6 ms·

> SVG supports JavaScript. I didn’t know that, probably because I never had to use JS in a SVG. Probably it works only when SVGs are rendered by a browser, so

> SVG supports JavaScript. I didn’t know that, probably because I never had to use JS in a SVG. Probably it works only when SVGs are rendered by a browser, so this feature is a bit of a hack for me.

dpwm 9y ago
It appears to be part of the spec, [0] although there appears to be no requirement for the SVG User Agent to support any particular scripting language. This is interesting from a security perspective, because it effectively means SVGs can contain scripts. If I was a security researcher, this sort of little-known but apparently widely supported feature is exactly where I would be looking for vulnerabilities. [0] https://www.w3.org/TR/SVG/script.html#ScriptElement https://www.w3.org/TR/SVG/script.html#ScriptElement
- microcolonel 9y ago
  > If I was a security researcher, this sort of little-known but apparently widely supported feature is exactly where I would be looking for vulnerabilities. Well, it's not really a thing. In a browser (the only kind of UA that'll really run any scripts in an SVG), SVG basically gets treated as SGML, all the DOM stuff is the same. If you include it as an image tag, the scripts don't tend to run, AFAIK. Added to clarify: The types of things you could exploit in a scripted SVG but which wouldn't also apply to HTML, would tend to be exploitable in a static SVG as well.
  - dpwm 9y ago
    That's a relief. I see from the page it's embedded as an iframe, which I didn't know you could do. Changing the iframe to an img on the page does indeed prevent the script executing, at least on Firefox.
    - edent 9y ago
      The script also works if the SVG is written inline. Sadly, WordPress makes that tricky on my site.
    - amenghra 9y ago
      Proof that it doesn't work when used in an <img> tag: https://jsfiddle.net/8rcrfpwn/ https://jsfiddle.net/8rcrfpwn/
      - four_serenity 9y ago
        It does work with an <object> tag, though: https://jsfiddle.net/8rcrfpwn/4/ https://jsfiddle.net/8rcrfpwn/4/
  - sebcat 9y ago
    > Well, it's not really a thing. In a browser (the only kind of UA that'll really run any scripts in an SVG), SVG basically gets treated as SGML, all the DOM stuff is the same. If you include it as an image tag, the scripts don't tend to run, AFAIK It breaks expectations. I havn't looked at it recently, but it used to be the case that hosting user submitted SVGs on your domain could yield JS-execution in the origin context. Embedding or iframing without sandbox isn't/wasn't secure either. A quick google search on SVG XSS gives: https://hackerone.com/reports/148853 https://hackerone.com/reports/148853 https://phabricator.wikimedia.org/T85850 https://phabricator.wikimedia.org/T85850
    - noblethrasher 9y ago
      On the other hand, I think it would be straightforward to strip script tags and attributes using XSLT. You could even go as far as just whitelisting certain tags/attributes.
    - microcolonel 9y ago
      > I havn't looked at it recently, but it used to be the case that hosting user submitted SVGs on your domain could yield JS-execution in the origin context. Oh definitely, though that's more an administrative problem than a platform problem. Webmasters (or whoever that task falls to today) must understand that SVGs are exactly as dangerous as HTML files in exactly the same ways. Where you would sanitize user-provided HTML, you must also sanitize user-provided SVG.
  - im3w1l 9y ago
    So let's say you use an img tag, but the user right clicks and chooses view image? Navigating to the svg seems to run the scripts.
    - zaroth 9y ago
      > Navigating to the svg seems to run the scripts. Now that seems like a bug.
      - WorldMaker 9y ago
        If I recall correctly, Mozilla thought so too and recently disabled scripts in direct SVG navigation and data: URIs in recent versions of Firefox.
        WorldMaker 9y ago
        Well, direct navigation worked when I tried it now and I can't seem to turn up the blog post I thought commented on the situation, so my recollection here may be in question.
  - deleted 9y ago
    [deleted]
- imaginenore 9y ago
  That's pretty insane and irresponsible to just drop this bombshell in the middle of the spec. "Oh, by the way, our image format allows for embedded scripts. We won't tell you what the scope is, or even what languages are allowed. Deal with it.".
petters 9y ago
JS in SVGs is very useful. See for example Gregg's flame graphs: http://www.brendangregg.com/flamegraphs.html http://www.brendangregg.com/flamegraphs.html
billyhoffman 9y ago
Normally this isn’t a problem. If the SVG is included with an <img> Tag the JS won’t execute. If you directly embed the SVG tags into the HTML it will. But surely no one does this with SVGs they receive from untrustworthy 3rd parties right? Except a web performance best practice (for non-H2 sites) is to inline images into the HTML... and there are some auto-optimizing libraries that will embed SVG as <SVG> tags instead of as a data: URI... Yep. I know of a few persistent XSS vulns in a few top Alexa sites because of this :-D
- cbr 9y ago
  a web performance best practice (for non-H2 sites) is to inline images into the HTML... and there are some auto- optimizing libraries that will embed SVG as <SVG> tags instead of as a data: URI... When I worked on mod_pagespeed (a website auto-optimizer) we wouldn't have it touch SVGs at all because there are so many potential security issues there. (And it wasn't widely used enough to be worth it given the risks.)
  - billyhoffman 9y ago
    Thanks for your work on mod_pagespeed. It was an awesome approach for FEO. It's not that problem. I see this issue primarily in some semi-popular build/publishing plugins.
- djsumdog 9y ago
  I think back to the XML readers and XSLT parsers that could read arbitrary files or the fact that you can technically embed flash in a PDF (which is just a wrapper around Postscript). It's things like these that lead to security issues. Honestly when I read that first line about SVG and Javascript, I was a little horrified. Who the fuck on what planet ever though that could possible be a good idea?
  - pcwalton 9y ago
    > Honestly when I read that first line about SVG and Javascript, I was a little horrified. Who the fuck on what planet ever though that could possible be a good idea? Adobe, when they were trying to position SVG as a competitor to Macromedia's Flash.
ogig 9y ago
I got a bug bounty [1] on Shopify because of a similar feature in SVG, external sources. I've been saying this for a while: if you are parsing svg server side you are most likely vulnerable to these type of attacks. SVG files should be considered programs from a security standpoint. [1] https://hackerone.com/reports/97501 https://hackerone.com/reports/97501
- dannyw 9y ago
  I wish there was a SVG-lite / SVGL standard that is a subset of SVG.
  - edent 9y ago
    There is SVG Tiny - https://www.w3.org/TR/SVGTiny12/ https://www.w3.org/TR/SVGTiny12/ - designed when phones were less powerful than today.
shilch 9y ago
Yeah, probably another entrypoint for xss.
soheil 9y ago
How does something like SVG even end up in HTML spec? It seems like there could be millions of such technologies but we end up with things like JS, SVG, XSLT.
bringtheaction 9y ago
I once started building a web application based on SVG, but if I remember correctly I gave up because it was harder to debug JavaScript running inside of an SVG compared to JavaScript running from HTML. The thing I was trying to build was a graph drawing tool, so that’s why I figured SVG made sense. It was quite a while ago though and I am much more experienced now than I was back then so maybe I could have pulled it off better now but honestly I don’t see a reason to try. If I were to make my graph editor tool I’d just build it as a regular web app now. I might still utilize SVG, or I might do it with canvas but either way I wouldn’t do it with the JavaScript running inside of the SVG this time I think.
tristanb 9y ago
I immediately thought what about webmail, like gmail? Gmail doesn't run JS in emails, would it in an SVG?

