OpenSSL 4.0 is in experimental. This package fails to build against it:

| libtool: link: x86_64-linux-gnu-gcc -I../include -I../../../../include -pedantic -Wall -Wcast-qual -Wextra -Wformat-security -Winline -Wmissing-declarations -Wmissing-prototypes -Wnested-externs -Wpointer-arith -Wshadow -Wstrict-prototypes -Wswitch-default -Wswitch-enum -Wunused -Wwrite-strings -Wlogical-op -Wunused-result -std=c99 -g -O2 -Werror=implicit-function-declaration -ffile-prefix-map=/build/reproducible-path/libcoap3-4.3.5=. -fstack-protector-strong -fstack-clash-protection -Wformat -Werror=format-security -fcf-protection -fdiagnostics-color -D_GNU_SOURCE -Wl,-z -Wl,relro -Wl,-z -Wl,now -o .libs/coap-client-openssl coap-client.o  -lssl -lcrypto ../.libs/libcoap-3-openssl.so
| /usr/bin/x86_64-linux-gnu-ld.bfd: ../.libs/libcoap-3-openssl.so: undefined reference to `ENGINE_init'
| /usr/bin/x86_64-linux-gnu-ld.bfd: ../.libs/libcoap-3-openssl.so: undefined reference to `ENGINE_ctrl_cmd'
| /usr/bin/x86_64-linux-gnu-ld.bfd: ../.libs/libcoap-3-openssl.so: undefined reference to `ENGINE_ctrl_cmd_string'
| /usr/bin/x86_64-linux-gnu-ld.bfd: ../.libs/libcoap-3-openssl.so: undefined reference to `ENGINE_load_dynamic'
| /usr/bin/x86_64-linux-gnu-ld.bfd: ../.libs/libcoap-3-openssl.so: undefined reference to `ENGINE_free'
| /usr/bin/x86_64-linux-gnu-ld.bfd: ../.libs/libcoap-3-openssl.so: undefined reference to `ENGINE_finish'
| /usr/bin/x86_64-linux-gnu-ld.bfd: ../.libs/libcoap-3-openssl.so: undefined reference to `ENGINE_by_id'
| /usr/bin/x86_64-linux-gnu-ld.bfd: ../.libs/libcoap-3-openssl.so: undefined reference to `ENGINE_set_default'
| /usr/bin/x86_64-linux-gnu-ld.bfd: ../.libs/libcoap-3-openssl.so: undefined reference to `ENGINE_load_private_key'
| collect2: error: ld returned 1 exit status
| make[4]: *** [Makefile:596: coap-client-openssl] Error 1

Full buildlog
https://breakpoint.cc/openssl-rebuild/logs-4/attempted/libcoap3_4.3.5-2_amd64-2026-04-19T07:47:12Z

Sebastian

Dear Maintainer,

The fix is available upstream https://github.com/obgm/libcoap/commit/5ea8b7d5e956d1574d78115da007bf23d118f1ae

We believe that the bug you reported is fixed in the latest version of
libcoap3, which is due to be installed in the Debian FTP archive.

A summary of the changes between this version and the previous one is
attached.

Thank you for reporting the bug, which will now be closed.  If you
have further comments please address them to 1138305@bugs.debian.org,
and the maintainer will reopen the bug report if appropriate.

Debian distribution maintenance software
pp.
Thorsten Alteholz <debian@alteholz.de> (supplier of updated libcoap3 package)

(This message was generated automatically at their request; if you
believe that there is a problem with it please contact the archive
administrators by mailing ftpmaster@ftp-master.debian.org)
Format: 1.8
Date: Mon, 22 Jun 2026 10:23:22 +0200
Source: libcoap3
Architecture: source
Version: 4.3.5-4
Distribution: unstable
Urgency: medium
Maintainer: Debian IoT Maintainers <debian-iot-maintainers@lists.alioth.debian.org>
Changed-By: Thorsten Alteholz <debian@alteholz.de>
Closes: 1138305
Changes:
 libcoap3 (4.3.5-4) unstable; urgency=medium
 .
   * support openssl 4.0 (Closes: #1138305)
     (thanks to Ravi Kant Sharma for pointing to the upstream commit)
Checksums-Sha1:
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 4f652109f730eb7494c0880fecd46b049fdabb47 588595 libcoap3_4.3.5.orig.tar.bz2
 d976f235515c535354d3de0c7603b2c6f3a98a28 17908 libcoap3_4.3.5-4.debian.tar.xz
 cde6a3753d2abfd4c5f3a06f65ce15e6dc0f126e 11027 libcoap3_4.3.5-4_amd64.buildinfo
Checksums-Sha256:
 5f45a276e6ad31e0fb24d9a0ecf9d8e34e2ff73f6e8d47490fa2c7280d824a01 2392 libcoap3_4.3.5-4.dsc
 a332b682ceacef4c3130b2fb17851db02020c3f64b8a562c1ffd8d9b8a9320d4 588595 libcoap3_4.3.5.orig.tar.bz2
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 7744031fee22da8d91d831ca29d78383896fdd7b50266f0aadb958a6f3c8be48 11027 libcoap3_4.3.5-4_amd64.buildinfo
Files:
 dc24285a010d6e077b54908163b15777 2392 libs optional libcoap3_4.3.5-4.dsc
 86c5364cd4c5a7d7eb94e560ec777969 588595 libs optional libcoap3_4.3.5.orig.tar.bz2
 bda9342b358b745e725b9e1b0d415658 17908 libs optional libcoap3_4.3.5-4.debian.tar.xz
 b0d5663f91ffd900cc71c1b687ad656d 11027 libs optional libcoap3_4.3.5-4_amd64.buildinfo
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