Log corruption on multiple webservers, log analyzers,... Mar 04 2003 05:39PM
Hugo Vázquez Caramés (overclocking_a_la_abuela hotmail com)


something that could be interesting...

We have decided not to contact any vendor (many vendors are vulnerable and

we have not enough time...sorry) and made this advisory public in this


ILLC - Inverse Lookup Log Corruption

We are using a technique that we have called ?ILLC? (Inverse Lookup Log

Corruption) that allows us to corrupt the logs generated by many web

servers that are doing inverse address resolution.

Impact of this technique:

- ?IP spoofing? on the logs

- Code execution (XSS) on boxes that are running log analyzers (web

servers that have buit-in report analisys tools,etc.)

On some specific scenarios, we have been able to hide the entire http

request to the log viewer.

Most of the actions were possible because of the lack of filtering when

parsing host names between different applications.

Related RFC´s about Internet Host Names convention:

RFC 952:


1. A "name" (Net, Host, Gateway, or Domain name) is a text string up

to 24 characters drawn from the alphabet (A-Z), digits (0-9), minus sign (-

), and period (.). Note that periods are only allowed when they serve to

delimit components of "domain style names". (See RFC-921, "Domain Name

System Implementation Schedule", for background). No blank or space

characters are permitted as part of a name. No distinction is made between

upper and lower case. The first character must be an alpha character. The

last character must not be a minus sign or period.... Single character

names or nicknames are not allowed....


RFC 1034:


3.5. Preferred name syntax

... The labels must follow the rules for ARPANET host names. They must

start with a letter, end with a letter or digit, and have as interior

characters only letters, digits, and hyphen. There are also some

restrictions on the length. Labels must be 63 characters or less. (...)?

RFC 1123:


2.1 Host Names and Numbers

The syntax of a legal Internet host name was specified in RFC-952 [DNS:4].

One aspect of host name syntax is hereby changed: the restriction on the

first character is relaxed to allow either a letter or a digit. Host

software MUST support this more liberal the RFCs. Note that under BIND 8,

you may need to add "check-names master ignore" to the zone definition

when defining these names.(?)?

RFC 2181:


11. Name syntax

Occasionally it is assumed that the Domain Name System serves only the

purpose of mapping Internet host names to data, and mapping Internet

addresses to host names. This is not correct, the DNS is a general (if

somewhat limited) hierarchical database, and can store almost any kind of

data, for almost any purpose.

The DNS itself places only one restriction on the particular labels that

can be used to identify resource records. That one restriction relates to

the length of the label and the full name. The length of any one label is

limited to between 1 and 63 octets. A full domain name is limited to 255

octets (including the separators).(?)?

Independently of what should be the legal host name syntax, it seems that

operating systems allows host names with arbitrary characters.

To succesfully attack a server with ?ILLC? technique is mandatory that web

server/log analyzer,etc., will be doing inverse address resolution and

that the attacker could control in any way the responses to those inverse

lookup requests.


Exploiting web server/log analyzers through ?ILLC?


Examples of attacks:

-Log ?IP Spoofing?

(exploited succesfully on Apache 2.0.44 on Windows/Linux, and Iplanet 6

on Windows)

Scenario: a machine with a host name as "" makes a request

to an Apache server. If the server dosn`t generate any error, on the

access log you will see an access request from a client

called "", what apparently seems to be a valid request from

a client that server was unable to resolve to a host name. So the real IP

wouldn't appear in the access log file.

access.log - - [28/Feb/2003:10:39:01 +0100] "GET / HTTP/1.1" 200

1786 "-" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0)" - - [28/Feb/2003:10:39:46 +0100] "GET /badrequest.html

HTTP/1.1" 404 294 "-" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0)"

If the request produces some error, you will see an entry in the error log

file were you could see the real IP, although the web server has the

inverse lookup activated.


[Fri Feb 28 10:39:46 2003] [error] [client] File does not

exist: C:/Archivos de programa/Apache Group/Apache2/htdocs/badrequest.html

So, while there aren?t errors, the real IP is not showed. This can lead

in a complete anonymous http access for a client in a usual web surfing

activity, that is, if there are not broken links,etc.

In the case of Iplanet 6, the real IP wouldn?t appear in the ?access?

preview (see link below):



Neither in the ?errors? preview (see link below):




(Succesfully exploited on Apache 2.0.44 on Windows/Linux, on IIS 6.0 and

Iplanet 6 on Windows)

Scenario: a machine with a hostname as ?<scrip>alert(?a?)</script>? that

makes an HTTP request leaves javascript code on the log.

When generating a report, with some log analyzers (that show results in

html), the script will be executed.