dpwm 9y ago

It appears to be part of the spec, [0] although there appears to be no requirement for the SVG User Agent to support any particular scripting language. This is interesting from a security perspective, because it effectively means SVGs can contain scripts. If I was a security researcher, this sort of little-known but apparently widely supported feature is exactly where I would be looking for vulnerabilities. [0] https://www.w3.org/TR/SVG/script.html#ScriptElement https://www.w3.org/TR/SVG/script.html#ScriptElement

microcolonel 9y ago
> If I was a security researcher, this sort of little-known but apparently widely supported feature is exactly where I would be looking for vulnerabilities. Well, it's not really a thing. In a browser (the only kind of UA that'll really run any scripts in an SVG), SVG basically gets treated as SGML, all the DOM stuff is the same. If you include it as an image tag, the scripts don't tend to run, AFAIK. Added to clarify: The types of things you could exploit in a scripted SVG but which wouldn't also apply to HTML, would tend to be exploitable in a static SVG as well.
- dpwm 9y ago
  That's a relief. I see from the page it's embedded as an iframe, which I didn't know you could do. Changing the iframe to an img on the page does indeed prevent the script executing, at least on Firefox.
  - edent 9y ago
    The script also works if the SVG is written inline. Sadly, WordPress makes that tricky on my site.
  - amenghra 9y ago
    Proof that it doesn't work when used in an <img> tag: https://jsfiddle.net/8rcrfpwn/ https://jsfiddle.net/8rcrfpwn/
    - four_serenity 9y ago
      It does work with an <object> tag, though: https://jsfiddle.net/8rcrfpwn/4/ https://jsfiddle.net/8rcrfpwn/4/
- sebcat 9y ago
  > Well, it's not really a thing. In a browser (the only kind of UA that'll really run any scripts in an SVG), SVG basically gets treated as SGML, all the DOM stuff is the same. If you include it as an image tag, the scripts don't tend to run, AFAIK It breaks expectations. I havn't looked at it recently, but it used to be the case that hosting user submitted SVGs on your domain could yield JS-execution in the origin context. Embedding or iframing without sandbox isn't/wasn't secure either. A quick google search on SVG XSS gives: https://hackerone.com/reports/148853 https://hackerone.com/reports/148853 https://phabricator.wikimedia.org/T85850 https://phabricator.wikimedia.org/T85850
  - noblethrasher 9y ago
    On the other hand, I think it would be straightforward to strip script tags and attributes using XSLT. You could even go as far as just whitelisting certain tags/attributes.
  - microcolonel 9y ago
    > I havn't looked at it recently, but it used to be the case that hosting user submitted SVGs on your domain could yield JS-execution in the origin context. Oh definitely, though that's more an administrative problem than a platform problem. Webmasters (or whoever that task falls to today) must understand that SVGs are exactly as dangerous as HTML files in exactly the same ways. Where you would sanitize user-provided HTML, you must also sanitize user-provided SVG.
- im3w1l 9y ago
  So let's say you use an img tag, but the user right clicks and chooses view image? Navigating to the svg seems to run the scripts.
  - zaroth 9y ago
    > Navigating to the svg seems to run the scripts. Now that seems like a bug.
    - WorldMaker 9y ago
      If I recall correctly, Mozilla thought so too and recently disabled scripts in direct SVG navigation and data: URIs in recent versions of Firefox.
      - WorldMaker 9y ago
        Well, direct navigation worked when I tried it now and I can't seem to turn up the blog post I thought commented on the situation, so my recollection here may be in question.
- deleted 9y ago
  [deleted]
imaginenore 9y ago
That's pretty insane and irresponsible to just drop this bombshell in the middle of the spec. "Oh, by the way, our image format allows for embedded scripts. We won't tell you what the scope is, or even what languages are allowed. Deal with it.".

