MediaWiki security bump

Did a security release yesterday: MediaWiki 1.11.1, 1.10.3, and 1.9.5. I noticed that some of the output formats for api.php are susceptible to HTML injection through a longstanding problem with Internet Explorer’s content-type autodetection.

We’ve had protection against this in action=raw mode for years, but it didn’t make it into the API as nobody had quite noticed that some output formats were vulnerable. JSON and XML-based formats aren’t, but PHP serialization and YAML don’t escape strings as much as we might like.

If the format lets you pass some raw HTML tags, and you can stick an additional fake path after the script name in the URL (as allowed by most configurations), MSIE opens up a big XSS hole on your site.

Path components in URLs are supposed to be opaque; the HTTP content-type header is the only thing that’s supposed to specify what kind of resource you’re loading. Microsoft thinks it knows better, though — if it recognizes one of several pre-defined “extension”s at the end of the “filename” on the URL, it sniffs the file’s actual content to try to determine the file type. If it sees certain HTML tags, it’ll interpret it as HTML — even for valid GIF and PNG files!

(Rumor is that last hole has finally been fixed in recent Windows security updates; GIF and PNG headers will override the HTML detection. I haven’t tested to confirm this though.)

For “raw” and “API” type stuff where we have to pass through user data, we can protect against the autodetection by ensuring the URL hasn’t been tampered with. Having both an unrecognized URL and an unrecognized content-type keeps the content sniffy away… That’s why you currently get a ‘403 – Forbidden’ if you just toss ?action=raw on the end of a page URL.

One thought on “MediaWiki security bump”

  1. My favorite example of content sniffing gone wrong (in this case, innocuously) is this one of Safari and XHTML. It’s a text/plain document that mentions XHTML and text/html in the first couple of lines, and has an email address delimited by angle brackets. One, or some combination of these was enough to convince the then-current version of Safari that it must be an XHTML document. It’s since been fixed.

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