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HttpInpsect/HTTP preprocessor: false positives + parsing of header/body?
From: Bram <bram-fabeg () mail wizbit be>
Date: Mon, 12 Aug 2013 14:50:07 +0200

Hi,


There appear to be several false positives in the HTTP preprocessor.

Attempting to find the source of the false positives led me to the 'HttpResponseInspection' in preprocessors/HttpInspect/server/hi_server.c.

What *appears* to be happening is that 'HttpResponseInspection' makes the distinction between header and body based on the first five bytes in the buffer. When the buffer starts with 'HTTP/' then it seems to parse the packet as HTTP Headers, when it does not start with 'HTTP/' then it seems to parse the packet as HTTP Body.

This looks incorrect... there is no reason for all the HTTP headers to be in a single packet nor is there a reason for the HTTP body to be in a separate packet... The end of the HTTP header is indicated by an empty line.. (i.e., a line with nothing preceding the CRLF)


Configuration used:
        dynamicpreprocessor directory /usr/lib/snort_dynamicpreprocessor/
        preprocessor stream5_global: \
           track_tcp yes, \
           track_udp no, \
           track_icmp no
        preprocessor stream5_tcp: policy first, ports 80 8080

preprocessor http_inspect: global iis_unicode_map unicode.map 1252 compress_depth 65535 decompress_depth 65535
        preprocessor http_inspect_server: server default \
http_methods { GET POST PUT SEARCH MKCOL COPY MOVE LOCK UNLOCK NOTIFY POLL BCOPY BDELETE BMOVE LINK UNLINK OPTIONS HEAD DELETE TRACE TRACK CONNECT SOURCE SUBSCRIBE UNSUBSCRIBE PROPFIND PROPPATCH BPROPFIND BPROPPATCH RPC_CONNECT PROXY_SUCCESS BITS_POST CCM_POST SMS_POST RPC_IN_DATA RPC_OUT_DATA RPC_ECHO_DATA } \
            chunk_length 500000 \
            server_flow_depth 0 \
            client_flow_depth 0 \
            post_depth 65495 \
            oversize_dir_length 500 \
            max_header_length 4096 \
            max_headers 100 \
            max_spaces 0 \
            small_chunk_length { 10 5 } \
            ports { 80 7000 8080 } \
            non_rfc_char { 0x00 0x01 0x02 0x03 0x04 0x05 0x06 0x07 } \
            enable_cookie \
            extended_response_inspection \
            inspect_gzip \
            normalize_utf \
            unlimited_decompress \
            normalize_javascript \
            apache_whitespace no \
            ascii no \
            bare_byte no \
            directory no \
            double_decode no \
            iis_backslash no \
            iis_delimiter no \
            iis_unicode no \
            multi_slash no \
            utf_8 no \
            u_encode yes \
            webroot no

alert ( msg: "HI_SERVER_INVALID_STATCODE"; sid: 2; gid: 120; rev: 1; metadata: rule-type preproc ; ) alert ( msg: "HI_SERVER_NO_CONTLEN"; sid: 3; gid: 120; rev: 1; metadata: rule-type preproc ; )

        output alert_fast: stdout


Running it:
$ snort -v -l /var/log -c /etc/ips/snort.conf --daq-dir /lib/daq/ -r /tmp/120_2_1.cap 2>&1 | grep '120:' 07/15-15:27:54.244754 [**] [120:2:1] (http_inspect) INVALID STATUS CODE IN HTTP RESPONSE [**] [Priority: 0] {TCP} 192.168.173.1:80 -> 192.168.173.153:54940

$ snort -v -l /var/log -c /etc/ips/snort.conf --daq-dir /lib/daq/ -r /tmp/120_3_1_A.cap 2>&1 | grep '120:' 08/18-16:46:53.189936 [**] [120:3:1] (http_inspect) NO CONTENT-LENGTH OR TRANSFER-ENCODING IN HTTP RESPONSE [**] [Priority: 0] {TCP} 192.168.173.1:80 -> 192.168.173.153:43674

$ snort -v -l /var/log -c /etc/ips/snort.conf --daq-dir /lib/daq/ -r /tmp/120_3_1_B.cap 2>&1 | grep '120:' 08/18-16:54:58.860230 [**] [120:3:1] (http_inspect) NO CONTENT-LENGTH OR TRANSFER-ENCODING IN HTTP RESPONSE [**] [Priority: 0] {TCP} 192.168.173.1:80 -> 192.168.173.153:43675


Capture files:

120_2_1.cap:
        This triggers the 'HI_SERVER_INVALID_STATCODE' alert.
        The dump consist out of 1 request and 1 response.
'Special' about the response is that the HTTP body happens to contain the string 'HTTP/2.0' and this happens to be at the start of a TCP packet. [This false positive was detected due to a visit to the HTTP/2.0 draft, which contains the string 'HTTP/2.0' many times]

Analyzing it with gdb and breaking on hi_server.c line 1480 (right before '/*If this is the next expected pac ...') shows: * packet 6: data starts with 'HTTP/1.1', expected_pkt is 0 => parsed as HTTP headers => OK * packet 7: data starts with 'HTTP/2.0', expected_pkt is 0 => parsed as HTTP headers => NOT OK, this is part of the body

120_3_1_A.cap:
        This triggers the 'HI_SERVER_NO_CONTLEN' alert.
        In the dump the HTTP Headers are split over two TCP packets.
The first packets consist of 'HTTP/1.1 200 OK', the second packet contains the remainder of the headers, the third packet contains the body

Analyzing it with gdb and breaking on hi_server.c line 1480 (right before '/*If this is the next expected pac ...') * packet 8: data starts with 'HTTP/1.1', expected_pkt is 0 => parsed as HTTP headers => OK * packet 10: data starts with 'Content-Length...', expected_pkt is 1 => parsed as HTTP body => NOT OK, this packet contains headers * packet 12: data starts with 'foo', expected_pkt is 1 => parsed as HTTP body => OK

120_3_1_B.cap:
        This triggers the 'HI_SERVER_NO_CONTLEN' alert.
        In the dump the HTTP Headers are split over two TCP packets.
The first packets consist of 'HTTP/1.1 200 OK' while the second packet contains the remainder of the headers + the body

Analyzing it with gdb and breaking on hi_server.c line 1480 (right before '/*If this is the next expected pac ...') * packet 8: data starts with 'HTTP/1.1', expected_pkt is 0 => parsed as HTTP headers => OK * packet 10: data starts with 'Content-Length...', expected_pkt is 1 => parsed as HTTP body => NOT OK, this packet contains a part of the headers and a part of the body



Best regards,

Bram


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