Apache Struts vulnerability (CVE-2017-5638) and the importance of RFC standards compliance

The recent Apache Struts vulnerability (CVE-2017-5638) has highlighted how quickly a vulnerability disclosure can morph into an exploitation frenzy.

From the initial disclosure announcement on March 6th, signs of testing and exploitation attempts in the wild materialised in less than 24 hours as the gravity of the situation sunk in.

The issue, being a specially crafted “Content-Type:” MIME header, was found to permit remote code execution (RCE) through calling Java to create a new java.lang.ProcessBuilder() object, executing the required arbitrary command on the web server without authentication. Further details about the exploit in action can be found here.

The challenge for many organisations facing this issue, is to be able to react in a timely fashion, to mitigate the risks of their own systems being exploited at the cost of loss of service, loss of confidentiality, and loss of own or customer data.

Under a backdrop of service management procedures and development safeguards, for many companies, simply upgrading the affected component may take days or weeks to implement, and meanwhile, attackers are all over your services like a rash.

The exploit used a specially crafted “Content-Type:” header, such as detailed here and the same as or simplar to the following example:

curl -i -v -s -k  -X 'GET' -H 'User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:51.0) Gecko/20100101 Firefox/51.0' -H 'Content-Type:%{(#nike=\'multipart/form-data\').(#dm=@ognl.OgnlContext@DEFAULT_MEMBER_ACCESS).(#_memberAccess?(#_memberAccess=#dm):((#container=#context[\'com.opensymphony.xwork2.ActionContext.container\']).(#ognlUtil=#container.getInstance(@com.opensymphony.xwork2.ognl.OgnlUtil@class)).(#ognlUtil.getExcludedPackageNames().clear()).(#ognlUtil.getExcludedClasses().clear()).(#context.setMemberAccess(#dm)))).(#cmd=\'echo "add malicious commands here"\').(#iswin=(@java.lang.System@getProperty(\'os.name\').toLowerCase().contains(\'win\'))).(#cmds=(#iswin?{\'cmd.exe\',\'/c\',#cmd}:{\'/bin/bash\',\'-c\',#cmd})).(#p=new java.lang.ProcessBuilder(#cmds)).(#p.redirectErrorStream(true)).(#process=#p.start()).(#ros=(@org.apache.struts2.ServletActionContext@getResponse().getOutputStream())).(@org.apache.commons.io.IOUtils@copy(#process.getInputStream(),#ros)).(#ros.flush())}' \   'http://vulnerable.web.site.com/struts2-blank/example/HelloWorld.action'

Now while many people are focusing on the "#cmd=" string to identify this being exploited, this is just a variable within the exploit, the real dodgyness happens when “new java.lang.ProcessBuilder(#cmds)” happens. So the problem is being able to call the java method to arbitrarily execute a command.

But is that it? maybe not. You might think ah! i’ve cracked the problem – prevent java from being used, but the problem is actually far more simpler than that.

Now what does this exploit have to do with standards compliance?

Back in June 1992, two people, Nathaniel Borenstein and Ned Freed devised RFC1341 – entitled “MIME (Multipurpose Internet Mail Extensions): Mechanisms for Specifying and Describing the Format of Internet Message Bodies”.

Within this document, in section 4, it describes in Backus-Nour-Form the structure of a “Content-Type:” declaration (as also detailed by W3C here).

That extract is as follows:

Content-Type := type "/" subtype *[";" parameter]

type := "application" / "audio" / "image" / "message" / "multipart" / "text" / "video" / x-token

x-token :=

subtype := token

parameter := attribute "=" value

attribute := token

value := token / quoted-string

token := 1*

tspecials := "(" / ")" / "<" / ">" / "@" / "," / ";" / ":" / "\" / <"> / "/" / "[" / "]" / "?" / "." / "="

The above tspecials must be in quoted-string, to use within parameter values.
Note that the definition of "tspecials" is the same as the RFC 822 definition of "specials" with the addition of the three characters "/", "?", and "=".

You might be thinking…”so what?”…well, the interesting thing is that the exploit was permitted due to lack of validation.

The exploit opened up its “Content-Type” value with the characters “%{“. Note that neither of these characters are permitted within the BNF specification for what a tspecial within a “Content-Type:” declaration should look like in RFC1341.

As such – it is evident that the impact of the exploit was accelerated due to lack of input validation, partly because the exploit was materialised using characters outside of the permitted structure.

While this validation itself would not have resolved the vulnerability in Apache Struts from existing, such validation would have significantly mitigated the impact of its exploitability.

Standards such as the RFC’s provide some great insight into what should be performed in terms of input validation. I am sure there are many other vulnerabilities and examples of where well defined constructs can be abused by using inputs not specified within the documented construct. It is for this reason that input validation is so important.

For those developing web application firewall rules, simply detecting a curly brace within the “Content-Type:” declaration would be sufficient to have mitigated this exploit and probably many more, I am sure this kind of logic could be applied to a whole host of declarations to improve security.

Seeing this exploit proven in testing brought only a sense of awe of how easy it was using this exploit to effectively walk through an open door. I am sure this exploit will have many reprocussions in weeks to come. The show is not over by far.


