diff --git a/readme-vars.yml b/readme-vars.yml
index 787c6622..5a0514e8 100644
--- a/readme-vars.yml
+++ b/readme-vars.yml
@@ -31,7 +31,11 @@ cap_add_param_vars:
 opt_param_usage_include_env: true
 opt_param_env_vars:
   - {env_var: "SUBDOMAINS", env_value: "www,", desc: "Subdomains you'd like the cert to cover (comma separated, no spaces) ie. `www,ftp,cloud`. For a wildcard cert, set this *exactly* to `wildcard` (wildcard cert is available via `dns` validation only)"}
-  - {env_var: "CERTPROVIDER", env_value: "", desc: "Optionally define the cert provider. Set to `zerossl` for ZeroSSL certs (requires existing [ZeroSSL account](https://app.zerossl.com/signup) and the e-mail address entered in `EMAIL` env var). Otherwise defaults to Let's Encrypt."}
+  - {env_var: "CERTPROVIDER", env_value: "", desc: "Optionally define the cert provider. Set to `zerossl` for ZeroSSL certs (requires existing [ZeroSSL account](https://app.zerossl.com/signup) and the e-mail address entered in `EMAIL` env var). Set to `custom` to use a custom/internal ACME server (e.g. [step-ca](https://smallstep.com/docs/step-ca/)), which requires `CERTPROVIDERURL` (see below). Otherwise defaults to Let's Encrypt."}
+  - {env_var: "CERTPROVIDERURL", env_value: "", desc: "Only used when `CERTPROVIDER=custom`. The ACME directory URL of your custom ACME server, ie. `https://ca.example.com/acme/acme/directory`."}
+  - {env_var: "ACMECABUNDLE", env_value: "", desc: "Only used when `CERTPROVIDER=custom`. Trust an internal CA so SWAG can connect to your ACME server over TLS. Either a path to a CA bundle PEM file mounted into the container, or the base64-encoded contents of that PEM."}
+  - {env_var: "EAB_KID", env_value: "", desc: "Only used when `CERTPROVIDER=custom`. External Account Binding key identifier, if your custom ACME server requires EAB. Must be set together with `EAB_HMAC_KEY`."}
+  - {env_var: "EAB_HMAC_KEY", env_value: "", desc: "Only used when `CERTPROVIDER=custom`. External Account Binding HMAC key, if your custom ACME server requires EAB. Must be set together with `EAB_KID`."}
   - {env_var: "DNSPLUGIN", env_value: "cloudflare", desc: "Required if `VALIDATION` is set to `dns`. Options are `acmedns`, `aliyun`, `azure`, `bunny`, `cloudflare`, `cpanel`, `desec`, `digitalocean`, `directadmin`, `dnsimple`, `dnsmadeeasy`, `dnspod`, `do`, `domeneshop`, `dreamhost`, `duckdns`, `dynu`, `freedns`, `gandi`, `gehirn`, `glesys`, `godaddy`, `google`, `he`, `hetzner`, `hetzner-cloud`, `infomaniak`, `inwx`, `ionos`, `linode`, `loopia`, `luadns`, `namecheap`, `netcup`, `njalla`, `nsone`, `ovh`, `porkbun`, `rfc2136`, `route53`, `sakuracloud`, `standalone`, `transip`, and `vultr`. Also need to enter the credentials into the corresponding ini (or json for some plugins) file under `/config/dns-conf`."}
   - {env_var: "PROPAGATION", env_value: "", desc: "Optionally override (in seconds) the default propagation time for the dns plugins."}
   - {env_var: "EMAIL", env_value: "", desc: "Optional e-mail address used for cert expiration notifications (Required for ZeroSSL)."}
@@ -219,6 +223,7 @@ init_diagram: |
   "swag:latest" <- Base Images
 # changelog
 changelogs:
+  - {date: "19.06.26:", desc: "Add support for custom/internal ACME servers via `CERTPROVIDER=custom` with `CERTPROVIDERURL`, optional `ACMECABUNDLE` for internal CA trust, and optional `EAB_KID`/`EAB_HMAC_KEY` for External Account Binding."}
   - {date: "01.06.26:", desc: "Remove obsolete old cert check logic."}
   - {date: "23.01.26:", desc: "Reorder init to fix proxy conf version checks."}
   - {date: "21.12.25:", desc: "Add support for hetzner-cloud dns validation."}
diff --git a/root/app/le-renew.sh b/root/app/le-renew.sh
index c597f359..5ac4ca8c 100755
--- a/root/app/le-renew.sh
+++ b/root/app/le-renew.sh
@@ -5,5 +5,10 @@ echo "<------------------------------------------------->"
 echo
 echo "<------------------------------------------------->"
 echo "cronjob running on $(date)"
+# Trust the custom/internal CA (CERTPROVIDER=custom) when renewing, since
+# REQUESTS_CA_BUNDLE is an env var and is not persisted in cli.ini.
+if [[ -f /config/cabundle.pem ]]; then
+    export REQUESTS_CA_BUNDLE="/config/cabundle.pem"
+fi
 echo "Running certbot renew"
 certbot renew --non-interactive --config-dir /config/etc/letsencrypt --logs-dir /config/log/letsencrypt --work-dir /tmp/letsencrypt --config /config/etc/letsencrypt/cli.ini
diff --git a/root/etc/s6-overlay/s6-rc.d/init-certbot-config/run b/root/etc/s6-overlay/s6-rc.d/init-certbot-config/run
index e018d62e..df4a4351 100755
--- a/root/etc/s6-overlay/s6-rc.d/init-certbot-config/run
+++ b/root/etc/s6-overlay/s6-rc.d/init-certbot-config/run
@@ -12,6 +12,7 @@ EXTRA_DOMAINS=${EXTRA_DOMAINS}\\n\
 ONLY_SUBDOMAINS=${ONLY_SUBDOMAINS}\\n\
 VALIDATION=${VALIDATION}\\n\
 CERTPROVIDER=${CERTPROVIDER}\\n\
+CERTPROVIDERURL=${CERTPROVIDERURL}\\n\
 DNSPLUGIN=${DNSPLUGIN}\\n\
 EMAIL=${EMAIL}\\n\
 STAGING=${STAGING}\\n"
@@ -23,6 +24,12 @@ for i in "${SANED_VARS[@]}"; do
     export echo "${i}"="$(echo "${!i}" | tr '[:upper:]' '[:lower:]')"
 done
 
