8 ms·

Or one of the best. Switch wen browsers to strict processong and you will hardly find working web page.

ygjb 10y ago
Yeah, imagine if processors gave best effort to processing binaries... what could possibly go wrong :/ The decision to allow this was made early and the liberal accept/strict transmit paradigm has in general made the web a mess. On the plus side, the consistent failure of browser vendors to apply strict controls to input means that as an application security person I will probably never be out of work :D Even though this pattern of behavior is starting to change, legacy support means that I will still be dealing with these issues well into my retirement!
- dpark 10y ago
  > as an application security person I will probably never be out of work How many security flaws are the result of malformed HTML?
  - Chernobog 10y ago
    Malformed HTML may escape sanitization on input in a vulnerable web app, and still render on the victim's browser because their browser wants to be helpful. (Yes, the output should have been escaped, but that is sadly not always the case)
    - dpark 10y ago
      I don't see how this has anything to do with malformed HTML or lenient rendering rules. In the scenario you're describing, well-formed but malicious HTML could also escape sanitization.
  - JimDabell 10y ago
    Malformed HTML? None. Browsers attempting to be lenient in what they accept? Loads. To take this article as an example, according to the HTTP specification, the `Content-Type` header is supposed to have the final say in what media type is being served. Internet Explorer decided it would be better to use heuristics. I think the idea was that if a web host was misconfigured, rather than have the web developer fix their bug, it would try to guess its way out of the error. Which kinda worked. The problem was, it opened it up to abuse. If you had a web host that allowed untrusted people to upload images (e.g. profile photos), you could construct an image that tricked Internet Explorer into thinking that it was an HTML document, even if the server explicitly told clients that it was an image. The main difference between images and HTML, of course, is that HTML can contain JavaScript, which would now execute in the security context of your web page. So all of these web hosts, thinking they were only giving people the ability to upload images, were now letting people execute JavaScript on their domain – simply because Internet Explorer tried to be lenient. The workaround ended up being forcing downloads with `Content-Disposition` headers instead of displaying inline. That's why, for example, visiting the URL of an image on Blogger directly triggers a download instead of showing the image. Other examples that spring to mind: Netscape interpreting certain Unicode characters as less than signs. People were correctly escaping `<` as `<` but the Unicode characters slipped through and caused XSS vulnerabilities in that browser. Browsers ignoring newlines in pseudo-protocols. Want to strip `href="javascript:…"` out of comments? No problem… except some browsers also executed JavaScript when an attacker placed a newline anywhere within the `javascript` token. Being lenient in what you accept has caused security vulnerabilities over and over again and there's no reason to think that it will stop now.
    - dpark 10y ago
      > To take this article as an example, according to the HTTP specification, the `Content-Type` header is supposed to have the final say in what media type is being served. Internet Explorer decided it would be better to use heuristics. I think the idea was that if a web host was misconfigured, rather than have the web developer fix their bug, it would try to guess its way out of the error. Which kinda worked. The problem was, it opened it up to abuse. If you had a web host that allowed untrusted people to upload images (e.g. profile photos), you could construct an image that tricked Internet Explorer into thinking that it was an HTML document, even if the server explicitly told clients that it was an image. The main difference between images and HTML, of course, is that HTML can contain JavaScript, which would now execute in the security context of your web page. So all of these web hosts, thinking they were only giving people the ability to upload images, were now letting people execute JavaScript on their domain – simply because Internet Explorer tried to be lenient. This is an interesting example, though I think this is a fair bit different than being lenient on HTML interpretation. The topic was strict HTML. Accepting malformed HTML doesn't seem to pose much of a problem. Blindly executing a non-executable file seems like a much different problem. > Netscape interpreting certain Unicode characters as less than signs. People were correctly escaping `<` as `<` but the Unicode characters slipped through and caused XSS vulnerabilities in that browser. This doesn't sound like being lenient. This just sounds like a bug. > Browsers ignoring newlines in pseudo-protocols. Want to strip `href="javascript:…"` out of comments? No problem… except some browsers also executed JavaScript when an attacker placed a newline anywhere within the `javascript` token. Huh? I don't understand the scenario being described here. It again sounds like a bug rather than lenient acceptance of data, though.
