diff --git a/pkgs/build-support/fetchurl/builder.sh b/pkgs/build-support/fetchurl/builder.sh
index ae6c372208e1..9add2b27a072 100644
--- a/pkgs/build-support/fetchurl/builder.sh
+++ b/pkgs/build-support/fetchurl/builder.sh
@@ -1,6 +1,16 @@
 source "$NIX_ATTRS_SH_FILE"
 source $mirrorsFile
 
+# Normalize `curlOpts` as a string.
+# If defined as a list (deprecated), it would be a bash array.
+if [[ "$(declare -p curlOpts 2&>/dev/null || true)" =~ ^"declare -a" ]]; then
+    unset _temp
+    _temp="${curlOpts[*]}"
+    unset curlOpts
+    curlOpts=$_temp
+    unset _temp
+fi
+
 curlVersion=$(curl -V | head -1 | cut -d' ' -f2)
 
 # Curl flags to handle redirects, not use EPSV, handle cookies for
@@ -23,17 +33,23 @@ if ! [ -f "$SSL_CERT_FILE" ]; then
     curl+=(--insecure)
 fi
 
-curl+=("${curlOptsList[@]}")
+# NOTE:
+# `netrcPhase` should not attempt to access builder.sh implementation details (e.g., the `${curl[@]}` array),
+# The implementation detail could change in any Nixpkgs revision, including backports.
+if [[ -n "${netrcPhase-}" ]]; then
+    runPhase netrcPhase
+    curl+=(--netrc-file "$PWD/netrc")
+fi
 
 curl+=(
-    ${curlOpts[*]}
+    "${curlOptsList[@]}"
+    $curlOpts
     $NIX_CURL_FLAGS
 )
 
 downloadedFile="$out"
 if [ -n "$downloadToTemp" ]; then downloadedFile="$TMPDIR/file"; fi
 
-
 tryDownload() {
     local url="$1"
     local target="$2"
diff --git a/pkgs/build-support/fetchurl/default.nix b/pkgs/build-support/fetchurl/default.nix
index 763bf5429cc1..7747a3a8f0fe 100644
--- a/pkgs/build-support/fetchurl/default.nix
+++ b/pkgs/build-support/fetchurl/default.nix
@@ -333,15 +333,6 @@ lib.extendMkDerivation {
 
       inherit preferLocalBuild;
 
-      postHook =
-        if netrcPhase == null then
-          null
-        else
-          ''
-            ${netrcPhase}
-            curlOpts="$curlOpts --netrc-file $PWD/netrc"
-          '';
-
       inherit meta;
       passthru = {
         inherit url resolvedUrl;
diff --git a/pkgs/build-support/fetchurl/tests.nix b/pkgs/build-support/fetchurl/tests.nix
index b4baeec9a9e7..c89368a6d209 100644
--- a/pkgs/build-support/fetchurl/tests.nix
+++ b/pkgs/build-support/fetchurl/tests.nix
@@ -1,11 +1,93 @@
 {
+  lib,
   testers,
   fetchurl,
+  writeShellScriptBin,
   jq,
   moreutils,
+  emptyFile,
   ...
 }:
+let
+  testFlagAppending =
+    args:
+    testers.invalidateFetcherByDrvHash
+      (fetchurl.override (previousArgs: {
+        curl = (
+          writeShellScriptBin "curl" ''
+            set -eu -o pipefail
+            hasFoo=
+            hasBar=
+            echo "curl-mock-expecting-flags: get flags: $*" >&2
+            for arg; do
+              case "$arg" in
+              -V|--version)
+                ${lib.getExe previousArgs.curl} "$arg"
+                exit "$?"
+                ;;
+              --foo)
+                echo "curl-mock-expecting-flags: \`--foo' found in the argument list passed to \`curl'." >&2
+                hasFoo=1
+                ;;
+              --bar)
+                echo "curl-mock-expecting-flags: \`--bar' found in the argument list passed to \`curl'." >&2
+                hasBar=1
+                ;;
+              esac
+            done
+            if [[ -z "$hasFoo" ]]; then
+              echo "ERROR: curl-mock-expecting-flags: \`--foo' missing in the argument list passed to \`curl'." >&2
+            fi
+            if [[ -z "$hasBar" ]]; then
+              echo "ERROR: curl-mock-expecting-flags: \`--bar' missing in the argument list passed to \`curl'." >&2
+            fi
+            if [[ -n "$hasFoo" ]] && [[ -n "$hasBar" ]]; then
+              touch $out
+            else
+              exit 1
+            fi
+          ''
+        );
+      }))
+      (
+        {
+          url = "https://www.example.com/source";
+          hash = emptyFile.outputHash;
+          recursiveHash = true; # aligned with emptyFile
+        }
+        // args
+      );
+in
 {
+  flag-appending-curlOpts = testFlagAppending {
+    name = "test-fetchurl-flag-appending-curlOpts";
+    curlOpts = "--foo --bar";
+  };
+
+  flag-appending-curlOptsList = testFlagAppending {
+    name = "test-fetchurl-flag-appending-curlOptsList";
+    curlOptsList = [
+      "--foo"
+      "--bar"
+    ];
+  };
+
+  flag-appending-netrcPhase-curlOpts = testFlagAppending {
+    name = "test-fetchurl-flag-appending-netrcPhase-curlOpts";
+    netrcPhase = ''
+      touch netrc
+      curlOpts="$curlOpts --foo --bar"
+    '';
+  };
+
+  flag-appending-netrcPhase-curlOptsList = testFlagAppending {
+    name = "test-fetchurl-flag-appending-netrcPhase-curlOptsList";
+    netrcPhase = ''
+      touch netrc
+      curlOptsList+=("--foo" "--bar")
+    '';
+  };
+
   # Tests that we can send custom headers with spaces in them
   header =
     let