+# Custom ACME vars are case-sensitive (URL paths, base64 EAB keys / CA bundle),
+# so only strip surrounding quotes; do not lowercase them.
+for i in CERTPROVIDERURL ACMECABUNDLE EAB_KID EAB_HMAC_KEY; do
+    export "${i}=${!i//\"/}"
+done
+
 # Check for and install requested DNS plugins
 if grep -q "universal-package-install" <<< "${DOCKER_MODS}" && grep -q "certbot-dns" <<< "${INSTALL_PIP_PACKAGES}"; then
     echo "**** Installing requested dns plugins ****"
@@ -80,7 +87,7 @@ if [[ -f "/config/donoteditthisfile.conf" ]]; then
     mv /config/donoteditthisfile.conf /config/.donoteditthisfile.conf
 fi
 if [[ ! -f "/config/.donoteditthisfile.conf" ]]; then
-    echo -e "ORIGURL=\"${URL}\" ORIGSUBDOMAINS=\"${SUBDOMAINS}\" ORIGONLY_SUBDOMAINS=\"${ONLY_SUBDOMAINS}\" ORIGEXTRA_DOMAINS=\"${EXTRA_DOMAINS}\" ORIGVALIDATION=\"${VALIDATION}\" ORIGDNSPLUGIN=\"${DNSPLUGIN}\" ORIGPROPAGATION=\"${PROPAGATION}\" ORIGSTAGING=\"${STAGING}\" ORIGCERTPROVIDER=\"${CERTPROVIDER}\" ORIGEMAIL=\"${EMAIL}\"" >/config/.donoteditthisfile.conf
+    echo -e "ORIGURL=\"${URL}\" ORIGSUBDOMAINS=\"${SUBDOMAINS}\" ORIGONLY_SUBDOMAINS=\"${ONLY_SUBDOMAINS}\" ORIGEXTRA_DOMAINS=\"${EXTRA_DOMAINS}\" ORIGVALIDATION=\"${VALIDATION}\" ORIGDNSPLUGIN=\"${DNSPLUGIN}\" ORIGPROPAGATION=\"${PROPAGATION}\" ORIGSTAGING=\"${STAGING}\" ORIGCERTPROVIDER=\"${CERTPROVIDER}\" ORIGCERTPROVIDERURL=\"${CERTPROVIDERURL}\" ORIGEMAIL=\"${EMAIL}\"" >/config/.donoteditthisfile.conf
     echo "Created .donoteditthisfile.conf"
 fi
 
