[exim] "allow_insecure_tainted_data = yes" - was: tainted da…

Hi,

finally a follow-up.

> In one word "upvote".
>
> I am all for improved security but a single "step change" that breaks
> existing configurations is IMHO going too far.
>
>     taint_mode = off | warn | enforce

        .ifdef _OPT_MAIN_ALLOW_INSECURE_TAINTED_DATA
        allow_insecure_tainted_data = yes
        .endif

The EDITME contains a new build time option
"ALLOW_INSECURE_TAINTED_DATA", currently enabled. Using this build time
option provides a new runtime option "allow_insecure_tainted_data", which
turns taint errors into warnings (and spams your log file). If you do
not want the warnings logged, you can use the "tainted" log selector to
switch off the warnings.

The *allow_insecure_tainted_data" is deprecated already today and future
versions of Exim (no schedule yet) will ignore this option. It's purely
meant as mitigation during upgrades.

I hope we can introduce this mitigation into 4.94+fixes and into the
upcoming 4.95. But we need testing.

For now I'm doing the work on my own but public Exim repos:

- https://gitea.schlittermann.de/HeikoSchlittermann/exim/src/branch/exim-4.94+fixes+taintwarn
- https://git.exim.org/users/heiko/exim.git/shortlog/refs/heads/exim-4.94+fixes+taintwarn

But as soon as the work stabilizes, it will be merged into the upstream
source. (For now, please expect changes in the commit history!)

Currently I'm running this on a production systems without any issues so
far. You're invited to do tests in your systems too.


(The above mentioned branch is cherry-picked and squashed from the
"hs/wip/taintwarn" branch, which is based on the current master.

- https://gitea.schlittermann.de/HeikoSchlittermann/exim/src/branch/hs/wip/taintwarn
- https://git.exim.org/users/heiko/exim.git/shortlog/refs/heads/hs/wip/taintwarn

Same here, please expect rewrites of the Git history, as long as I'm
working on it.

Suggestions, question, remarks are welcome.

    Best regards from Dresden/Germany
    Viele Grüße aus Dresden
    Heiko Schlittermann
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