BIND DNS query log shipping into a MySQL database

BIND DNS query log shipping into a MySQL database

Yay!, I’ve been wanting to do this for a while! Here it goes:-

Documented herein is a method for shipping BIND DNS query logs into a MySQL database and then reporting upon them!

Note: SSH keys are used for all password-less log-ons to avoid prompt issues

BIND logging configuration

BIND named.conf query logging directive should be set to simple logging:-


  # Your other log directives here

  channel query_log {
    file "/var/log/query.log";
    severity info;
    print-time yes;
    print-severity yes;
    print-category yes;

  category queries {

The reason why a simple log is needed is because the built-in BIND log rotation only allows rotation granularity of 1 day if based on time, hence an external log rotation method is required for granularity of under 24 hours.

BIND query log rotation

My external BIND log rotation script is scheduled from within cron and it looks like this:-


if [ -e $LOCK_FILE ]; then
  if [ ` ps -p $OLD_PID > /dev/null 2>&1 ` ]; then
    exit 0
echo $$ > $LOCK_FILE

cat $QLOG > $QLOG.`date '+%Y%m%d%H%M%S'`
if [ $? -eq 0 ]; then
  > $QLOG
service named reload

rm -f $LOCK_FILE

Place this in the crontab, working at between one and six hours, ensure it is not run on the hour or at the same time as other instances of this job on associated servers

make sure /var/named/chroot/var/log/old exists for file rotation, used in the data pump script later on.

From here, I create a MySQL table, called dnslogs with the following structure:-

create table dnslog (
  q_server   VARCHAR(255),
  q_date     VARCHAR(11),
  q_time     VARCHAR(8),
  q_client   VARCHAR(15),
  q_view     VARCHAR(64),
  q_text     VARCHAR(255),
  q_class    VARCHAR(8),
  q_type     VARCHAR(8),
  q_modifier VARCHAR(8)

You can either define a database user with a password and configure it such in the scripts, or you can configure a database user which can only connect and insert into the dnslogs table.

Then I use the following shell script to pump the rotated log data into the MySQL database:-

PATH=/path/to/specific/mysql/bin:$PATH export PATH
NAME_SERVERS="your name server list here"


if [ -e $LOCK_FILE ]; then
  if [ ` ps -p $OLD_PID > /dev/null 2>&1 ` ]; then
    exit 0
echo $$ > $LOCK_FILE
for host in $NAME_SERVERS; do
  REMOTE_LOGS="`ssh -l $SSH_USER $host find $LOG_DIR -maxdepth 1 -name $LOG_REGEX | sort -n`"
  test -n "$REMOTE_LOGS" && for f in $REMOTE_LOGS ; do
    ssh -C -l $SSH_USER $host "cat $f" | \
      sed 's/\./ /; s/#[0-9]*://; s/: / /g; s/\///g; s/'\''//g;' | \
        awk -v h=$host '{ printf("insert into '$DEST_TABLE' values ( 
STR_TO_DATE('\''%s %s.%06s'\'','\''%s'\''), 
$3 * 1000, 
"%d-%b-%Y %H:%i:%S.%f", 
); }' | mysql -A -S $DB_SOCK -u $DB_USER --password=$DB_PASS $DB_NAME 2> $ERROR_LOG
    if [ $RETVAL -ne 0 ]; then
      echo "Import of $f returned non-zero return code $RETVAL"
      test -s $ERROR_LOG && cat $ERROR_LOG
    ssh -l $SSH_USER $host mv $f ${f%/*}/old/

Put this script into a file and schedule from within crontab, running some time after the rotate job suffice to allow it to complete, but before the next rotate job.

Note that the last operation of the script is to move the processed log file into $LOG_DIR/old/.

This will take each file in /var/named/chroot/var/log/query.\* and ship it into the dnslogs table as frequently as is defined in the crontab.

From here, it is possible to report from the db with a simple query method such as:-

PATH=/path/to/specific/mysql/bin:$PATH export PATH


if [ -e $LOCK_FILE ]; then
  if [ ` ps -p $OLD_PID > /dev/null 2>&1 ` ]; then
    exit 0
echo $$ > $LOCK_FILE

echo "select * from dnslogs where q_text like '$SQL_REGEX';" | \
  mysql -A -S $DB_SOCK -u $DB_USER --password=$DB_PASS $DB_NAME

rm -f $LOCK_FILE

And there it is! SQL reporting from DNS query logs! You can turn this into whatever report you like.

From there, you may wish to script solutions to partition the database and age the data.

Database partitioning should be done upon the q_timestamp value, dividing the table into periods which align with the expectation of the depth for which reporting is expected. On a minimal basis, I would recommend keeping at least 4 days of data in partitions of between 24 hours and 1 hour, depending upon the reporting expectations. If reports are upon the previous day’s data only, then 1 partition per day will do, while reports which are only interested in the past hour or so will benefit from having partitions of an hour. in MySQL, sub-partitions are not worthwhile because they give you nothing more than partitions but adds a layer of complexity on what is otherwise a linear data set.
Once partitioning is established, it should be possible to fulfill reports by querying only the relevant partitions to cover the time span of interest.
Partitioning also has another benefit, which is data aging. Instead of deleting old records, it is possible to drop entire partitions which cover select periods of time without having to create a huge temporary table to hold the difference as would be required by a delete operation. This becomes an extremely useful feature if you have a disk with a table size which is greater than the amount of free space available.

