cbcvebase.
CVE-2009-4444
published 2009-12-29

CVE-2009-4444: Microsoft Internet Information Services (IIS) 5.x and 6.x uses only the portion of a filename before a ; (semicolon) character to determine the file extension…

PriorityP343medium6CVSS 2.0
AVNACMAuSCPIPAP
EPSS
63.63%
99.1th percentile
Microsoft Internet Information Services (IIS) 5.x and 6.x uses only the portion of a filename before a ; (semicolon) character to determine the file extension, which allows remote attackers to bypass intended extension restrictions of third-party upload applications via a filename with a (1) .asp, (2) .cer, or (3) .asa first extension, followed by a semicolon and a safe extension, as demonstrated by the use of asp.dll to handle a .asp;.jpg file.

Affected

2 ranges
VendorProductVersion rangeFixed in
microsoftinternet_information_services
microsoftinternet_information_services

Detection & IOCsextracted from sources · hover to see the quote

filename.asp;.jpg
snort
alert http $EXTERNAL_NET any -> $HTTP_SERVERS any (msg:"ET WEB_SERVER Possible Microsoft Internet Information Services (IIS) .asp Filename Extension Parsing File Upload Security Bypass Attempt (asp)"; flow:established,to_server; http.uri; content:".asp|3B 2E|"; nocase; reference:url,www.securityfocus.com/bid/37460/info; reference:url,www.securityfocus.com/bid/37460/info; reference:url,soroush.secproject.com/downloadable/iis-semicolon-report.pdf; reference:cve,2009-4444; classtype:web-application-attack; sid:2010592; rev:9; metadata:created_at 2010_07_30, cve CVE_2009_4444, confidence Medium, signature_severity Major, updated_at 2020_08_20;)
snort
alert http $EXTERNAL_NET any -> $HTTP_SERVERS any (msg:"ET WEB_SERVER Possible Microsoft Internet Information Services (IIS) .aspx Filename Extension Parsing File Upload Security Bypass Attempt (aspx)"; flow:established,to_server; http.uri; content:".aspx|3B 2E|"; nocase; reference:url,www.securityfocus.com/bid/37460/info; reference:url,www.securityfocus.com/bid/37460/info; reference:url,soroush.secproject.com/downloadable/iis-semicolon-report.pdf; reference:cve,2009-4444; classtype:web-application-attack; sid:2010593; rev:10; metadata:created_at 2010_07_30, cve CVE_2009_4444, confidence Medium, signature_severity Major, updated_at 2020_09_14;)
bytes
.asp|3B 2E|
bytes
.aspx|3B 2E|
  • Detect HTTP requests to IIS servers where the URI contains a semicolon-delimited double extension pattern (e.g., .asp;.jpg, .cer;.jpg, .asa;.jpg). IIS parses only the portion before the semicolon as the file extension, causing asp.dll to execute the file.
  • Inspect HTTP URI for the byte sequence 0x3B 0x2E (semicolon followed by a dot) immediately after a dangerous extension (.asp, .aspx, .cer, .asa) — this is the canonical bypass pattern used in upload exploitation.
  • Focus detection on inbound HTTP POST requests (file uploads) to_server where the filename in the URI or multipart body contains the semicolon bypass pattern, as the attack vector is third-party upload applications on IIS.
  • ·The bypass affects IIS 5.x and 6.x only. The semicolon extension-truncation behavior is a server-side parsing quirk specific to these versions; later IIS versions are not confirmed affected.
  • ·The vulnerability is exploitable only when a third-party upload application (not IIS itself) enforces extension restrictions — IIS will still execute the file as ASP/ASPX/ASA/CER regardless of the suffix after the semicolon.
  • ·Snort/Suricata rules (ET SID 2010592/2010593) carry only 'Medium' confidence metadata, meaning they may produce false positives on legitimate URIs containing semicolons; tune accordingly.
Stop checking back — get the weekly exploitation signal.

Every Monday: what got weaponized or added to CISA KEV in the last seven days — each CVE cross-linked to its PoC, Nuclei template, and detection rule. Free, one email a week, unsubscribe in one click.