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Jun 24, 2009, 9:30 AM
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[Bug 47417] New: Apache Web Server 2.2.11 Incomplete HTTP Header Resource Exhaustion Vulnerability
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https://issues.apache.org/bugzilla/show_bug.cgi?id=47417
Summary: Apache Web Server 2.2.11 Incomplete HTTP Header
Resource Exhaustion Vulnerability
Product: Apache httpd-2
Version: 2.2.11
Platform: All
URL: http://isc.sans.org/diary.html?storyid=6601
OS/Version: All
Status: NEW
Severity: major
Priority: P2
Component: Core
AssignedTo: bugs[at]httpd.apache.org
ReportedBy: sailesh_kyanam[at]fanniemae.com
This alleged vulnerability was reported to us by our internal security group.
Apparently, all versions of Apache 2.2, including the latest version 2.2.11 are
affected by a bug that can cause DoS attacks to be made very trivially.
While there are a lot of DoS tools available today, this one is particularly
interesting because it holds the connection open while sending incomplete HTTP
requests to the server.
In this case, the server will open the connection and wait for the complete
header to be received. However, the client (the DoS tool) will not send it and
will instead keep sending bogus header lines which will keep the connection
allocated.
The initial part of the HTTP request is completely legitimate:
GET / HTTP/1.1\r\n
Host: host\r\n
User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; Trident/4.0;
.NET CLR 1.1.4322; .NET CLR 2.0.503l3; .NET CLR 3.0.4506.2152; .NET CLR
3.5.30729; MSOffice 12)\r\n
Content-Length: 42\r\n
After sending this the client waits for certain time – notice that it is
missing one CRLF to finish the header which is otherwise completely legitimate.
The bogus header line the tools sends is currently:
X-a: b\r\n
Which obviously doesn't mean anything to the server so it keeps waiting for the
rest of the header to arrive.
This link at iDefense labs has the code required to run teh exploit:
https://ialert.idefense.com/idcontent/2009/exploit_code/487469-Web_Server_HTTP_Header_DoS.php.txt
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[Bug 47417] New: Apache Web Server 2.2.11 Incomplete HTTP Header Resource Exhaustion Vulnerability