Tag Archives: java

Logback and space in the XML configuration

Please see this issue that I created for LogBack and was accepted to be taken care of in v1.3.0:


The fix should allow for a stack trace or some warning message if the level value is incorrect as it was when I had inadvertently used a space character as a suffix as in “WARN “ instead of “WARN” and it led to the library defaulting to “DEBUG”.



InetAddress, InetSocketAddress and multiple IP addresses assignment to a single host

InetAddress: This address maps to a single IP address for a host name even though the host name might resolve to multiple IP addresses.

One way of creating an InetAddress:

Here the “host” argument can be either a string representation of the IP Address (as in “x.x.x.x.” for IPv4) or a host name. If it is an IP Address then an InetAddress instance is returned with that IP. In such cases, the InetAddress instance would be bound to the IP specified in the host. If that host has multiple IP addresses then this InetAddress instance would remain bound to the IP address passed in. That returned instance could be an instance of the two subclasses of InetAddress for IPv4 and IPv6.

=> This constructor, if passed an IP address would invoke the InetAddress.getByName(String host) with the IP address and there would not be any reverse lookups.

Java 8 Streams: difference between map and flatMap

Difference between map() and flatMap():
The map() operates on the individual elements of the stream and returns a value as a Stream with the results. The flatMap() operates on the individual elements of the stream and returns a value as a Stream with the results. In this they are similar.
However the map() takes in a function that returns any R (could be a Stream as well) whereas the flatMap() operation takes in a function that returns a Stream.


Streams employ the CRTP (Curiously Recurring Template Pattern).

For instance:


Levenshtein Part 2: Dynamic Programming

Proceeding from the earlier post on the same topic, we have the following code for a Dynamic Programming bottom-up approach to the Levenshtein “Edit Distance” algorithm where all the weights are simply “1” for insert, delete, substitute operations. Note that this would be also referred to the “tabular” approach as well since we fill up a matrix as we go along.

In a later post, we would attempt to reduce the space requirement from O(m.n) to a constant. In another post, we would provide a way to configure the weights and thereby providing weighted edges to the DAG.


And the test code:

Levenshtein: elementary recursion – a gateway to DP

This is my first post on Levenshtein Algorithm: a way to calculate the edit distance between two strings. A way to calculate the cost to transmute a string1 to a string2.

The comments in the codebase are useful for comprehension on their own. I start with the recursing and building a matrix where the answer (the cost to transmute or convert str1 into str2) lies at index [m][n] for the first implementation. For the second case, the cost would be at index [0][0].

This is the gateway to elucidating DP through the implementation of this algorithm. We would implement in ways that classify as DP (both memoization and tabulation) as well as plain and simple recursion or iteration with no DP components. We start with vanilla recursion in the following codebase. Please review the comments in the codebase to understand why this is not DP. A future post would elucidate DP.


A snippet of the test code:

Spring 4.1 (Web MVC) and earlier versions and the MVC trailing slashes issue on Tomcat

There is an issue with Spring Web component in terms of handling slashes “forward slash” when Tomcat is in the mix. Found the issue when our web application in Production received GET requests with 2 trailing slashes in the path info and the Spring Dispatcher Servlet was not able to map it to a controller even though the Spring Dispatcher Servlet is the root servlet.

Filed an issue with the Spring Jira and it has been corroborated and it is scheduled to be fixed in an upcoming release.

Please see the following issue in the Spring Jira:


BTW: the quick fix is to map the Spring Dispatcher Servlet with a wildcard as in:

OpenSSL Heartbeat vulnerability – Heartbleed and Java, BouncyCastle, How to write a program to check

To write a program to validate if a particular web server that runs on a version susceptible to “heartbleed”, one could use the plethora of free tests on the web such as the ones linked from the Wikipedia link on the subject. I am the author of the heartbleed test for Symantec SSL at: “https://ssltools.websecurity.symantec.com/checker/views/certCheck.jsp“.  Note: the codebase in that is completely different and follows a completely different approach to what I am going to release to the general public: a sample code that demonstrates heartbeat requests with BouncyCastle.

However, if the server that is to be verified is not accessible from the outside, you would need to write your own or download one. There are quite a few of python, Go and even one that details the changes to be made to OpenSSL s_client and use them to discover the vulnerability. Some of them work, some some of the time and when they do not, one need to understand why.

The following sections would briefly explain the OpenSSL vulnerability and the fix and how to write one of your own.

