• iPhone zombie

    This is how the end began. Over two years ago I unwittingly planted the seeds of an undead outbreak into the pages of my book, Seven Deadliest Web Application Attacks.

    Only recently did I discover the decaying fruit of those seeds festering within the pages of Amazon. The book had been translated into Korean and I was curious about the translation of a few sentences. So, I went to check a few words in the English version, which was easy to do on Amazon:

    • Visit the book’s Amazon page.
    • Click on the “Look Inside!” feature. (Although apparently no longer available for this title.)
    • Use the “Search Inside This Book” feature to search for zombie.
    • Cower before the approaching horde of flesh-hungry brutes – or just click OK a few times.

    On page 16 of the book there is an example of an HTML element’s syntax that forgoes the typical whitespace used to separate attributes. The element’s name is followed by a valid token separator, albeit one rarely used in hand-written HTML. The printed text contains this line:


    onerror alert zombie

    The “Search Inside” feature lists the matches for a search term. It makes the search term bold (i.e. adds <b> markup) and includes the context in which the search term was found (hence the surrounding text with the full <img/src="."alt="" /> element). Then it just pops the contextual find into the list, to be treated as any other “text” extracted from the book.

    <img src="." alt="" onerror="alert('<b>zombie</b>')"/>

    Finally, the matched term is placed within an anchor so you can click on it to find the relevant page. Notice that the <img> tag hasn’t been inoculated with HTML entities; it’s a classic HTML injection attack.

    <a ... href="javascript:void(0)">
    <span class="sitbReaderSearch-result-page">Page 16 ...t require spaces to
        delimit their attributes.
        **<img src="." alt="" onerror="alert('<b>zombie</b>')"/>** JavaScript
        doesn't have to...

    (You can also use Google Books to see similar results, minus the XSS flaw.)

    This has actually happened before. In December 2010 a researcher in Germany, Dr. Wetter, reported the same effect via <script> tags when searching for content in different security books. He even found <script> tags whose src attribute pointed to a live host, which made the flaw infinitely more entertaining.

    iPad zombie

    In fact, this was such a clever example of an unexpected vector for HTML injection that I included Dr. Wetter’s findings in the new Hacking Web Apps book (pages 40 and 41, the same <img...onerror> example shows up a little later on page 59).

    Behold, there’s a different infestation on page 31 (see also the Google Books result). Try searching for zombie again. This time the server responds with a JSON object that contains <script> tags. This one was harder to track down. The <script> tags don’t appear in the search listing, but they do exist in the excerpt property of the JSON object that contains the results of search queries:

    {...,"totalResults":2,"results":[[52,"Page 31","... encoded characters with
    their literal values:  <a href=\"http://\"/>**<script>alert('<b>zombie</b>')
    </script>**@some.site/\">search again</a>   Abusing the authority component
    of a ...", ...}

    I only discovered this injection flaw when I recently searched the older book for references to the living dead. (Yes, a weird – but true – reason.)

    How did this happen?

    One theory is that an anti-XSS filter relied on a deny list to catch potentially malicious tags. In this case, the <img> tag used a valid, but uncommon, token separator that would have confused any filter expecting whitespace delimiters.

    One common approach to regexes is to build a pattern based on what we think browsers know. For example, a quick filter to catch <script> tags or other opening tags like <iframe src...> or <img src...> might look like this – note the required space character:


    A payload like <img/src> bypasses that regex. The browser correctly parses its valid syntax to create an image element. Of course, the src attribute fails to resolve, which triggers the onerror event handler, leading to yet another banal alert() declaring the presence of an HTML injection flaw.

    The second <script>-based example is less clear without knowing more about the internals of the site. Perhaps a sequence of stripping quotes plus poor regexes misunderstood the href to actually contain an authority section? I don’t have a good guess for this one.

    This highlights one problem of relying on regexes to parse a grammar like HTML. Yes, it’s possible to create strong, effective regexes. However, a regex does not represent the parsing state machine of browsers, including their quirks, exceptions, and “fix-up” behaviors.

    Fortunately, HTML5 brings a degree of sanity to this mess by clearly defining rules of interpretation. On the other hand, web history foretells that we’ll be burdened with legacy modes and outdated browsers for years to come. So, be wary of those regexes.

    Or maybe misusing regexes as parsers wasn’t the real flaw.

    How did this really happen?

    Well, I listen to all sorts of music while I write. You might argue that it was the demonic influence and Tony Iommi riffs of Black Sabbath that ensorcelled the book’s pages or that Judas Priest made me do it. Or that on March 30, 2010 – right around the book’s release – there was a full moon. Maybe in one of Amazon’s vast, randomly-stocked warehouses an oil drum with military markings spilled over, releasing a toxic gas that infected the books. We’ll never know for sure.

    Maybe one day we’ll be safe from this kind of attack. HTML5 and Content Security Policy make more sandboxes and controls available for implementing countermeasures to HTML injection. But I just can’t shake the feeling that somehow, somewhere, there will always be more lurking about.