petters 9y ago

JS in SVGs is very useful. See for example Gregg's flame graphs: http://www.brendangregg.com/flamegraphs.html http://www.brendangregg.com/flamegraphs.html

billyhoffman 9y ago

Normally this isn’t a problem. If the SVG is included with an <img> Tag the JS won’t execute. If you directly embed the SVG tags into the HTML it will. But surely no one does this with SVGs they receive from untrustworthy 3rd parties right? Except a web performance best practice (for non-H2 sites) is to inline images into the HTML... and there are some auto-optimizing libraries that will embed SVG as <SVG> tags instead of as a data: URI... Yep. I know of a few persistent XSS vulns in a few top Alexa sites because of this :-D

cbr 9y ago
a web performance best practice (for non-H2 sites) is to inline images into the HTML... and there are some auto- optimizing libraries that will embed SVG as <SVG> tags instead of as a data: URI... When I worked on mod_pagespeed (a website auto-optimizer) we wouldn't have it touch SVGs at all because there are so many potential security issues there. (And it wasn't widely used enough to be worth it given the risks.)
- billyhoffman 9y ago
  Thanks for your work on mod_pagespeed. It was an awesome approach for FEO. It's not that problem. I see this issue primarily in some semi-popular build/publishing plugins.
djsumdog 9y ago
I think back to the XML readers and XSLT parsers that could read arbitrary files or the fact that you can technically embed flash in a PDF (which is just a wrapper around Postscript). It's things like these that lead to security issues. Honestly when I read that first line about SVG and Javascript, I was a little horrified. Who the fuck on what planet ever though that could possible be a good idea?
- pcwalton 9y ago
  > Honestly when I read that first line about SVG and Javascript, I was a little horrified. Who the fuck on what planet ever though that could possible be a good idea? Adobe, when they were trying to position SVG as a competitor to Macromedia's Flash.

ogig 9y ago

I got a bug bounty [1] on Shopify because of a similar feature in SVG, external sources. I've been saying this for a while: if you are parsing svg server side you are most likely vulnerable to these type of attacks. SVG files should be considered programs from a security standpoint. [1] https://hackerone.com/reports/97501 https://hackerone.com/reports/97501

dannyw 9y ago
I wish there was a SVG-lite / SVGL standard that is a subset of SVG.
- edent 9y ago
  There is SVG Tiny - https://www.w3.org/TR/SVGTiny12/ https://www.w3.org/TR/SVGTiny12/ - designed when phones were less powerful than today.

shilch 9y ago

Yeah, probably another entrypoint for xss.

soheil 9y ago

How does something like SVG even end up in HTML spec? It seems like there could be millions of such technologies but we end up with things like JS, SVG, XSLT.

bringtheaction 9y ago

I once started building a web application based on SVG, but if I remember correctly I gave up because it was harder to debug JavaScript running inside of an SVG compared to JavaScript running from HTML. The thing I was trying to build was a graph drawing tool, so that’s why I figured SVG made sense. It was quite a while ago though and I am much more experienced now than I was back then so maybe I could have pulled it off better now but honestly I don’t see a reason to try. If I were to make my graph editor tool I’d just build it as a regular web app now. I might still utilize SVG, or I might do it with canvas but either way I wouldn’t do it with the JavaScript running inside of the SVG this time I think.

tristanb 9y ago

I immediately thought what about webmail, like gmail? Gmail doesn't run JS in emails, would it in an SVG?