Script updates for add and drop partition to follow….

Shell Tricks Part 1 – Substituting basename, dirname and ls commands

Substituting basename, dirname and ls commands

In Bourne shell, it is possible to use the following variable expansions as substitutes for the basename, dirname and ls commands

$ MYVAR=/path/to/basename
$ echo ${MYVAR##*/}
$ MYVAR=/path/to/dirname
$ echo ${MYVAR%/*}
$ echo *
bin boot dev etc home lib lost+found mnt opt proc root sbin tmp usr var

Hows That?

Quick and dirty data wiping

How to wipe a disk with pre-determined bit patterns

Many of you who know me may have heard of my ‘mythical’ data wiping script, which I have maintained can be done in just a few lines shell script on pretty much *any* UNIX box. Well, here it is:-

for pattern in 85 170 ; do

awk -v p=$pattern ‘END {while (1) printf(“%c”,p);};’ \

< /dev/null > /path/to/device/or/file


The patterns 85 and 170 represent 01010101 and 10101010 in 8-bit binary. These patterns can be replaced with any sequence which can be generated or pre-defined prior to the wiping run.

This command converts the decimal value of $pattern into a binary pattern using awk. The output is then generated until the target device reaches the end of the media or fills the containing filesystem.

Please use with caution, my overly simplified version does not check what it is writing to – I accept no responsibility for damages arising as a result of using this script or any derivative works.

Real Security: A GravityLight in the darkness

Guys and Gals,

Today I present to you something far more important than dealing with a technology disaster. The need for light and energy.

Everyone should get behind this project:-



These guys have developed an amazing product! a light which works on gravity alone!, while not a completely novel concept, these guys have packaged it into something portable, simple and hopefully reliable.

Intended for developing countries to reduce dependence on relatively expensive and unhealthy kerosene lamps, it represents an essential survival tool for all because when the candles have all burned out and the batteries are all flat, having some light source can be essential.

Give them some ca$h and help them on their way.

AV Comparatives

Today, let me introduce you to AV Comparatives, a trusty AV testing lab which will open your eyes to how good your anti-virus is. I have used these guys for many years to consider my options on AV.

Disclaimer: Don’t be fooled by the sell of McAfee and Symantec – they are *NOT* the best AV products by a country mile.

The reports from AV Comparatives shows the difference between “out-of-the-box” and “configured-for-security” effectiveness. This provides an interesting and sometimes scary revelation.

Please pay special attention to the historical reviews for proactive tests. The teams that score best consistently on this test do better overall because if they are on-top for 0day threats then the historical virus detection is, as they say, “history”. You can see developer drain happen when a product slips from it’s ranking where a developer leaves or the company generally lags.

For Windows, I normally use Avira Free with secure-start and detection of all categories including jokes and games. Just taking another look, I guess I might reconsider…..maybe Avast?

I’d like to try QiHoo but I can’t read Chinese and I’m not sure i trust a ‘free’ product which is difficult to find on Google and intended for a single-country only market (you can’t even find it easily on Baidu!). Chinese users – please leave comment on this point and let me know what your experience with QiHoo AV is like!

Meanwhile, I’m on Linux, so ClamAV will do for now.

sendmail relaying nightmare!

While I’m hot on the topic – I’ve just spent a whole afternoon/evening trying to figure out why my sendmail installation keeps on becoming an open-relay every time i configure my desired domains! – which I have now figured out!

While listing my desired domains in the access file, or in the relay-domains file, it seemed to turn my sendmail host into an open-relay.

It turns out that access and relay-domains supports relay for all valid hosts and sub-domains within the DNS domains permitted for relay, hence all hosts with a valid DNS A record within the defined domains becomes a valid source of mail! As my testing point had a valid DNS record within the permitted domain (and I did check to see whether it was an open-relay), the host allowed relay based on membership to the permitted domains.
This effectively made my sendmail box an open-relay to all internal hosts with a DNS name.

This was fixed with a FEATURE:-


This sanitised my security from internal abuse! and made my access file work as intended, supporting explicitly listed hosts and domains only.


Update: I later realised that the domain names I was configuring also had ‘A’ records in DNS for the top-level domain. As these hosts were not valid mail sources for this relay, I had to explicitly configure a REJECT action within the access file for all of the IPs named in an ‘A’ record lookups on the given domain names within the access or relay-hosts file in order to deny an implicit behavior which is the consequence of permitting a given domain.


So….some things to remember for Sendmail:-


Any domain listed in the access file or relay-domains file will allow ‘open’ relay for all hosts :-


1) Within the visible DNS structure beneath the defined domain (unless you use “FEATURE(`relay_hosts_only’)dnl”)

2) Defined as an ‘A’ Record for the given domain name as returned by DNS.
Does your Sendmail MTA relay to the hosts you intend?