Install User Certificate Via ADB

I figured out a way to do this, thus i was able to trust charles proxy certificate. it will be added as trusted SSL root certificate.

First you need to get the certificate hash

openssl x509 -inform PEM -subject_hash_old -in charles-proxy-ssl-proxying-certificate.pem | head -1>toto

i use windows, store it in a var in a matter to automate the process set /p totoVar=<toto

set totoVar=%totoVar%.0 && DEL toto

cat charles-proxy-ssl-proxying-certificate.pem > %totoVar%

openssl x509 -inform PEM -text -in charles-proxy-ssl-proxying-certificate.pem -out nul >> %totoVar%

adb shell mount -o rw,remount,rw /system

adb push %totoVar% /system/etc/security/cacerts/

adb shell mount -o ro,remount,ro /system

adb reboot

Thanks to this answer Install User Certificate Via ADB I was able to adapt a script that works on a bash shell:

PEM_FILE_NAME=logger-charles-cert.pem
hash=$(openssl x509 -inform PEM -subject_hash_old -in $PEM_FILE_NAME | head -1)
OUT_FILE_NAME="$hash.0"

cp $PEM_FILE_NAME $OUT_FILE_NAME
openssl x509 -inform PEM -text -in $PEM_FILE_NAME -out /dev/null >> $OUT_FILE_NAME

echo "Saved to $OUT_FILE_NAME"
adb shell mount -o rw,remount,rw /system
adb push $OUT_FILE_NAME /system/etc/security/cacerts/
adb shell mount -o ro,remount,ro /system
adb reboot

(Yes, I know this should probably be a comment, but I don't have enough reputation to post it as a comment yet)

2022: httptoolkit has a good solution to inject a custom cert without rebooting into rooted devices/emulators

Details here: https://httptoolkit.tech/blog/intercepting-android-https/#injecting-ca-certificates-into-rooted-devices

    set -e # Fail on error
    # Create a separate temp directory, to hold the current certificates
    # Without this, when we add the mount we can't read the current certs anymore.

    mkdir -m 700 /data/local/tmp/htk-ca-copy
    # Copy out the existing certificates

    cp /system/etc/security/cacerts/* /data/local/tmp/htk-ca-copy/
    # Create the in-memory mount on top of the system certs folder

    mount -t tmpfs tmpfs /system/etc/security/cacerts
    # Copy the existing certs back into the tmpfs mount, so we keep trusting them

    mv /data/local/tmp/htk-ca-copy/* /system/etc/security/cacerts/
    # Copy our new cert in, so we trust that too

    mv ${certificatePath} /system/etc/security/cacerts/
    # Update the perms & selinux context labels, so everything is as readable as before

    chown root:root /system/etc/security/cacerts/*
    chmod 644 /system/etc/security/cacerts/*
    chcon u:object_r:system_file:s0 /system/etc/security/cacerts/*
    # Delete the temp cert directory & this script itself

    rm -r /data/local/tmp/htk-ca-copy
    rm ${injectionScriptPath}
    echo "System cert successfully injected"

Source

I was able to get a server cert to show up under the Trusted Credential -> User tab (rather than the system tab, which other answers show) with the following steps:

#!/bin/bash
subjectHash=`openssl x509 -inform PEM -subject_hash_old -in server.crt | head -n 1`
openssl x509 -in server.crt -inform PEM -outform DER -out $subjectHash.0
adb root
adb push ./$subjectHash.0 /data/misc/user/0/cacerts-added/$subjectHash.0
adb shell "su 0 chmod 644 /data/misc/user/0/cacerts-added/$subjectHash.0"
adb reboot