Here's the fix, which is a downgrade to 2.4.25-3+deb9u4, with dependencies: apt-get install apache2=2.4.25-3+deb9u4 apache2-bin=2.4.25-3+deb9u4 apache2-utils=2.4.25-3+deb9u4 apache2-data=2.4.25-3+deb9u4
Here it is: root@hostname:/home/username# dpkg-query -l apache2 Desired=Unknown/Install/Remove/Purge/Hold | Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend |/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad) ||/ Name Version Architecture Description +++-=============================-===================-===================-=============================================================== ii apache2 2.4.25-3+deb9u5 amd64 Apache HTTP Server root@hostname:/home/username# $ echo | openssl s_client -alpn h2 -connect ServerInQuestion:443 | grep ALPN depth=2 O = Digital Signature Trust Co., CN = DST Root CA X3 verify return:1 depth=1 C = US, O = Let's Encrypt, CN = Let's Encrypt Authority X3 verify return:1 depth=0 CN = ServerInQuestion verify return:1 DONE ALPN protocol: http/1.1 root@hostname:/home/username# apt-get install apache2=2.4.25-3+deb9u4 apache2-bin=2.4.25-3+deb9u4 apache2-utils=2.4.25-3+deb9u4 apache2-data=2.4.25-3+deb9u4 Reading package lists... Done Building dependency tree Reading state information... Done Suggested packages: www-browser apache2-doc apache2-suexec-pristine | apache2-suexec-custom The following packages will be DOWNGRADED: apache2 apache2-bin apache2-data apache2-utils 0 upgraded, 0 newly installed, 4 downgraded, 0 to remove and 4 not upgraded. Need to get 1797 kB of archives. After this operation, 9216 B disk space will be freed. Do you want to continue? [Y/n] y $ echo | openssl s_client -alpn h2 -connect ServerInQuestion:443 | grep ALPN depth=2 O = Digital Signature Trust Co., CN = DST Root CA X3 verify return:1 depth=1 C = US, O = Let's Encrypt, CN = Let's Encrypt Authority X3 verify return:1 depth=0 CN = ServerInQuestion verify return:1 ALPN protocol: h2 DONE