      - JimDabell 10y ago
        > I think this is a fair bit different than being lenient on HTML interpretation. It's not. There are two areas where the leniency was a problem here. Firstly, the leniency in rendering one media type as a completely different media type because the browser heuristic thought it was being lenient. Secondly, the leniency in parsing HTML out of an image file – you can't do that with valid HTML. > Accepting malformed HTML doesn't seem to pose much of a problem. I've literally just given three specific examples of it causing security vulnerabilities. > This doesn't sound like being lenient. This just sounds like a bug. No, it was intentional. It was specifically Unicode characters that looked like less than and greater than signs, but weren't. > I don't understand the scenario being described here. Somebody noticed that href="java\nscript:…" wasn't being parsed as JavaScript, and it was causing some malformed pages to fail to work properly. Rather than let it fail, they tried to fix it by stripping out the whitespace, and caused a security vulnerability. If these three examples aren't enough, take a look at OWASP's XSS filter evasion cheat sheet. There's plenty of examples in there of lenient parsing causing security problems: https://www.owasp.org/index.php/XSS_Filter_Evasion_Cheat_Sheet https://www.owasp.org/index.php/XSS_Filter_Evasion_Cheat_She...
danbruc 10y ago
Which are the faults of the authors. No one expects malformed source code to compile, a video with corrupted headers to play properly or a binary containing invalid instructions not to crash. This decision allowed people to get away with broken web pages instead of forcing them to fix their mistakes.
- eyelidlessness 10y ago
  It's a better web environment than it was years ago, but there was a time that even simple layouts required invalid code to display similarly across the browsers of the time.
- dwaltrip 10y ago
  The web would not have been as successful if it wasn't for this leniency. Full correctness is only worthwhile if both attaining it does not excessively harm other goals and the cost of not attaining it is severe enough. The cost of a malformatted HTML document rendering despite the errors is not that severe compared to the benefits it provides, as we have seen.
nolok 10y ago
Well of course if correctness is not a requirement then it's not being paid attention to. That doesn't in any way indicate that it was a good decision not to require it in the first place.
- notahacker 10y ago
  If it had been made as difficult as possible for enthusiasts learning a markup language to get what was essentially text document to actually display anything, it's probably not too much of an exaggeration to say the World Wide Web wouldn't have existed in its current form. It's not as if many of the web's security holes are related to whether a page displays valid HTML markup or not.
  - danbruc 10y ago
    I don't think the assumption is justified that enforcing well-formed HTML documents would have been a significant barrier. To me it actually seems easier to have a few simple and strictly enforced rules than having a more or less random assortment of exception to save a handful of key strokes.
    - notahacker 10y ago
      The failure of XHTML Strict suggests otherwise. Ultimately it's less about saving keystrokes and more about amateur enthusiasts having the opportunity to start with the browser rendering their unformatted document rather than an "Error at line 1" warning, and changes they introduce being considerably less likely to break the entire page
      - sievebrain 10y ago
        XHTML Strict only solved one set of leniencies in the web platform, which were also the least important kinds. Malformed HTML makes writing browsers complicated but doesn't generally seem to cause security issues or other visible, obvious, must-fix-now problems. The real security due to leniency problems in the web platform revolve around the handling of data and how JavaScript works, which were not addressed by XHTML. In that sense it's not surprising it went nowhere.