@@ -186,15 +193,23 @@ if [[ ! "${URL}" = "${ORIGURL}" ]] ||
     [[ ! "${DNSPLUGIN}" = "${ORIGDNSPLUGIN}" ]] ||
     [[ ! "${PROPAGATION}" = "${ORIGPROPAGATION}" ]] ||
     [[ ! "${STAGING}" = "${ORIGSTAGING}" ]] ||
-    [[ ! "${CERTPROVIDER}" = "${ORIGCERTPROVIDER}" ]]; then
+    [[ ! "${CERTPROVIDER}" = "${ORIGCERTPROVIDER}" ]] ||
+    [[ ! "${CERTPROVIDERURL}" = "${ORIGCERTPROVIDERURL}" ]]; then
     echo "Different validation parameters entered than what was used before. Revoking and deleting existing certificate, and an updated one will be created"
-    if [[ "${ORIGCERTPROVIDER}" = "zerossl" ]]; then
+    if [[ "${ORIGCERTPROVIDER}" = "custom" ]]; then
+        REV_ACMESERVER=("${ORIGCERTPROVIDERURL}")
+    elif [[ "${ORIGCERTPROVIDER}" = "zerossl" ]]; then
         REV_ACMESERVER=("https://acme.zerossl.com/v2/DV90")
     elif [[ "${ORIGSTAGING}" = "true" ]]; then
         REV_ACMESERVER=("https://acme-staging-v02.api.letsencrypt.org/directory")
     else
         REV_ACMESERVER=("https://acme-v02.api.letsencrypt.org/directory")
     fi
+    # if the previous provider was a custom internal CA, trust its bundle so the
+    # revocation request can verify TLS
+    if [[ "${ORIGCERTPROVIDER}" = "custom" ]] && [[ -f /config/cabundle.pem ]]; then
+        export REQUESTS_CA_BUNDLE="/config/cabundle.pem"
+    fi
     if [[ -f /config/etc/letsencrypt/live/"${ORIGDOMAIN}"/fullchain.pem ]]; then
         certbot revoke --config-dir /config/etc/letsencrypt --logs-dir /config/log/letsencrypt --work-dir /tmp/letsencrypt --config /config/etc/letsencrypt/cli.ini --non-interactive --cert-path /config/etc/letsencrypt/live/"${ORIGDOMAIN}"/fullchain.pem --key-path /config/etc/letsencrypt/live/"${ORIGDOMAIN}"/privkey.pem --server "${REV_ACMESERVER[@]}" || true
     else
@@ -204,13 +219,34 @@ if [[ ! "${URL}" = "${ORIGURL}" ]] ||
 fi
 
 # saving new variables
-echo -e "ORIGURL=\"${URL}\" ORIGSUBDOMAINS=\"${SUBDOMAINS}\" ORIGONLY_SUBDOMAINS=\"${ONLY_SUBDOMAINS}\" ORIGEXTRA_DOMAINS=\"${EXTRA_DOMAINS}\" ORIGVALIDATION=\"${VALIDATION}\" ORIGDNSPLUGIN=\"${DNSPLUGIN}\" ORIGPROPAGATION=\"${PROPAGATION}\" ORIGSTAGING=\"${STAGING}\" ORIGCERTPROVIDER=\"${CERTPROVIDER}\" ORIGEMAIL=\"${EMAIL}\"" >/config/.donoteditthisfile.conf
+echo -e "ORIGURL=\"${URL}\" ORIGSUBDOMAINS=\"${SUBDOMAINS}\" ORIGONLY_SUBDOMAINS=\"${ONLY_SUBDOMAINS}\" ORIGEXTRA_DOMAINS=\"${EXTRA_DOMAINS}\" ORIGVALIDATION=\"${VALIDATION}\" ORIGDNSPLUGIN=\"${DNSPLUGIN}\" ORIGPROPAGATION=\"${PROPAGATION}\" ORIGSTAGING=\"${STAGING}\" ORIGCERTPROVIDER=\"${CERTPROVIDER}\" ORIGCERTPROVIDERURL=\"${CERTPROVIDERURL}\" ORIGEMAIL=\"${EMAIL}\"" >/config/.donoteditthisfile.conf
 
