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HttpInpsect/HTTP preprocessor: false positives HI_CLISRV_MSG_SIZE_EXCEPTION
From: Bram <bram-fabeg () mail wizbit be>
Date: Fri, 09 Aug 2013 14:45:27 +0200

Hi,


There appears to be a false positive with the 'HI_CLISRV_MSG_SIZE_EXCEPTION' alert.
At the very least it displays some confusing/strange behaviour.

Attached are three TCP dump files.
The TCP session in the three dumps is exactly the same, the only difference is in the port number (and the squence/ack numbers). (NOTE: this is a minimal version of the dump - this was triggered on other traffic but the dump was made as tiny as possible)

In the config file:
* port 80 and port 8080 are handled the same.
* port 7000 is listed as port for the 'http_inspect' preprocessor but not listed in the 'stream5_tcp' preprocessor.


Full configuration file:
        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 HEAD 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_CLISRV_MSG_SIZE_EXCEPTION"; sid: 8; gid: 120; rev: 2; 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 shows:
$ snort -v -l /var/log -c /etc/ips/snort.conf --daq-dir /lib/daq/ -r /tmp/120_8_2_80.cap 2>&1 | grep 120: 08/12-18:21:01.997452 [**] [120:3:1] (http_inspect) NO CONTENT-LENGTH OR TRANSFER-ENCODING IN HTTP RESPONSE [**] [Priority: 0] {TCP} 192.168.173.1:80 -> 192.168.173.153:43668

$ snort -v -l /var/log -c /etc/ips/snort.conf --daq-dir /lib/daq/ -r /tmp/120_8_2_7000.cap 2>&1 | grep 120: 08/12-18:19:21.036304 [**] [120:3:1] (http_inspect) NO CONTENT-LENGTH OR TRANSFER-ENCODING IN HTTP RESPONSE [**] [Priority: 0] {TCP} 192.168.173.1:7000 -> 192.168.173.153:36671

$ snort -v -l /var/log -c /etc/ips/snort.conf --daq-dir /lib/daq/ -r /tmp/120_8_2_8080.cap 2>&1 | grep 120: 08/09-10:00:25.993618 [**] [120:8:2] (http_inspect) INVALID CONTENT-LENGTH OR CHUNK SIZE [**] [Priority: 0] {TCP} 192.168.173.153:54958 -> 192.168.173.1:8080 08/09-10:00:25.993471 [**] [120:3:1] (http_inspect) NO CONTENT-LENGTH OR TRANSFER-ENCODING IN HTTP RESPONSE [**] [Priority: 0] {TCP} 192.168.173.1:8080 -> 192.168.173.153:54958

The 'NO CONTENT-LENGTH OR TRANSFER-ENCODING IN HTTP RESPONSE' alert is expected.
The 'INVALID CONTENT-LENGTH OR CHUNK SIZE' alert is unexpected...


Looking further:

* 120_8_2_80.cap:
        'HI_SERVER_NO_CONTLEN' shown on packet 6

* 120_8_2_7000.cap:
        'HI_SERVER_NO_CONTLEN' shown on packet 6

* 120_8_2_8080.cap:
        'HI_CLISRV_MSG_SIZE_EXCEPTION' shown on packet 8
        'HI_SERVER_NO_CONTLEN' shown on packet 10

Why is the behaviour for port 80 different from the behaviour for port 8080?
As far as I can see it's configured the same...


Breaking in 'CheckFlushPolicyOnAck' with gdb shows:


* 120_8_2_80.cap:
        For packets 1, 2 and 10: 'CheckFlushPolicyOnAck' never called
For packets 3, 4, 5, 6, 7, 8 and 9: 'talker->flush_mgr.flush_policy' is set to 5 (STREAM_FLPOLICY_IGNORE)

* 120_8_2_7000.cap:
        For packets 1, 2 and 10: 'CheckFlushPolicyOnAck' never called
For packets 3, 4, 5, 6, 7, 8 and 9: 'talker->flush_mgr.flush_policy' is set to 5 (STREAM_FLPOLICY_IGNORE)


* 120_8_2_8080.cap:
        For packets 1 and 2: 'CheckFlushPolicyOnAck' never called
For packets 3, 4, 5, 6 and 7: 'talker->flush_mgr.flush_policy' is set to 6 (STREAM_FLPOLICY_PROTOCOL) For packets 8: 'CheckFlushPolicyOnAck' called twice, first time 'talker->flush_mgr.flush_policy' is set to 6 (STREAM_FLPOLICY_PROTOCOL), second time it's set to 1 (STREAM_FLPOLICY_FOOTPRINT) For packets 9 and 10: 'talker->flush_mgr.flush_policy' is set to 1 (STREAM_FLPOLICY_FOOTPRINT)



Best regards,

Bram


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