diff --git a/Library/Homebrew/brew.sh b/Library/Homebrew/brew.sh
index 3be0d36d88..839d057546 100644
--- a/Library/Homebrew/brew.sh
+++ b/Library/Homebrew/brew.sh
@@ -429,6 +429,20 @@ then
     printf "\\n" >&2
   fi
 
+  # Versions before Sierra don't handle custom cert files correctly, so need a full brewed curl.
+  if [[ "${HOMEBREW_MACOS_VERSION_NUMERIC}" -lt "101200" ]]
+  then
+    HOMEBREW_SYSTEM_CURL_TOO_OLD="1"
+    HOMEBREW_FORCE_BREWED_CURL="1"
+  fi
+
+  # The system libressl has a bug before macOS 10.15.6 where it incorrectly handles expired roots.
+  if [[ -z "${HOMEBREW_SYSTEM_CURL_TOO_OLD}" && "${HOMEBREW_MACOS_VERSION_NUMERIC}" -lt "101506" ]]
+  then
+    HOMEBREW_SYSTEM_CA_CERTIFICATES_TOO_OLD="1"
+    HOMEBREW_FORCE_BREWED_CA_CERTIFICATES="1"
+  fi
+
   # The system Git on macOS versions before Sierra is too old for some Homebrew functionality we rely on.
   HOMEBREW_MINIMUM_GIT_VERSION="2.14.3"
   if [[ "${HOMEBREW_MACOS_VERSION_NUMERIC}" -lt "101200" ]]
@@ -530,6 +544,12 @@ Your Git executable: $(unset git && type -p ${HOMEBREW_GIT})"
   fi
 fi
 
+if [[ -n "${HOMEBREW_FORCE_BREWED_CA_CERTIFICATES}" && -f "${HOMEBREW_PREFIX}/etc/ca-certificates/cert.pem" ]]
+then
+  export SSL_CERT_FILE="${HOMEBREW_PREFIX}/etc/ca-certificates/cert.pem"
+  export GIT_SSL_CAINFO="${HOMEBREW_PREFIX}/etc/ca-certificates/cert.pem"
+fi
+
 # A bug in the auto-update process prior to 3.1.2 means $HOMEBREW_BOTTLE_DOMAIN
 # could be passed down with the default domain.
 # This is problematic as this is will be the old bottle domain.
@@ -567,6 +587,7 @@ export HOMEBREW_DEFAULT_TEMP
 export HOMEBREW_TEMP
 export HOMEBREW_CELLAR
 export HOMEBREW_SYSTEM
+export HOMEBREW_SYSTEM_CA_CERTIFICATES_TOO_OLD
 export HOMEBREW_CURL
 export HOMEBREW_CURL_WARNING
 export HOMEBREW_SYSTEM_CURL_TOO_OLD
diff --git a/Library/Homebrew/cmd/update.sh b/Library/Homebrew/cmd/update.sh
index 8d496d1220..57206086d5 100644
--- a/Library/Homebrew/cmd/update.sh
+++ b/Library/Homebrew/cmd/update.sh
@@ -375,11 +375,21 @@ user account:
 EOS
   fi
 
