this post was submitted on 18 Nov 2023
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Piracy: ꜱᴀɪʟ ᴛʜᴇ ʜɪɢʜ ꜱᴇᴀꜱ
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I work on a Web app and we recently decided that we're just not gonna support double quotes in free text fields because oh holy balls what a thing it is to try to deal with those in a way that doesn't open you up to multiple encoding vulnerabilities.
That's... Surprising. If you're doing things right, double quotes should be no trouble at all:
They are usually only trouble if you're doing SQL queries wrong (concatenation etc.) or if you're not escaping your output.
The issue is the filter that we're using to avoid multiple encoding attacks de-escapes everything via multiple rounds, then tries to pass it to the next layer of filtering with the de-escaped request body as a json string. Your absolutely right that this is a silly way of doing it, but sometimes we have to live with decisions that were made before we were onboarded to a project. In this particular case, I pushed to improve the filters but all our PO heard was "spend development time weakening security" and at the end of the day they decide what to do and we do it.
The filter you're using to avoid multiple encoding attacks creates multiple encoding attacks.
You should tell that to OWASP then, they wrote it. org.owasp.esapi 2.5.2.0, class is Encoder, method is canonicalize(String, bool, bool)
This method is a band-aid patch when your downstream code is all messed up and you can't fix it. Instead of treating the input string correctly, it just removes anything that might possibly trigger some vulnerability in wrong code.