ygjb 10y ago

Yeah, imagine if processors gave best effort to processing binaries... what could possibly go wrong :/ The decision to allow this was made early and the liberal accept/strict transmit paradigm has in general made the web a mess. On the plus side, the consistent failure of browser vendors to apply strict controls to input means that as an application security person I will probably never be out of work :D Even though this pattern of behavior is starting to change, legacy support means that I will still be dealing with these issues well into my retirement!

dpark 10y ago
> as an application security person I will probably never be out of work How many security flaws are the result of malformed HTML?
- Chernobog 10y ago
  Malformed HTML may escape sanitization on input in a vulnerable web app, and still render on the victim's browser because their browser wants to be helpful. (Yes, the output should have been escaped, but that is sadly not always the case)
  - dpark 10y ago
    I don't see how this has anything to do with malformed HTML or lenient rendering rules. In the scenario you're describing, well-formed but malicious HTML could also escape sanitization.
- JimDabell 10y ago
  Malformed HTML? None. Browsers attempting to be lenient in what they accept? Loads. To take this article as an example, according to the HTTP specification, the `Content-Type` header is supposed to have the final say in what media type is being served. Internet Explorer decided it would be better to use heuristics. I think the idea was that if a web host was misconfigured, rather than have the web developer fix their bug, it would try to guess its way out of the error. Which kinda worked. The problem was, it opened it up to abuse. If you had a web host that allowed untrusted people to upload images (e.g. profile photos), you could construct an image that tricked Internet Explorer into thinking that it was an HTML document, even if the server explicitly told clients that it was an image. The main difference between images and HTML, of course, is that HTML can contain JavaScript, which would now execute in the security context of your web page. So all of these web hosts, thinking they were only giving people the ability to upload images, were now letting people execute JavaScript on their domain – simply because Internet Explorer tried to be lenient. The workaround ended up being forcing downloads with `Content-Disposition` headers instead of displaying inline. That's why, for example, visiting the URL of an image on Blogger directly triggers a download instead of showing the image. Other examples that spring to mind: Netscape interpreting certain Unicode characters as less than signs. People were correctly escaping `<` as `<` but the Unicode characters slipped through and caused XSS vulnerabilities in that browser. Browsers ignoring newlines in pseudo-protocols. Want to strip `href="javascript:…"` out of comments? No problem… except some browsers also executed JavaScript when an attacker placed a newline anywhere within the `javascript` token. Being lenient in what you accept has caused security vulnerabilities over and over again and there's no reason to think that it will stop now.
  - dpark 10y ago
    > To take this article as an example, according to the HTTP specification, the `Content-Type` header is supposed to have the final say in what media type is being served. Internet Explorer decided it would be better to use heuristics. I think the idea was that if a web host was misconfigured, rather than have the web developer fix their bug, it would try to guess its way out of the error. Which kinda worked. The problem was, it opened it up to abuse. If you had a web host that allowed untrusted people to upload images (e.g. profile photos), you could construct an image that tricked Internet Explorer into thinking that it was an HTML document, even if the server explicitly told clients that it was an image. The main difference between images and HTML, of course, is that HTML can contain JavaScript, which would now execute in the security context of your web page. So all of these web hosts, thinking they were only giving people the ability to upload images, were now letting people execute JavaScript on their domain – simply because Internet Explorer tried to be lenient. This is an interesting example, though I think this is a fair bit different than being lenient on HTML interpretation. The topic was strict HTML. Accepting malformed HTML doesn't seem to pose much of a problem. Blindly executing a non-executable file seems like a much different problem. > Netscape interpreting certain Unicode characters as less than signs. People were correctly escaping `<` as `<` but the Unicode characters slipped through and caused XSS vulnerabilities in that browser. This doesn't sound like being lenient. This just sounds like a bug. > Browsers ignoring newlines in pseudo-protocols. Want to strip `href="javascript:…"` out of comments? No problem… except some browsers also executed JavaScript when an attacker placed a newline anywhere within the `javascript` token. Huh? I don't understand the scenario being described here. It again sounds like a bug rather than lenient acceptance of data, though.
    - JimDabell 10y ago
      > I think this is a fair bit different than being lenient on HTML interpretation. It's not. There are two areas where the leniency was a problem here. Firstly, the leniency in rendering one media type as a completely different media type because the browser heuristic thought it was being lenient. Secondly, the leniency in parsing HTML out of an image file – you can't do that with valid HTML. > Accepting malformed HTML doesn't seem to pose much of a problem. I've literally just given three specific examples of it causing security vulnerabilities. > This doesn't sound like being lenient. This just sounds like a bug. No, it was intentional. It was specifically Unicode characters that looked like less than and greater than signs, but weren't. > I don't understand the scenario being described here. Somebody noticed that href="java\nscript:…" wasn't being parsed as JavaScript, and it was causing some malformed pages to fail to work properly. Rather than let it fail, they tried to fix it by stripping out the whitespace, and caused a security vulnerability. If these three examples aren't enough, take a look at OWASP's XSS filter evasion cheat sheet. There's plenty of examples in there of lenient parsing causing security problems: https://www.owasp.org/index.php/XSS_Filter_Evasion_Cheat_Sheet https://www.owasp.org/index.php/XSS_Filter_Evasion_Cheat_She...

danbruc 10y ago

Which are the faults of the authors. No one expects malformed source code to compile, a video with corrupted headers to play properly or a binary containing invalid instructions not to crash. This decision allowed people to get away with broken web pages instead of forcing them to fix their mistakes.

eyelidlessness 10y ago
It's a better web environment than it was years ago, but there was a time that even simple layouts required invalid code to display similarly across the browsers of the time.
dwaltrip 10y ago
The web would not have been as successful if it wasn't for this leniency. Full correctness is only worthwhile if both attaining it does not excessively harm other goals and the cost of not attaining it is severe enough. The cost of a malformatted HTML document rendering despite the errors is not that severe compared to the benefits it provides, as we have seen.

nolok 10y ago

Well of course if correctness is not a requirement then it's not being paid attention to. That doesn't in any way indicate that it was a good decision not to require it in the first place.

notahacker 10y ago
If it had been made as difficult as possible for enthusiasts learning a markup language to get what was essentially text document to actually display anything, it's probably not too much of an exaggeration to say the World Wide Web wouldn't have existed in its current form. It's not as if many of the web's security holes are related to whether a page displays valid HTML markup or not.
- danbruc 10y ago
  I don't think the assumption is justified that enforcing well-formed HTML documents would have been a significant barrier. To me it actually seems easier to have a few simple and strictly enforced rules than having a more or less random assortment of exception to save a handful of key strokes.
  - notahacker 10y ago
    The failure of XHTML Strict suggests otherwise. Ultimately it's less about saving keystrokes and more about amateur enthusiasts having the opportunity to start with the browser rendering their unformatted document rather than an "Error at line 1" warning, and changes they introduce being considerably less likely to break the entire page
    - sievebrain 10y ago
      XHTML Strict only solved one set of leniencies in the web platform, which were also the least important kinds. Malformed HTML makes writing browsers complicated but doesn't generally seem to cause security issues or other visible, obvious, must-fix-now problems. The real security due to leniency problems in the web platform revolve around the handling of data and how JavaScript works, which were not addressed by XHTML. In that sense it's not surprising it went nowhere.