How to Use Content switching to Mitigate against MS15-034 Vulnerability
Summary: An exploit has been found effecting IIS server MS15-034. The exploit involves using the Range Header to potentially allow remote code execution.
Details:
It is recommended that anyone effected install the latest Microsoft patch to mitigate against this.
If you are using the Kemp Loadmaster you may also use the Content Switching feature to Block/Modify requests which trigger the vulnerability.
As the exploit involves accessing memory at "-1" location using the value 18446744073709551615, one method to block is to restrict the length of values in the range header. The following rule will match any request with a Range header which includes a value >10 digits and remove this value from the request.
Rule Name: MS15034
Rule Type: Replace Header
Header Field: Range
Match String: /(bytes\s*=)(.*?)(([0-9]){10,})(.*)/
Value of Header Field to be replaced: \1\2\4
Perform If flag Set: unset
Test
curl -v http://10.111.0.171/welcome.png -H "Host: domain.com" -H "Range: bytes = 10-18446744073709551615" -k
Client -> Loadmaster
HTTP Header
GET /welcome.png HTTP/1.1
User-Agent: curl/7.37.1
Accept: */*
Host: domain.com
Range: bytes = 10-18446744073709551615
User-Agent: curl/7.37.1
Accept: */*
Host: domain.com
Range: bytes = 10-18446744073709551615
Loadmaster -> Real Server
HTTP Header:
GET /welcome.png HTTP/1.1
User-Agent: curl/7.37.1
Accept: */*
Host: domain.com
Range: bytes = 10-
User-Agent: curl/7.37.1
Accept: */*
Host: domain.com
Range: bytes = 10-
Test with range header values < 10 digits
curl -v http://10.111.0.171/welcome.png -H "Host: domain.com" -H "Range: bytes = 10-123456789" -k
Client -> Loadmaster
HTTP Header
GET /welcome.png HTTP/1.1
User-Agent: curl/7.37.1
Accept: */*
Host: domain.com
Range: bytes = 10-123456789
User-Agent: curl/7.37.1
Accept: */*
Host: domain.com
Range: bytes = 10-123456789
Loadmaster -> Real Server
HTTP Header:
GET /welcome.png HTTP/1.1User-Agent: curl/7.37.1
Accept: */*
Host: domain.com
Range: bytes = 10-123456789
To Apply this rule:
1. Create the content rule as above
Rules & Checking-Content Rules-Create New
2. Under the Virtual Service apply as a Header modification rule
Virtual Services-View/Modify Services-Advanced Properties-Content Switching-Enable
Virtual Services-View/Modify Services-Advanced Properties-Content Switching-Enable-HTTP Header Modifications
3. Apply the rule Created as a request rule.
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