    Until then, the most secure solution is to –

    – huh, what’s that noise at the door…?

    They're coming to get you, Barbara.

    • • •
  • LinkedIn, HashedOut

    The Foole Rides Mee

    Linked – “Be great at what you do” – In, bringing you modern social networking with less than modern password protection – like, about 1970s UNIX modern. The passwords in this dump not only rejected a robust, well-known password hashing scheme like PBKDF2, they didn’t even salt the passwords. As a historical reference, salts are something FreeBSD introduced around 1994.

    It also appears some users are confused as to what constitutes a good password. Length? Characters? Punctuation? Phrases? An unfortunate number of users went for length, but neglected the shift key, space bar, or one of those numbers above qwerty.

    I sat down for 20 minutes with shasum and grep – plus my bookshelf for inspiration – to guess some possible passwords without resorting to a brute-force dictionary crack.

    grep `echo -n myownpassword | shasum | cut -c6-40` SHA1.txt

    The grep/shasum trick works on Unix-like command lines. John the Ripper is the usual tool for password cracking without entering the super assembly of GPU customization.

    I love sci-fi and fantasy. I still run an RPG session on a weekly basis; there’s no dust on my polyhedrals. Speaking of RPGs. I started the guesswork with 1st Edition AD&D terms only to strike out after a dozen tries, but the 2nd edition references fared better:

    waterdeep – Under Mountain was awesome, unlike your password.

    menzoberranzan – Yeah, mister dual-scimitars shows up in the list, too. This single-handedly killed the Ranger class for me. (Er, not before I had about three rangers with dual longswords. ‘Cause that was totally different…)

    No one seems to have taken “1stEditionAD&D”. Maybe that’ll be my new password – 14 characters, a number, a symbol, what’s not to love? Aside from this retroactive revelation?

    tardis – Come on, that’s not even eight characters. Would tombaker or jonpertwee approve? I don’t think so. But no Wiliam Hartnell? Have you no sense of history? Even for a timelord?

    doctorwho – Longer, but…um…we just covered this.

    badwolf – Cool, some Jack Harkness fans out there, but still not eight characters.

    torchwood – Love the show, but your anagram improves nothing.

    kar120c – I’m glad there’s a fan of The Prisoner out there. It was a cool series with a mind-blowingly bizarre, pontificating, intriguing ending that demands discussion. However, not only is that password short, it even shows up in my book. I should find out who it was and send them a signed copy.

    itsatrap – Seriously? You chose a cliched, meme-laden movie quote that short? And you couldn’t be bothered with an exclamation point at the end? At least you chose a line from the best of the three movies.

    myprecious – Not anymore.

    onering – Onering? While you were out onering your oner a password cracker was cracking your comprehension of LotR. By the way, hackers have also read earthsea, theshining and darktower. Hey, they’ve got good taste.

    I adore the Dune books. Dune is near the top of my favorites. Seems I’m not the only fan:

    benegesserit – Don’t they have some other quotes? Something about fear?

    fearisthemindkiller – Heh, even the hackers hadn’t cracked that one yet. Referencing The Litany Against Fear would have been a nice move except that if “fear is the mind killer” then “obvious is the password.”

    entersandman, blackened, dyerseve – What are you going to do when you run out of Metallica tracks? Use megadeath? It’s almost sadbuttrue. And jethrotull beat them at the Grammy’s. So, there.

    loveiskind – Love is patient, love is kind, hackers aren’t stupid, passwords they find.

    h4xx0r – No, probably not.

    notmypassword – Actually, it is. At least you didn’t choose a 14-character secretpassword. That would just be dumb.

    stevejobs – Now how is he going to change his password?

    • • •
  • An exposition on the first of the Twelve Web (In)Security Truths

    #1 – Software execution is less secure than software design, but running code has more users.

    A Chronograph

    A running site is infinitely more useable than one that only exists in design. Talk all you want, but eventually someone wants you to deliver that design.

    Sure, you could describe Twitter as a glorified event loop around an echo server. You might even replicate it in a weekend with a few dozen lines of Python and an EC2 instance. Now scale that napkin design to a few hundred million users preserving security and privacy controls. That’s a testament to implementing a complex design – or scaling a simple design if you boil it down to sending and viewing tweets.

    It’s possible to have impressive security through careful design. A prominent example in cryptography is the “perfect secrecy”1 of the One-Time Pad (OTP). The first OTP appeared in 1882, designed in an era without the codified information theory or cryptanalysis of Claude Shannon and Alan Turing.2 Never the less, its design understood the threats to confidential communications when telegraphs and Morse code carried secrets instead of fiber optics and TCP/IP. Sadly, good designs are sometimes forgotten or their importance unrecognized. The OTP didn’t gain popular usage until its re-invention in 1917, along with a more rigorous proof of its security.

