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back to article Anatomy of a killer bug: How just 5 characters can murder iPhone, Mac apps

There has been much sniggering into sleeves after wags found they could upset iOS 6 iPhones and iPads, and Macs running OS X 10.8, by sending a simple rogue text message or email. A bug is triggered when the CoreText component in vulnerable Apple operating systems tries to render on screen a particular sequence of Unicode …

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        1. Anonymous Coward
          Reply Icon
          Anonymous Coward

          Re: Compulsory casts.

          What compiler doesn't know the types of arguments?

          A C compiler :-)

          Well it knows what they are, but it won't save you when you decide you know better.

          5 0 Reply
      2. Tridac
        Reply Icon

        Re: Compulsory casts.

        Ime, it's typical of modern programming idiom in some areas, where people are sloppy about typing and sloppy about algorithm design, or just plain cut and paste bad code from elsewhere. K&R and other programming texts actually encourage this by using signed types (char, int etc) as the default, but signed types should only be used where actually needed, ie, for arithmetic ops, with the default unsigned everywhere else. It's the same story with the original C standard library, which tended to use (signed) int or char for everything. There, the function typing is a real mess, but may have a historical context excuse..

        Back in the days of assembler, it was quite common to find bugs eg: compare or increment / decrement op, followed by a signed branch when processing unsigned values. The bug was perhaps discovered much later when the value flipped the overflow or sign bit. If you do a lot of embedded work, failure to use of the correct type is something you ignore at your peril and standards like Misra specifically warn against this sort of thing.

        Programming != Software engineering :-)...

        Chris

        2 0 Reply
        1. Stevie
          Reply Icon

          Re: Compulsory casts.

          "Programming != Software engineering"

          True, but when I was a Cobol database programmer we knew enough to guard against this stupidity when zipping in and out of separately collected subroutines via the Linkage Section.

          Tsk! Baby out with the bathwater again, youth of today, general lack of wherewithal, three world wars, shrapnel in the head, wouldn't've happened in my day, flogging too good for them etc etc.

          2 0 Reply
    2. fajensen
      Reply Icon
      Coat

      Re: Compulsory casts.

      Thats what the #pragma was designed to fix ....

      0 0 Reply
  2. Anonymous Coward
    Anonymous Coward

    "Perhaps Apple could stop being so stingy with its billions and set up a bug bounty program"

    Bounty? Well I thought you got a free Mars bar for your million dollar saving discoveries.

    1 0 Reply
    1. Anonymous Dutch Coward
      Reply Icon
      Coat

      Bounty?

      Or what about some KitKats ;)

      0 0 Reply
    2. fajensen
      Reply Icon

      Perhaps apple developers would all retire rich then?

      0 0 Reply
  3. Graham Dawson Silver badge

    BLIT

    wink

    0 0 Reply
    1. cordwainer 1
      Reply Icon
      Pint

      Re: BLIT

      I was so, so tempted to reply, "Don't be a blittering idiot."

      0 0 Reply
  4. Anonymous Coward
    Anonymous Coward

    " If someone is this lazy then god knows what else is lurking in their code."

    3rd party piece work possibly. You get no rewards for putting your guilty hand up in the air.

    0 0 Reply
  5. Anonymous Coward
    Anonymous Coward

    So that's what it was ..

    I noticed this on a couple of sites I frequent but due to it occuring in both safari and chrome, I concluded it was a webkit bug and would therefore remain unpatched for ages.

    Nice to know what's happening ... +1

    0 0 Reply
  6. davemcwish
    IT Angle

    Technical Article in ElReg ?

    Looks like a Mac equivalent to a Mark Russinovich’s technical blog posting. very interesting, even for a non Mac owner.

    Must lie down now as am more used to clickbait posts ...

    4 0 Reply
  7. Phil O'Sophical Silver badge

    Brings back memories

    Of using VMS mail to send <ESC>[2;9y to people with VT100 terminals. Lots of hilarity (for the rest of the office) until DEC blocked escape sequences in mail.

    3 0 Reply
    1. BristolBachelor Gold badge
      Reply Icon

      Re: Brings back memories

      Talking of VMS, and VT100 terminals, reminds me of much fun in the old days. I remember a certain PAD that would drop the connection if sent something like ZZZZZ (as well as certain control sequences). People used to put them everywhere - including nicknames and in finger IDs on the unix boxes. If you hacked appropriated someone's account, you might set it as their prompt too.

      Oh, it's good to see how far we have come.

      1 0 Reply
      1. Phil O'Sophical Silver badge
        Reply Icon

        Re: Brings back memories

        VT132s were fun, they had a "readback screen" escape sequence. If you crafted the right string in mail you could write something like

        DELETE<CR>EXIT<CR> SYS$SYSTEM:SHUTDOWN<CR> to the system admin's terminal, and instruct the terminal to playback the string. By the time he had rebooted the system all the evidence had gone...

