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Date: Tue, 11 May 2021 20:55:17 +0000
From: Qualys Security Advisory <>
To: null p0int3r <>
CC: "" <>
Subject: Re: [CVE-2020-28018] Use-After-Free on Exim Question


On Tue, May 11, 2021 at 01:23:43PM +0200, null p0int3r wrote:
> So I suppose that command is the first you send after the second
> "STARTTLS" command being sent right?

Yes! After the second STARTTLS we send an invalid MAIL FROM command (for
example, "MAIL FROM:(\"${run{...}}\")\n"). Exim then responds with a 501
error message that includes our "${run{...}}" string, and since corked
in tls_write() is still non-NULL, this string is written to where the
used-after-free corked points to.

Hopefully this helps!

> PD: Congrats for those nice bugs discovered.

Thank you very much for your mail!

With best regards,

the Qualys Security Advisory team


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