Make WordPress Core

Opened 9 months ago

Last modified 8 days ago

#28699 new defect (bug)

\0 (backslash+zero) gets stripped from post content for users without "unfiltered_html"

Reported by: azaozz Owned by:
Milestone: Future Release Priority: normal
Severity: normal Version: 1.0
Component: Formatting Keywords: kses has-patch commit
Focuses: Cc:


Happens when saving posts from both the Visual and Text editors. Seems caused by kses.

Attachments (3)

miqro-28699.patch (460 bytes) - added by miqrogroove 9 months ago.
Make the filter even more greedy.
miqro-28699.2.patch (1.2 KB) - added by miqrogroove 9 months ago.
... but don't always run the filter.
miqro-28699.3.patch (3.5 KB) - added by miqrogroove 9 months ago.
Add unit tests.

Download all attachments as: .zip

Change History (26)

comment:1 @azaozz9 months ago

  • Keywords needs-patch needs-unit-tests added
  • Milestone changed from Awaiting Review to 4.0

Moving to 4.0 for investigation and eventual patch.

comment:3 @SergeyBiryukov9 months ago

  • Component changed from General to Formatting
  • Version set to 1.0

Caused by the second line in wp_kses_no_null():

$string = preg_replace('/(\\\\0)+/', '', $string);

Introduced in [649].

comment:4 @miqrogroove9 months ago

So according to that original kses.php file, \0 has always been stripped, even though the function comments say it was intended to strip NUL bytes only.

Can we call this a bug, or can we identify any concerns in HTML, JS, or CSS related to having that phrase appear in the output?

comment:5 @ircbot9 months ago

This ticket was mentioned in IRC in #wordpress-dev by mauteri. View the logs.

comment:6 @miqrogroove9 months ago

Looks like \0 is special (octal) in JS in at least two contexts.

\0 is also special (hex) in CSS content attributes.

These might also prefix any non-zero integer.

Any security implications there?

comment:7 @miqrogroove9 months ago

I think there's a significant concern with the XSS Cheat Sheet example:

<DIV STYLE="background-image:\0075\0072\006C\0028'\006a\0061\0076\0061\0073\0063\0072\0069\0070\0074\003a\0061\006c\0065\0072\0074\0028.1027\0058.1053\0053\0027\0029'\0029">

Because, wp_kses_no_null() is used inside of safecss_filter_attr(). If the latter function has any usage outside of the usual wp_kses_attr() calls, then someone could be depending on the removal of hex codes for security.

comment:8 @miqrogroove9 months ago

A secondary concern would be that the kses filter incorrectly removes multiple chars from the middle of user input. This could be exploited to form other unwanted strings, including \0 itself by simply re-encoding as \\00.

@miqrogroove9 months ago

Make the filter even more greedy.

@miqrogroove9 months ago

... but don't always run the filter.

comment:9 @miqrogroove9 months ago

  • Keywords kses added

Patch attached. I'll try to write the unit tests before the first beta.

@miqrogroove9 months ago

Add unit tests.

comment:10 @miqrogroove9 months ago

  • Keywords has-patch added; needs-patch needs-unit-tests removed

In miqro-28699.3.patch:

  • Do not strip \0 from user's CDATA.
  • Do not strip \0 from most attributes.
  • Do strip \0 in STYLE attributes.
  • \0 and \\00 are now identical and removed greedily.
  • STYLE and SCRIPT elements unaffected; kses removes them by default.
  • Add unit tests.

comment:11 @ircbot9 months ago

This ticket was mentioned in IRC in #wordpress-dev by SergeyBiryukov. View the logs.

comment:12 @ircbot8 months ago

This ticket was mentioned in IRC in #wordpress-dev by DrewAPicture. View the logs.

comment:13 @DrewAPicture8 months ago

  • Keywords 4.1-early added
  • Milestone changed from 4.0 to Future Release

Too late in the cycle for something like this. Let's try to hit it in 4.1 early. Patch still applies.

comment:14 @SergeyBiryukov7 months ago

  • Milestone changed from Future Release to 4.1

comment:15 @miqrogroove4 months ago

  • Keywords 4.2-early added; 4.1-early removed

Please punt to Future Release. I need to discuss this more with azaozz, and it looks like we can get it ready for 4.2.

comment:16 @azaozz4 months ago

  • Milestone changed from 4.1 to Future Release

Right. We probably can also explore removing it in some contexts for users with unfiltered_html.

comment:17 @iseulde2 months ago

  • Milestone changed from Future Release to 4.2

has-patch 4.2-early so moving to 4.2.

comment:18 @DrewAPicture10 days ago

@miqrogroove, @azaozz: What's left here? The latest patch still applies and the unit tests pass.

comment:19 follow-up: @miqrogroove10 days ago

It needs commit love.

comment:20 in reply to: ↑ 19 @DrewAPicture10 days ago

  • Keywords commit added

Replying to miqrogroove:

It needs commit love.

Let's make that happen :-)

comment:21 @slackbot9 days ago

This ticket was mentioned in Slack in #core by drew. View the logs.

comment:22 @nacin9 days ago

  • Keywords 4.2-early removed
  • Milestone changed from 4.2 to Future Release

@mdawaffe or @duck_, can you evaluate this for 4.3 inclusion?

comment:23 @mdawaffe8 days ago

wp_kses_no_null( $string, $strip_slash_zero = true ) is a so-called "boolean trap" and violates our coding standards: https://make.wordpress.org/core/handbook/coding-standards/php/#self-explanatory-flag-values-for-function-arguments.

Other than that, I don't see any problems.

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