        0 0 Reply
      2. Gunnar Wolf
        Reply Icon

        Re: Brings back memories

        Right, killer sequences... "ZZZZZ as well as certain control sequences", you mention... Well, there was this Hayes patent on inserting a "two second pause" between parts of a command to make their modems not hang up on the (otherwise valid) +++ATH0\n sequence. Of course, most manufacturers did license this, and several manufacturers found ways not to... But I do remember seeing a modem that could be pushed to hang up this way. Very fun for us BBS (ab)users! :)

        0 0 Reply
    2. Ken Hagan Gold badge
      Reply Icon

      Re: Brings back memories

      Ah yes, executable content in messaging systems. It's been a crap idea every single time it has been invented (which appears to have happened almost annually for the past half century).

      4 0 Reply
    3. fajensen
      Reply Icon

      Re: Brings back memories

      Old WYSE graphic terminals, "high-ASCII" chars were magick. One could set the refresh frequency to Zero letting the smoke out. Dumping a binary by mistake to the victims terminal would usually do the trick.

      0 0 Reply
  8. Thomas 4
    Devil

    Reminds me

    ...of the problem with the Google G1 when it first came out. Texting the word "reset" did exactly what it said, much to the ire of my G1 owning friend. =3

    1 0 Reply
    1. Ian 55
      Reply Icon

      Re: Reminds me

      Wasn't that the one that copied everything to a terminal session, so if you typed something that was a valid command, it executed?

      0 0 Reply
  9. John Smith 19 Gold badge
    Happy

    It'll be intersting to see how Apple handles this.

    Given all that "Premium product, because it just works" BS I'd expect them to a) Issue a bug fix b)Scan all their code to find similar code sequences c)Update those as well.

    IOW not just the bug, but the pattern of the bug.

    Let's see if we revisit this gag in the future? Different string, different function, same epic fail.

    Should not be possible, and yet......

    Excellent article and very detailed low level stuff.

    4 0 Reply
    1. ThomH
      Reply Icon

      Re: It'll be intersting to see how Apple handles this.

      It's actually a frustratingly easy mistake to make with Apple's APIs — those CFIndexes pop up in quite a few places — and Xcode ships with the implicit signedness conversions warning disabled. It's one of the things I always enable when I'm starting a new project. Just enabling that would probably help them catch stuff like this.

      That said, if it's a latent problem in initial table setup then the true diagnosis is probably that whomever guarantees the signed value would always be positive needs fixing, so they'd probably just have thrown the explicit cast in and forgotten about it.

      1 0 Reply
  10. M Gale

    Could be worse

    Could have been the unicode bug in all versions of NT4/IIS up to service pack 6a.

    I remember a friend telling me how awesome the BackOffice server he'd set up for his boss was. I told him to get that shit off the Internet now. He refused.

    An hour later I came back to him with a printed directory listing of the server's hard drive and pointed out that I could execute a "format c: /y" just as easily.

    Within an hour, the server had been firewalled.

    3 3 Reply
    1. NumptyScrub
      Reply Icon

      Re: Could be worse

      quote: "An hour later I came back to him with a printed directory listing of the server's hard drive and pointed out that I could execute a "format c: /y" just as easily."

      And if you did that today, you'd be arrested for unauthorised intereference with a computer system, an offense under The Computer Misuse Act 1990. In fact since you're talking about NT4, it would have been a criminal act at the time... hopefully you didn't get collared for the 12 to 24 months custodial sentence you could have been liable to.

      Interesting thought that; using a flaw to point out the flaw is a criminal offense, however a failure to act (failing to secure the system after being notified of the flaw) is deemed a civil offense...

      1 0 Reply

      1. This post has been deleted by its author

  11. William Boyle

    Shame on Apple!

    This is just such a fundamentally elementary bug that the fact it ended up in OSX and the iPhone product lines is just (to me at least) inconceivable! Truly incompetent! When dealing with buffer sizes/lengths, one NEVER uses signed variables, for just this reason... A true FAIL moment for the Apple software team!

    FWIW, I have been writing software for large-scale systems for 30+ years. I am a senior systems engineer for a tier-one hardware/software manufacturer. And I was writing software to support Unicode back in the late 1980's when it was still in the development stages.

    3 0 Reply
  12. phuzz Silver badge
    Thumb Up

    PS, great article, more like this please!

    0 0 Reply
  13. Popup

    > void vDSP_sveD (double *__vDSP_A, vDSP_Stride __vDSP_I,

    > double *__vDSP_C, vDSP_Length __vDSP_N);

    Why on earth would you use freaking floating point arithmetics to calculate string lengths?