+  # we may want to use Homebrew CA certificates
+  if [[ -n "${HOMEBREW_FORCE_BREWED_CA_CERTIFICATES}" && ! -f "${HOMEBREW_PREFIX}/etc/ca-certificates/cert.pem" ]]
+  then
+    # we cannot install Homebrew CA certificates if homebrew/core is unavailable.
+    if [[ -d "${HOMEBREW_LIBRARY}/Taps/homebrew/homebrew-core" || -n "${HOMEBREW_INSTALL_FROM_API}" ]]
+    then
+      brew install ca-certificates || true
+    fi
+  fi
+
   # we may want to use a Homebrew curl
   if [[ -n "${HOMEBREW_FORCE_BREWED_CURL}" && ! -x "${HOMEBREW_PREFIX}/opt/curl/bin/curl" ]]
   then
     # we cannot install a Homebrew cURL if homebrew/core is unavailable.
-    if [[ ! -d "${HOMEBREW_LIBRARY}/Taps/homebrew/homebrew-core" ]] || ! brew install curl
+    if [[ ! -d "${HOMEBREW_LIBRARY}/Taps/homebrew/homebrew-core" && -z "${HOMEBREW_INSTALL_FROM_API}" ]] || ! brew install curl
     then
       odie "'curl' must be installed and in your PATH!"
     fi
@@ -389,7 +399,7 @@ EOS
      [[ -n "${HOMEBREW_FORCE_BREWED_GIT}" && ! -x "${HOMEBREW_PREFIX}/opt/git/bin/git" ]]
   then
     # we cannot install a Homebrew Git if homebrew/core is unavailable.
-    if [[ ! -d "${HOMEBREW_LIBRARY}/Taps/homebrew/homebrew-core" ]] || ! brew install git
+    if [[ ! -d "${HOMEBREW_LIBRARY}/Taps/homebrew/homebrew-core" && -z "${HOMEBREW_INSTALL_FROM_API}" ]] || ! brew install git
     then
       odie "'git' must be installed and in your PATH!"
     fi
diff --git a/Library/Homebrew/development_tools.rb b/Library/Homebrew/development_tools.rb
index 89fd30e568..2d3ab61043 100644
--- a/Library/Homebrew/development_tools.rb
+++ b/Library/Homebrew/development_tools.rb
@@ -98,6 +98,11 @@ class DevelopmentTools
       @non_apple_gcc_version = {}
     end
 
+    sig { returns(T::Boolean) }
+    def ca_file_handles_most_https_certificates?
+      true
+    end
+
     sig { returns(T::Boolean) }
     def curl_handles_most_https_certificates?
       true
diff --git a/Library/Homebrew/env_config.rb b/Library/Homebrew/env_config.rb
index 26ef31b11a..f75e3eeace 100644
--- a/Library/Homebrew/env_config.rb
+++ b/Library/Homebrew/env_config.rb
@@ -138,6 +138,11 @@ module Homebrew
         description: "A space-separated list of licenses. Homebrew will refuse to install a " \
                      "formula if it or any of its dependencies has a license on this list.",
       },
+      HOMEBREW_FORCE_BREWED_CA_CERTIFICATES:      {
+        description: "If set, always use a Homebrew-installed `ca-certificates` rather than the system version. " \
+                     "Automatically set if the system version is too old.",
+        boolean:     true,
+      },
       HOMEBREW_FORCE_BREWED_CURL:                 {
         description: "If set, always use a Homebrew-installed `curl`(1) rather than the system version. " \
                      "Automatically set if the system version of `curl` is too old.",
diff --git a/Library/Homebrew/extend/os/mac/development_tools.rb b/Library/Homebrew/extend/os/mac/development_tools.rb
index 353ae30428..f7c49645c1 100644
--- a/Library/Homebrew/extend/os/mac/development_tools.rb
+++ b/Library/Homebrew/extend/os/mac/development_tools.rb
@@ -10,7 +10,7 @@ class DevelopmentTools
 
     alias generic_locate locate
     undef installed?, default_compiler, curl_handles_most_https_certificates?,
-          subversion_handles_most_https_certificates?
+          ca_file_handles_most_https_certificates?, subversion_handles_most_https_certificates?
 
     sig { params(tool: String).returns(T.nilable(Pathname)) }
     def locate(tool)
@@ -37,6 +37,13 @@ class DevelopmentTools
       :clang
     end
 
+    sig { returns(T::Boolean) }
+    def ca_file_handles_most_https_certificates?
+      # The system CA file is too old for some modern HTTPS certificates on
+      # older macOS versions.
+      ENV["HOMEBREW_SYSTEM_CA_CERTIFICATES_TOO_OLD"].nil?
+    end
+
     sig { returns(T::Boolean) }
     def curl_handles_most_https_certificates?
       # The system Curl is too old for some modern HTTPS certificates on
diff --git a/Library/Homebrew/formula_installer.rb b/Library/Homebrew/formula_installer.rb
index dc4975b617..0fbee44fca 100644
--- a/Library/Homebrew/formula_installer.rb
+++ b/Library/Homebrew/formula_installer.rb
@@ -796,6 +796,12 @@ class FormulaInstaller
     # let's reset Utils::Git.available? if we just installed git
     Utils::Git.clear_available_cache if formula.name == "git"
 
