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List: snort-sigs
Subject: [Snort-sigs] False positive on sid:2109 POP3 TOP overflow attempt
From: Ben Dugdale <ben.dugdale () sheepcamp ! com>
Date: 2005-01-31 22:59:18
Message-ID: 41FEB9F8.2050702 () sheepcamp ! com
[Download RAW message or body]
It seems that this rule looks for 10 or more non-newline characters
after the TOP command, and considers anything more a buffer overflow
attempt.
alert tcp $EXTERNAL_NET any -> $HOME_NET 110 (msg:"POP3 TOP overflow
attempt"; flow:to_server,established; content:"TOP"; nocase;
isdataat:10,relative; pcre:"/^TOP\s[^\n]{10}/smi";
classtype:attempted-admin; sid:2109; rev:3;)
I've got a fetchmail process that consistently sets off the rule with a
packet like this:
01/30-17:05:54.820048 0:30:7B:BC:48:1C -> 0:2:B9:45:A7:6E type:0x800
len:0x52
xxx.xxx.xxx.xxx:36186 -> xxx.xxx.xxx.xxx:110 TCP TTL:63 TOS:0x0 ID:25677
IpLen:20 DgmLen:68 DF
***AP*** Seq: 0x377E2A43 Ack: 0x37122A03 Win: 0x5B4 TcpLen: 32
TCP Options (3) => NOP NOP TS: 792484376 2669171
54 4F 50 20 31 20 39 39 39 39 39 39 39 39 0D 0A TOP 1 99999999..
I looked in the RFC ( http://www.faqs.org/rfcs/rfc1939.html ) for a
specification on the arguments given the top command, and see no
indication of a limit.
Arguments:
a message-number (required) which may NOT refer to to a
message marked as deleted, and a non-negative number
of lines (required)
Restrictions:
may only be given in the TRANSACTION state
Discussion:
If the POP3 server issues a positive response, then the
response given is multi-line. After the initial +OK, the
POP3 server sends the headers of the message, the blank
line separating the headers from the body, and then the
number of lines of the indicated message's body, being
careful to byte-stuff the termination character (as with
all multi-line responses).
Note that if the number of lines requested by the POP3
client is greater than than the number of lines in the
body, then the POP3 server sends the entire message.
Possible Responses:
+OK top of message follows
-ERR no such message
Examples:
C: TOP 1 10
S: +OK
S: <the POP3 server sends the headers of the
message, a blank line, and the first 10 lines
of the body of the message>
S: .
...
C: TOP 100 3
S: -ERR no such message
FWIW, man fetchmail didn't give me any info about specifying something
less, and my installation if pretty much default.
Something like:
alert tcp $EXTERNAL_NET any -> $HOME_NET 110 (msg:"POP3 TOP overflow
attempt"; flow:to_server,established; content:"TOP"; nocase;
isdataat:10,relative; pcre:"/^TOP\s[^\n]{12}/smi";
classtype:attempted-admin; sid:2109; rev:4;)
Would take care of my false positive, but I don't know about the
possibility of others. Where did the 10 character limit come from? Is
there a POP server that is vulnerable at that number, or have I missed a
specification, or just common sense and I found the exception?
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