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Don't replace regex / function placeholders within string literals #79

merged 2 commits into from May 20, 2020


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@Siebes Siebes commented Apr 1, 2020

credits JordanMilne ( #22

I confirm that this contribution is made under the terms of the license found in the root directory of this repository's source tree and that I have the authority necessary to make this contribution on behalf of its copyright owner.

Based on Jordan's comment here I believe he gave approval and I approve any merging I did :)
#22 (comment)

Previously we weren't checking if the quote that started the placeholder
was escaped or not, meaning an object like

    {"foo": /1"/, "bar": "a\"@__R-<UID>-0__@"}

Would be serialized as

    {"foo": /1"/, "bar": "a\/1"/}

meaning an attacker could escape out of `bar` if they controlled both
`foo` and `bar` and were able to guess the value of `<UID>`.

UID was generated once on startup, was chosen using `Math.random()` and
had a keyspace of roughly 4 billion, so within the realm of an online

Here's a simple example that will cause `console.log()` to be called when
the `serialize()`d version is `eval()`d

    eval('('+ serialize({"foo": /1" + console.log(1)/i, "bar": '"@__R-<UID>-0__@'}) + ')');

Where `<UID>` is the guessed `UID`.

This fixes the issue by ensuring that placeholders are not preceded by
a backslash.

We also switch to a higher entropy `UID` to prevent people from guessing it.

credits JordanMilne (
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@Siebes Siebes requested a review from okuryu Apr 9, 2020
okuryu approved these changes May 20, 2020
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@okuryu okuryu left a comment

Sorry for delay. I would like to try to merge this in. I will be working on a release that includes this as soon as possible. Thank you for your patience.

@okuryu okuryu merged commit f21a6fb into yahoo:master May 20, 2020
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@okuryu okuryu commented May 28, 2020

Published serialize-javascript@3.1.0 today. Thanks!

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3 participants