    0 0 Reply
    1. ThomH
      Reply Icon

      If I understood the article, it sounds like it's meant to be calculating something related to the string width, and getting that wrong because by passing from one place to another it interprets a glyph count of -1 as a glyph count of UINT64_MAX.

      So if you're asking why floating point arithmetic is used, it's because fonts are designed with floating point arithmetic and rendered with floating point arithmetic. The OS X graphics system uses the same drawing primitives as PDF and Postscript.

      1 0 Reply
  14. Anonymous Coward
    Anonymous Coward

    Perhaps Apple knew nothing about it -- whenever they received an email from someone telling them about it......

    5 0 Reply
  15. Gunnar Wolf
    Thumb Up

    Also chiming in to the happy people...

    I am also delighted to find this great example of technical content in The Register. I teach the Operating Systems class, and I will *surely* give this article to my students to read and appreciate the ways to find a real vulnerability. Thanks!

    2 0 Reply
  16. Spoonsinger
    Coat

    "-1is the loneliest number that you'll ever do

    zero(*) can be as bad as -1

    It's the loneliest number since the -1"

    (*) ok zero is actually a number.

    0 0 Reply
  17. Anonymous Coward
    Anonymous Coward

    Error in a Mac?

    This is unpossible. I think it was a deliberate attempt to invoke a singularity allowing His Jobbliness to return and to save the heathens from a world in which PC has Gone Mad.

    0 0 Reply
  18. Eric Kimminau TREG

    Ping of death

    I thought this stupidity stooped with the Ping of Death http://en.wikipedia.org/wiki/Ping_of_death and programming 101 taught proper bounds and error return code checking. Stupid programmers. Kix are for Trids.

    0 0 Reply
  19. Dennis Wilson

    Keeping mum

    As this has bug been in the wild for over six months I can see Apple has been true to form in ignoring it.

    0 0 Reply
    1. Anonymous Coward
      Reply Icon
      Anonymous Coward

      Re: Keeping mum

      Assuming Apple staff used macs -- how would they find out about it?

      0 0 Reply
  20. Disco Dance Donkey
    Holmes

    I'm sure the bug could possibly be trigged by any uneven number of characters. Not had full chance to play, but my assumption is they're using the direction of the text to mess up the counter; two forwards, three back. Personally I'd compute LTR direction then char direction (i.e. for the backspace char).

    0 0 Reply
    1. Benjol
      Reply Icon

      If that's the case surely one (reverse) character would suffice?

      0 0 Reply
      1. Steve Knox
        Reply Icon

        No, because the function would appear to get the direction right for a single character or any group of characters with the same direction.

        You'd need an initial set of characters going one direction, and then a wider reversing set of characters going the other direction.

        If we're talking proportional fonts and actual pixel width rather than character width, it could be done with one narrow LTR character followed by one wider RTL character.

        Otherwise, you'd need one more RTL character than LTR (or vice versa if you started with the RTL character).

        If this were the case, the correct answer would be either

        a) the absolute value of the result, or

        b) the sum of the absolute values of the character widths in each direction

        depending on whether the desired result is overlapping characters or each set of characters rendered side-by-side.

        0 0 Reply
  21. Mac Logo

    This article causes the Register Windows 8 App to crash on my PC. How ironic.

    0 0 Reply
    1. diodesign (Written by Reg staff) Silver badge
      Reply Icon

      Re: Windows 8 app

      Hrm. Is that a reproducible bug? It may be one of the embedded objects on the page? Feel free to drop me an email: cwilliams@thereg

      C.

      0 0 Reply
  22. Terry Cloth

    Schneier got there over a decade ago

    This little exploint rang a bell, so I searched Bruce Schneier's website. And, sure enough, on July 15, 2000, he observed ``Unicode is just too complex to ever be secure. '' See https://www.schneier.com/crypto-gram-0007.html#9. Doesn't exactly warm the cockles of the paranoid's heart.

    0 0 Reply
  23. WatAWorld

    Security by obscurity

    Security by obscurity: Governments use it. Jewelers use it. MS used to use it, before everything it did got put under microscopes. Apple uses it.

    It works fine for a while, until you get into the spotlight, and then it is totally worthless.

    0 0 Reply
  24. abueloeddie

    EE! I wish I knew WTF every one of the above messages were about. Personally I think that you are all making it up as you go

    WIIBIIFTBW Work that bugger out if you can! :-)

    0 0 Reply
  25. Anonymous Coward
    Anonymous Coward

    EE! I wish I knew WTF every one of the above messages were about. Personally I think that you are all making it up as you go

    WIIBIIFTBW Work that bugger out if you can! :-)

    0 0 Reply

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