    But security also suffers when designs have poor implementations or have complex requirements. The OTP fails miserably should a pad be reused or is insufficiently random. The pad must be as long as the input to be ciphered. So, if you’re able to securely distribute a pad (remember, the pad must be unknown to the attacker), then why not just distribute the original message? Once someone introduces a shortcut in the name of efficiency or cleverness the security breaks. (Remember the Debian OpenSSL debacle?) This is why it’s important to understand the reasons for a design rather than treat it as a logic table to be condensed like the singularity of a black hole. Otherwise, you might as well use two rounds of ROT13.

    Web security has its design successes. Prepared statements are a prime example of a programming pattern that should have relegated SQL injection to ancient CVEs. Only devotees of Advanced Persistent Ignorance continue to glue SQL statements together with string concatenation. SQL injection is so well-known (at least in appsec) and studied that a venerable tool like sqlmap has been refining exploitation for over six(teen) years.

    Yet the Internet loves to re-invent vulns. Whether or not SQL injection is in its death throes, NoSQL injection promises to reanimate its bloated corpse. Herbet West would be proud.

    Sometimes software repeats the mistakes, intentionally or not, of other projects without understanding the underlying reasons for those mistakes. The Ruby on Rails Mass Assignment feature is reminiscent of PHP’s register_globals issues. Both are open source projects with large communities. It’s unfair to label the entire group as ignorant of security. But the question of priorities has to be considered. Do you have a default stance of high or low security? Do you have language features whose behavior changes based on configuration settings outside the developer’s control, or that always have predictable behavior?

    Secure design is never easy. Apache’s reverse proxy/mod_rewrite bug went through a few iterations and several months of discussion before Apache developers arrived at an effective solution. You might argue that the problem lies with users who created poor rewrite rules that omitted a path component. But I prefer to see it as a design flaw because users had difficulty understanding its nuances and mistakes lead to security issues. Either way, the vuln proved how difficult it is to choose between trade-offs in security decisions.


    HTML injection is another bugbear of web security. (Which makes SQL injection the owlbear?) For the longest time there was no equivalent to prepared statements for building HTML on the fly. Developers had to create bespoke solutions for their programming language and web architecture. Then came frameworks like React. It did away with the string concatenation that lead to HTML injection and cross-site scripting. The framework knew how to correctly place arbitrary content so that it remained a text node rather than become a JavaScript execution context.

    React even preserved the ability to write insecure HTML, but it did so in a way that was obvious in its design as dangerouslySetInnerHTML. This also made it easier to run linters for identifying areas of risk in the code. Alas, React didn’t even appear until a year after the original version of this article.

    The Content Security Policy has been trying to bring a secure design against HTML injection with mechanisms that restrict how a page may load resources. CSP doesn’t prevent HTML injection – it mitigates its exploitability. So, developers must still invest in frameworks like React or other ways of preventing these XSS in the first place. CSP feels like a well-intentioned design, but it suffers from being placed at the point of exploitation (the browser) as opposed to the point where flaws are introduced (the app). Despite more than ten years of design and an iteration to CSP Level 3, this is the type of security design that places a lot of burden on developers to implement without a lot of compelling benefits to justify that burden.

    Secure design should be how we send whole groups of vulns to the graveyard. Good security models understand the threats a design counters as well as those it does not. Spend too much time on design and the site will never be implemented. Spend too much time on piecemeal security and you risk blocking obscure exploits rather than fundamental threats.

    Dune Cover

    As the ancient Fremen saying goes, “Truth suffers from too much analysis.”3 Design also suffers when its scrutiny is based on nonspecific or unreasonable threats. It’s important to question the reasons behind a design and the security claims it makes. Yes, HSTS relies on the frail security of DNS – it’s a trust on first use (TOFU) model where the browser assumes the header comes from an authentic source.

    However, HSTS improved the reliability of maintaing HTTPS connections and minimizing the impact of malicious CAs, but it also introduced risk. A misconfigured HSTS header could create a self-induced DoS by preventing browsers from connecting. And that misconfiguration might come from a developer mistake or an attacker with the ability to set headers from a compromised server.

    Design your way to a secure concept, code your way to a secure site. When vulns appear determine if they’re due to flaws in the design or mistakes in programming. A design that anticipates vulns, like parameterized queries, should be easy to implement and serve the developer’s needs. Vulns that surprise developers should lead to design changes that provide more flexibility for resolving the problem.

    Inflexibility, whether in design or implementation, is dangerous to security. As the Bene Gesserit say, “Any road followed precisely to its end leads precisely nowhere.”4

    1. In the sense of Claude Shannon’s “Communication Theory of Secrecy Systems”. 

    2. As Steven Bellovin notes in his paper, an 1882 codebook contains an amusingly familiar phrase regarding identity questions, “Identity can be established if the party will answer that his or her mother’s maiden name is…“ It seems identity proofs haven’t changed in 140 years! 

    3. Frank Herbert. Dune Messiah. p. 81. 

    4. Frank Herbert. Dune. p. 69. 

    • • •