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#28699 closed defect (bug) (fixed)

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

Reported by: azaozz Owned by: miqrogroove
Milestone: 4.3 Priority: normal
Severity: normal Version: 1.0
Component: Formatting Keywords: kses has-patch
Focuses: Cc:


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

Attachments (5)

miqro-28699.patch (460 bytes) - added by miqrogroove 22 months ago.
Make the filter even more greedy.
miqro-28699.2.patch (1.2 KB) - added by miqrogroove 22 months ago.
... but don't always run the filter.
miqro-28699.3.patch (3.5 KB) - added by miqrogroove 22 months ago.
Add unit tests.
miqro-28699.4.patch (3.0 KB) - added by miqrogroove 11 months ago.
miqro-28699.5.patch (3.2 KB) - added by miqrogroove 11 months ago.

Download all attachments as: .zip

Change History (36)

#1 @azaozz
22 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.

#3 @SergeyBiryukov
22 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].

#4 @miqrogroove
22 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?

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

22 months ago

#6 @miqrogroove
22 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?

#7 @miqrogroove
22 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.

#8 @miqrogroove
22 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.

22 months ago

Make the filter even more greedy.

22 months ago

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

#9 @miqrogroove
22 months ago

  • Keywords kses added

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

22 months ago

Add unit tests.

#10 @miqrogroove
22 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.

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

22 months ago

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

21 months ago

#13 @DrewAPicture
21 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.

#14 @SergeyBiryukov
20 months ago

  • Milestone changed from Future Release to 4.1

#15 @miqrogroove
18 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.

#16 @azaozz
18 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.

#17 @iseulde
16 months ago

  • Milestone changed from Future Release to 4.2

has-patch 4.2-early so moving to 4.2.

#18 @DrewAPicture
14 months ago

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

#19 follow-up: @miqrogroove
14 months ago

It needs commit love.

#20 in reply to: ↑ 19 @DrewAPicture
14 months ago

  • Keywords commit added

Replying to miqrogroove:

It needs commit love.

Let's make that happen :-)

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

14 months ago

#22 @nacin
14 months 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?

#23 @mdawaffe
14 months 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.

#24 @miqrogroove
11 months ago

  • Milestone changed from Future Release to 4.3

#25 @miqrogroove
11 months ago

  • Owner set to miqrogroove
  • Status changed from new to accepted

#26 @wonderboymusic
11 months ago

  • Keywords commit removed

@miqrogroove - can you offer an alternative to alleviate the boolean trap?

#27 @miqrogroove
11 months ago

I'm not understanding what is needed there. I realize it's just a style change. The logic requires a parameter for true/false when removing \0 versus not removing \0. So what is the alternative?

#28 @miqrogroove
11 months ago

Added a patch that uses $slash_zero = 'remove' instead of $strip_slash_zero = true Is that the better style?

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

11 months ago

#30 @mdawaffe
11 months ago

Yeah - you found the alternatives. I like attachment:miqro-28699.5.patch​ - when reading a call to the function, it's clear what the parameter does. Any of the following would pass the "no boolean trap" test.

wp_kses_no_null( $string, 'keep_slash_zero' ); // Not very WordPressy
wp_kses_no_null( $string, array( 'slash_zero' => 'keep' ) ); // miqro-28699.5.patch
wp_kses_no_null( $string, array( 'keep_slash_zero' => true ) ); // Seems most WordPressy to me

Either of the last two seem fine to me. Stylistically, I may have a slight preference for attachment:miqro-28699.5.patch, since it allows for the current "keep", "remove", and a possible future "urlencode", etc. (We'll never need/want such an option; I'm just talking about style habits, not the code.)

#31 @wonderboymusic
10 months ago

  • Resolution set to fixed
  • Status changed from accepted to closed

In 32860:

Don't strip \0 (backslash+zero) from post content for users without "unfiltered_html"

Adds unit tests.

Props miqrogroove.
Fixes #28699.

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