The vulnerability

An improper  heartbeat (HB) request would lead to a vulnerable web server leaking the content in its memory. This content could be a secret key or its password and so on.

While testing my HB tester program, noticed that there were attacks on my external facing web server at a rate of 2 every 10 minutes. There are a lot of attacks going on at this time.

The HB request

The request is in the following format:

HB request => ContentType (1 byte) : TLS Version (2 bytes: major and minor) : Record Size (2 bytes) : Encrypted or not Encrypted bytes

An example in hex: 18 0303 00cc and then the encrypted or not encrypted HB message follows.


Encrypted or not Encrypted bytes => HB message Type : Payload Size : Payload : Padding

An example in hex for a non encrypted HB request message: 1 00cf and the actual payload and padding follow.

An improper HB request

An improper or attack vector request would create a payload of size less than what it specifies in the “Payload”. As simple as that.

An improper HB request to a vulnerable OpenSSL installation would result in it returning a HB response. The same request to a patched / not vulnerable OpenSSL (or any other web server that is not susceptible to HB) would result in a TLS alert.

The vulnerability in a little more detail

The malformed message results in an affected OpenSSL version returning a payload of the same size that is specified in the HB request irrespective of the real payload in the request. The affected OpenSSL version does not validate this aspect and the response payload is read from the memory going way beyond and leaking memory contents.

 The patch

The patch is a test of the payload length specified in the request and the actual payload size.

if (1 + 2 + payload + 16 > s->s3->rrec.length) return 0;
/* silently discard per RFC 6520 sec. 4 */

If the 1 byte that specifies the size of the HB record type (request in this case) plus the 2 bytes of that specify the payload length plus the size of the payload that is specified plus the minimum size of the padding (that is 16 bytes) is GREATER than the record length then no HB response is to be returned.

Design of a program to test for this vulnerability using Java

  1. Create a Java Socket to the web server
  2. Get a sample of a TLS ClientHello from TCPDUMP (Use s_client to send in a HB request and capture the packets to get the sample) (Make sure that it has the Heartbeat Extension setup to allow for heartbeats).
  3. Write the TLS ClientHello bytes to the socket
  4. Read the ServerHello response bytes till the end
  5. Sent in the malformed HB request (construct it in the way that is described above)
  6. Check the response: if it not an TLS alert then the web server is vulnerable. The server could reset the connection or timeout and we can assume – in these cases – that the server is not vulnerable. However there is a infinitesimal chance of a false negative especially in case of a connection timeout (and consequently the server certified to be not vulnerable) then the connection time out could be due to an actual connection issue! Note that to circumvent the heartbleed issue, network adminstrators have deployed firewall rules that would time out a heartbeat request – valid or not.

There are a huge number of types of web servers out there. The above steps would in all probability work in properly diagnosing this vulnerability in Apache and Nginx on Linux but would fail with IBM HTTP Web Server or IIS. In such a scenario, one would parse the complete ServerHello and check whether it is an extended ServerHello and if it is would check for the existence of the heartbeat extension.

Also note, that the SSL version 3.0 RFC does not allow an extended ClientHello or a ServerHello so the suggestion is to use a TLS 1.0 ClientHello in this case.

The other alternate approach would be to use BouncyCastle and send in heartbeat requests after the TLS session has been established: check out TLSProtocolHandler. If I have time, I would post that piece of code for your perusal. However as of now, I have experimented with BouncyCastle and have been successful in:

  • Establishing a TLS session
  • Sending in an encrypted “valid” TLS heartbeat request and received an encrypted heartbeat response.
  • Sending in an encrypted “invalid” TLS heartbeart request and received an encrypted heartbeat response if the server is vulnerable to heartbleed. Else, we receive an Alert, connection reset or a socket read timeout that points to a patched or a server unaffected by heartbleed.

NB: It seems to me that a valid heartbeat request is only allowed by OpenSSL after a TLS session has been established. I have tested (and you can test either with s_client or your own tool perhaps using BouncyCastle) sending a valid heartbeat request after establishing a TLS session. I established a valid TLS session and sent in an encrypted heartbeat and was able to elicit a heartbeat response using java and bouncycastle. I have not cleaned out the code and once I do will post it. So empirically, it seems that even in OpenSSL versions that are broken, a valid heartbeat request right after ServerHelloDone is disallowed. That would be the reason that a heartbeat response is not forthcoming for a valid heartbeat request send before the TLS handshake is complete.

Happy testing.