 # if zerossl is selected or staging is set to true, use the relevant server
 if [[ "${CERTPROVIDER}" = "zerossl" ]] && [[ "${STAGING}" = "true" ]]; then
     echo "ZeroSSL does not support staging mode, ignoring STAGING variable"
 fi
-if [[ "${CERTPROVIDER}" = "zerossl" ]] && [[ -n "${EMAIL}" ]]; then
+if [[ "${CERTPROVIDER}" = "custom" ]]; then
+    if [[ -z "${CERTPROVIDERURL}" ]]; then
+        echo "CERTPROVIDER is set to custom, but CERTPROVIDERURL is not set. Please set CERTPROVIDERURL to the ACME directory URL of your custom ACME server (e.g. https://ca.example.com/acme/acme/directory). Sleeping."
+        sleep infinity
+    fi
+    echo "Using custom ACME server as the cert provider: ${CERTPROVIDERURL}"
+    if [[ "${STAGING}" = "true" ]]; then
+        echo "STAGING has no effect with a custom ACME server, ignoring STAGING variable"
+    fi
+    ACMESERVER="${CERTPROVIDERURL}"
+    # Trust a custom/internal CA so certbot can reach the ACME server over TLS.
+    # ACMECABUNDLE may be a path to a mounted PEM file or a base64-encoded PEM.
+    if [[ -n "${ACMECABUNDLE}" ]]; then
+        if [[ -f "${ACMECABUNDLE}" ]]; then
+            cp "${ACMECABUNDLE}" /config/cabundle.pem
+        else
+            echo "${ACMECABUNDLE}" | base64 -d - >/config/cabundle.pem
+        fi
+        lsiown abc:abc /config/cabundle.pem
+        echo "Custom CA bundle written to /config/cabundle.pem"
+    fi
+elif [[ "${CERTPROVIDER}" = "zerossl" ]] && [[ -n "${EMAIL}" ]]; then
     echo "ZeroSSL is selected as the cert provider, registering cert with ${EMAIL}"
     ACMESERVER="https://acme.zerossl.com/v2/DV90"
 elif [[ "${CERTPROVIDER}" = "zerossl" ]] && [[ -z "${EMAIL}" ]]; then
@@ -227,6 +263,20 @@ fi
 
 set_ini_value "server" "${ACMESERVER}" /config/etc/letsencrypt/cli.ini
 
+# Trust the custom CA for this issuance run; clean up custom remnants when not
+# using a custom provider so a previous internal CA / EAB is not reused.
+if [[ "${CERTPROVIDER}" = "custom" ]] && [[ -f /config/cabundle.pem ]]; then
+    export REQUESTS_CA_BUNDLE="/config/cabundle.pem"
+elif [[ "${CERTPROVIDER}" != "custom" ]]; then
+    unset REQUESTS_CA_BUNDLE
+    rm -f /config/cabundle.pem
+    if [[ "${ORIGCERTPROVIDER}" = "custom" ]]; then
+        # drop EAB creds left over from a custom provider so Let's Encrypt /
+        # ZeroSSL re-register cleanly
+        sed -i -e "/^eab-kid\b/d" -e "/^eab-hmac-key\b/d" /config/etc/letsencrypt/cli.ini
+    fi
+fi
+
 # figuring out domain only vs domain & subdomains vs subdomains only
 DOMAINS_ARRAY=()
 if [[ -z "${SUBDOMAINS}" ]] || [[ "${ONLY_SUBDOMAINS}" != true ]]; then
@@ -321,7 +371,22 @@ fi
 
 # generating certs if necessary
 if [[ ! -f "/config/keys/letsencrypt/fullchain.pem" ]]; then
-    if [[ "${CERTPROVIDER}" = "zerossl" ]] && [[ -n "${EMAIL}" ]]; then
+    if [[ "${CERTPROVIDER}" = "custom" ]]; then
+        # Optional External Account Binding for custom ACME servers (e.g. step-ca
+        # ACME provisioners configured to require EAB). Only consumed at account
+        # registration, so it has no effect on later renewals.
+        if [[ -n "${EAB_KID}" ]] && [[ -n "${EAB_HMAC_KEY}" ]]; then
+            echo "Registering with custom ACME server using provided EAB credentials"
+            set_ini_value "eab-kid" "${EAB_KID}" /config/etc/letsencrypt/cli.ini
+            set_ini_value "eab-hmac-key" "${EAB_HMAC_KEY}" /config/etc/letsencrypt/cli.ini
+        elif [[ -n "${EAB_KID}" ]] || [[ -n "${EAB_HMAC_KEY}" ]]; then
+            echo "Only one of EAB_KID / EAB_HMAC_KEY is set. Both are required to use External Account Binding. Sleeping."
+            sleep infinity
+        else
+            # remove any EAB values left over from a previous provider
+            sed -i -e "/^eab-kid\b/d" -e "/^eab-hmac-key\b/d" /config/etc/letsencrypt/cli.ini
+        fi
+    elif [[ "${CERTPROVIDER}" = "zerossl" ]] && [[ -n "${EMAIL}" ]]; then
         echo "Retrieving EAB from ZeroSSL"
         EAB_CREDS=$(curl -s https://api.zerossl.com/acme/eab-credentials-email --data "email=${EMAIL}")
         ZEROSSL_EAB_KID=$(echo "${EAB_CREDS}" | jq .eab_kid)