+    # use installed ca-certificates when it's needed and available
+    if formula.name == "ca-certificates" &&
+       !DevelopmentTools.ca_file_handles_most_https_certificates?
+      ENV["SSL_CERT_FILE"] = ENV["GIT_SSL_CAINFO"] = formula.pkgetc/"cert.pem"
+    end
+
     # use installed curl when it's needed and available
     if formula.name == "curl" &&
        !DevelopmentTools.curl_handles_most_https_certificates?
diff --git a/Library/Homebrew/sorbet/rbi/hidden-definitions/hidden.rbi b/Library/Homebrew/sorbet/rbi/hidden-definitions/hidden.rbi
index 2f3ce5b65d..c2568cc90c 100644
--- a/Library/Homebrew/sorbet/rbi/hidden-definitions/hidden.rbi
+++ b/Library/Homebrew/sorbet/rbi/hidden-definitions/hidden.rbi
@@ -4626,6 +4626,8 @@ module Homebrew::EnvConfig
 
   def self.forbidden_licenses(); end
 
+  def self.force_brewed_ca_certificates?(); end
+
   def self.force_brewed_curl?(); end
 
   def self.force_brewed_git?(); end
diff --git a/Library/Homebrew/utils/curl.rb b/Library/Homebrew/utils/curl.rb
index 6d3049d5d3..6d5ffdbf01 100644
--- a/Library/Homebrew/utils/curl.rb
+++ b/Library/Homebrew/utils/curl.rb
@@ -107,11 +107,9 @@ module Utils
         verbose:      verbose,
       }.compact
 
-      # SSL_CERT_FILE can be incorrectly set by users or portable-ruby and screw
-      # with SSL downloads so unset it here.
       result = system_command curl_executable(use_homebrew_curl: use_homebrew_curl),
                               args:    curl_args(*args, **options),
-                              env:     { "SSL_CERT_FILE" => nil }.merge(env),
+                              env:     env,
                               timeout: end_time&.remaining,
                               **command_options
 
diff --git a/docs/Manpage.md b/docs/Manpage.md
index f539da429f..e56fd566b7 100644
--- a/docs/Manpage.md
+++ b/docs/Manpage.md
@@ -1976,6 +1976,9 @@ example, run `export HOMEBREW_NO_INSECURE_REDIRECT=1` rather than just
 - `HOMEBREW_FORBIDDEN_LICENSES`
   <br>A space-separated list of licenses. Homebrew will refuse to install a formula if it or any of its dependencies has a license on this list.
 
+- `HOMEBREW_FORCE_BREWED_CA_CERTIFICATES`
+  <br>If set, always use a Homebrew-installed `ca-certificates` rather than the system version. Automatically set if the system version is too old.
+
 - `HOMEBREW_FORCE_BREWED_CURL`
   <br>If set, always use a Homebrew-installed `curl`(1) rather than the system version. Automatically set if the system version of `curl` is too old.
 
diff --git a/manpages/brew.1 b/manpages/brew.1
index 1ec0872291..94d48a3d6b 100644
--- a/manpages/brew.1
+++ b/manpages/brew.1
@@ -1,7 +1,7 @@
 .\" generated with Ronn/v0.7.3
 .\" http://github.com/rtomayko/ronn/tree/0.7.3
 .
-.TH "BREW" "1" "September 2021" "Homebrew" "brew"
+.TH "BREW" "1" "October 2021" "Homebrew" "brew"
 .
 .SH "NAME"
 \fBbrew\fR \- The Missing Package Manager for macOS (or Linux)
@@ -2826,6 +2826,12 @@ Output this many lines of output on formula \fBsystem\fR failures\.
 A space\-separated list of licenses\. Homebrew will refuse to install a formula if it or any of its dependencies has a license on this list\.
 .
 .TP
+\fBHOMEBREW_FORCE_BREWED_CA_CERTIFICATES\fR
+.
+.br
+If set, always use a Homebrew\-installed \fBca\-certificates\fR rather than the system version\. Automatically set if the system version is too old\.
+.
+.TP
 \fBHOMEBREW_FORCE_BREWED_CURL\fR
 .
 .br