http.sys vulnerability cve-2015-1635 ms15-034 johannes b. ullrich, ph.d. jullrich@sans.edu 1
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HTTP.sys VulnerabilityCVE-2015-1635 MS15-034
Johannes B. Ullrich, Ph.D.jullrich@sans.edu
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Outline
• What is HTTP.sys?• What does the “Range” header do?• How is it exploited?• How to test if you are vulnerable• Examples of Current Exploits in the
Wild
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No Logo? No Catchy Name
HTTPdeRANGEd
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HTTP.sys
• Parses HTTP Requests• Caches response using kernel caching• If a “Range” header is used, extracts
specific portion of page from Kernel Cache to pass to client
• Used in IIS 6 and later.• NOT JUST USED BY IIS (part of
Windows)
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Range Header (RFC 7233)
• Used for partial downloads• Often used to complete downloads• Mobile clients (podcast clients)
download pages in “chunks”.
GET / HTTP/1.1Host: testRange: bytes=0-5,10-15
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Range Header Response
HTTP/1.1 206 Partial ContentContent-Type: multipart/byteranges; boundary=513da661b3ac6e
--513da661b3ac6eContent-type: text/html; charset=UTF-8Content-range: bytes 0-5/15
<HTML> --513da661b3ac6eContent-type: text/html; charset=UTF-8
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No Upper Limit
“Since there is no predefined limit to the length of a payload, recipients must anticipate potentially large
decimal numerals and prevent parsing errors due to integer conversion
overflows.” (RFC 7233)
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Exploit
• IIS limits the range to a 64 Bit Unsigned number.
• Maximum Number: 2^64-1184467440737095516150xFFFFFFFFFFFFFF
If lower end 0 -> No exploit
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Exploit (2)
• Lower end > Size of file: No exploit• Lower end > 0 and <=Size of file:
Exploit!!
Integer Overflow
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Exploit Request
GET / HTTP/1.1Host: testRange: bytes=x-18446744073709551615
X=0 no exploitX>0 and X<Filesize Exploit
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Information Leak
• If “lower end” = “file size - 1”• Not reproducable in my testing• Dumps kernel memory (same
segment as “cache”?)• Maximum size depends on size of file
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Tests
• Send large HTTP Range request with lower end 0..
• Other Software using http.sysnetsh http show servicestate
• Check if patch is installedwmic qfe | find KB3042553
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Other Protections
• I(D|P)S: Does not work for SSL• Host based IPS
e.g. Symantec has signatures that block exploit
• WAF• Authentication: Disable Anonymous
Access
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Current Exploits
• Many vulnerability scans (range starts at “0”)
• Some random DoS exploit attempts• No Information Disclosure exploits in
honeypot so far• Reports of more targeted exploit
attempts.
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Risk
• Exposed Public Systems are at immediate risk of DoS
• Memory Disclosure likely “stable” in a couple days
• Remote Execution unlikely (in the near future)
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What to do next?
• Expedite Patching MS15-034• Consider “virtual patching” via WAF
until patch is applied and verified• Add IDS rules to detect exploit
attempts• Please… share anything you see!• Is it as bad as Heartbleed? No…
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Questions?
jullrich@sans.eduhttps://isc.sans.edu/presentations
@johullrichDaily Podcast…
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