.0 including English versions are unaffected. Exploit: ========== [Proof of concept code will be available soon] Workaround: =================== 1¡¢Remove unnecessary ISAPI mapping like HTR, HTW,IDQ etc. 2¡¢Turn on "Check that file exists" option in every necessary ISAPI mapping. 3¡¢If you are using vulnerable IIS 4.0 with prior SP6, update to SP6. Vendor Status: ============== Microsoft has been informed on Oct 8, 2000. Additional Information: ======================== The Common Vulnerabilities and Exposures (CVE) project has assigned the name CAN-2000-1090 to this issue. This is a candidate for inclusion in the CVE list (http://cve.mitre.org), which standardizes names for security problems. Candidates may change significantly before they become official CVE entries. DISCLAIMS: ========== THE INFORMATION PROVIDED IS RELEASED BY NSFOCUS "AS IS" WITHOUT WARRANTY OF ANY KIND. NSFOCUS DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, EXCEPT FOR THE WARRANTIES OF MERCHANTABILITY. IN NO EVENTSHALL NSFOCUS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL,CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF NSFOCUS HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. DISTRIBUTION OR REPRODUTION OF THE INFORMATION IS PROVIDED THAT THE ADVISORY IS NOT MODIFIED IN ANY WAY. ?Copyright 1999-2000 NSFOCUS. All Rights Reserved. Terms of use. NSFOCUS Security Team NSFOCUS INFORMATION TECHNOLOGY CO.,LTD (http://www.nsfocus.com)