*Note: in IIS 6.0 case we needed to restrict access on webserver by domain

name in order to force inverse lookup resolution.

*Note2: in the Iplanet case we needed to simulate a FQDN client host name

like this:


You can also set a host name were the script is only part of the entire

string label:


so when html formatted it will appear as a valid domain name:


Meanwhile the script will be executed?

Some log analyzers proved to be vulnerable to "ILLC":

WebTrends (see link below):



SurfStats (see link below):



WebLogExpert (see link below):



And probably many more?

Iplanet comes with a buil-in tool to generate html reports based on access

and error logs. This tool is part of the administration web interface.

Moreover, Iplanet log analyzer always uses a web broser to show the

results of the report, although the user selects ?Only text output?, so it

will be always exploitable.

Iplanet Log Analyzer (HTML report, see link below):



Iplanet Log Analyzer (?txt? report, see link below):



On the other hand we have to notice that the access log previewer(?View

Access Log?) in the Iplanet web interface is doing some kind of filtering

on some characters (for example <>).

Iplanet ?View Access Log? (see link below):



-HIDING REQUESTS (Iplanet 6 on Windows)

In the specific case of Iplanet 6, we coul realise that there`s a way to

trick the server on not showing the request in the log preview (?View

Acces Log? and ?View Error Log?). The requests from boxes whose host name

begins with ?format=? will not be showed, that is, those requests still

are visible in the access and error log files, but they would

be ?invisible? from the built-in access and error log viewer of the

administration web interface?(?Last 25 accesses to??). As an example we

made requests from a box with this host name:

?format=.infohacking.com?, and we realize that we can see the request in

the access log:

format=%Ses->client.ip% - %Req->vars.auth-user% [%SYSDATE%] "%Req-

>reqpb.clf-request%" %Req->srvhdrs.clf-status% %Req->srvhdrs.content-

length% "%Req->headers.referer%" "%Req->headers.user-agent%" %Req-

>reqpb.method% %Req->reqpb.uri% %Req->reqpb.query% "%Req->reqpb.protocol%"


format=.winmat.com - - [28/Feb/2003:10:22:25 +0100] "GET /evilrequest.html

HTTP/1.1" 404 292 "-" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0)"

GET /evilrequest.html - "HTTP/1.1" https-script.winmat.com

But on the ?View Access Log? nothing is showed (see link below):



We suppose that server is processing the first part of the host name

string ?format=? as it would be the directive that sets the log format,

and the rest of the string is not recognized as valid format, so nothing

is showed.

Combining the possibility of hiding a request and the Cross Site Scripting

technique we could execute scripts on the machine that runs the Report

Generator of the Iplanet Web Server in a ?stealth? way. We have done this

establishing a host name like this:


(See link below):



Many more evil actions can be done... it only depends on the attacker's


We haven?t checked ?ILLC? on other daemons as ftp, smtp, or firewalls,

IDSs, etc. We think that probably this technique could be used in the same

way in other scenarios.


Exploiting http headers for log corruption


Controlling inverse lookup responses is not always possible for the

attacker. We tried to figure out another, more generic attack to corrupt

web logs.

The first that came to us was to use faked http headers in order to

achieve the same result: execution of scripts by log analyzers.

There are a lot of http headers that can be used to inject code in a log

file. We are not going to discuss all of them in this paper, but only to

outline some generic ways to do it.

The main objective here is to choose the right header to inject code in

the http request? For example, the ?RequestResource? is always showed in

web logs, but probably it will be filtered by many application firewalls

or it will be detected by IDSs? On the other hand the ?UserAgent? header

usually is not being checked for suspicious secuence of characters, and

web masters usually like to have this info on their log files?

An example on how to trick a log analizer to execute a script we

set ?UserAgent? header of our client as:


The requests of our client with this faked ?UserAgent? will inject code in

the web server log. Some log analyzers reading this logs and generating

HTML formatted reports without filtering the output, will execute the


-Examples of vulnerables log analyzers-

WebExpert (see link below):



LoganPro (see link below):



To solve this kind of problems it would be nice a more aggressive

filtering on DNS responses and HTTP requests on all the headers.

To finish this short analisys we would like to make some questions:

Are log analyzers thrusting too mutch on log files?

Maybe, are web servers the ones that would have to filter what they write

to log files??

Is the operating system the one that have to filter the returned values

from DNS servers?

Are the actual legal domain name hosts allowed too mutch liberal?

Sorry for our bad english.

We would like to thank Martí Domenech, director of "Winmat Catalunya", for

letting us doing this "research".

Hugo Vázquez Caramés & Toni Cortés Martínez

Infohacking Research 2001-2003



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