diff --git a/pkgs/tools/archivers/p7zip/default.nix b/pkgs/tools/archivers/p7zip/default.nix
index 7f892a44da5d..5e92553b636c 100644
--- a/pkgs/tools/archivers/p7zip/default.nix
+++ b/pkgs/tools/archivers/p7zip/default.nix
@@ -8,7 +8,18 @@ stdenv.mkDerivation rec {
     owner  = "jinfeihan57";
     repo   = pname;
     rev    = "v${version}";
-    sha256 = "sha256-19F4hPV0nKVuFZNbOcXrcA1uW6Y3HQolaHVIYXGmh18=";
+    sha256 = {
+      free = "sha256-DrBuf2VPdcprHI6pMSmL7psm2ofOrUf0Oj0qwMjXzkk=";
+      unfree = "sha256-19F4hPV0nKVuFZNbOcXrcA1uW6Y3HQolaHVIYXGmh18=";
+    }.${if enableUnfree then "unfree" else "free"};
+    # remove the unRAR related code from the src drv
+    # > the license requires that you agree to these use restrictions,
+    # > or you must remove the software (source and binary) from your hard disks
+    # https://fedoraproject.org/wiki/Licensing:Unrar
+    extraPostFetch = lib.optionalString (!enableUnfree) ''
+      rm -r $out/CPP/7zip/Compress/Rar*
+      find $out -name makefile'*' -exec sed -i '/Rar/d' {} +
+    '';
   };
 
   # Default makefile is full of impurities on Darwin. The patch doesn't hurt Linux so I'm leaving it unconditional
@@ -25,11 +36,6 @@ stdenv.mkDerivation rec {
     substituteInPlace makefile.machine \
       --replace 'CC=gcc'  'CC=${stdenv.cc.targetPrefix}gcc' \
       --replace 'CXX=g++' 'CXX=${stdenv.cc.targetPrefix}g++'
-  '' + lib.optionalString (!enableUnfree) ''
-    # Remove non-free RAR source code
-    # (see DOC/License.txt, https://fedoraproject.org/wiki/Licensing:Unrar)
-    rm -r CPP/7zip/Compress/Rar*
-    find . -name makefile'*' -exec sed -i '/Rar/d' {} +
   '';
 
   makeFlags = [ "DEST_HOME=${placeholder "out"}" ];
@@ -46,13 +52,20 @@ stdenv.mkDerivation rec {
 
   NIX_CFLAGS_COMPILE = lib.optionalString stdenv.cc.isClang "-Wno-error=c++11-narrowing";
 
-  meta = {
+  passthru.updateScript = ./update.sh;
+
+  meta = with lib; {
     homepage = "https://github.com/jinfeihan57/p7zip";
     description = "A new p7zip fork with additional codecs and improvements (forked from https://sourceforge.net/projects/p7zip/)";
-    platforms = lib.platforms.unix;
-    maintainers = [ lib.maintainers.raskin ];
+    license = with licenses;
+      # p7zip code is largely lgpl2Plus
+      # CPP/7zip/Compress/LzfseDecoder.cpp is bsd3
+      [ lgpl2Plus /* and */ bsd3 ] ++
+      # and CPP/7zip/Compress/Rar* are unfree with the unRAR license restriction
+      # the unRAR compression code is disabled by default
+      lib.optionals enableUnfree [ unfree ];
+    maintainers = with maintainers; [ raskin jk ];
+    platforms = platforms.unix;
     mainProgram = "7z";
-    # RAR code is under non-free UnRAR license, but we remove it
-    license = if enableUnfree then lib.licenses.unfree else lib.licenses.lgpl2Plus;
   };
 }
diff --git a/pkgs/tools/archivers/p7zip/update.sh b/pkgs/tools/archivers/p7zip/update.sh
new file mode 100755
index 000000000000..0d4b91e56e80
--- /dev/null
+++ b/pkgs/tools/archivers/p7zip/update.sh
@@ -0,0 +1,47 @@
+#! /usr/bin/env nix-shell
+#! nix-shell -i bash -p coreutils gnused curl jq
+set -euo pipefail
+cd "$(dirname "${BASH_SOURCE[0]}")"
+
+DRV_DIR="$PWD"
+
+OLD_VERSION="$(sed -nE 's/\s*version = "(.*)".*/\1/p' ./default.nix)"
+
+NEW_VERSION="$(curl https://api.github.com/repos/jinfeihan57/p7zip/releases/latest | jq .tag_name -r | tr -d 'v')"
+
+echo "comparing versions $OLD_VERSION => $NEW_VERSION"
+if [[ "$OLD_VERSION" == "$NEW_VERSION" ]]; then
+    echo "Already up to date! Doing nothing"
+    exit 0
+fi
+
+NIXPKGS_ROOT="$(realpath "$DRV_DIR/../../../..")"
+
+echo "getting free source hash"
+OLD_FREE_HASH="$(nix-instantiate --eval --strict -E "with import $NIXPKGS_ROOT {}; p7zip.src.drvAttrs.outputHash" | tr -d '"')"
+echo "getting unfree source hash"
+OLD_UNFREE_HASH="$(nix-instantiate --eval --strict -E "with import $NIXPKGS_ROOT {}; (p7zip.override { enableUnfree = true; }).src.drvAttrs.outputHash" | tr -d '"')"
+
+
+NEW_FREE_HASH=$(nix-prefetch -f "$NIXPKGS_ROOT" -E "p7zip.src" --rev "v$NEW_VERSION")
+
+NEW_UNFREE_OUT=$(nix-prefetch -f "$NIXPKGS_ROOT" -E "(p7zip.override { enableUnfree = true; }).src" --rev "v$NEW_VERSION" --output raw --print-path)
+# first line of raw output is the hash
+NEW_UNFREE_HASH="$(echo "$NEW_UNFREE_OUT" | sed -n 1p)"
+# second line of raw output is the src path
+NEW_UNFREE_SRC="$(echo "$NEW_UNFREE_OUT" | sed -n 2p)"
+# make sure to nuke the unfree src from the updater's machine
+# > the license requires that you agree to these use restrictions, or you must remove the software (source and binary) from your hard disks
+# https://fedoraproject.org/wiki/Licensing:Unrar
+nix-store --delete "$NEW_UNFREE_SRC"
+
+
+echo "updating version"
+sed -i "s/version = \"$OLD_VERSION\";/version = \"$NEW_VERSION\";/" "$DRV_DIR/default.nix"
+
+echo "updating free hash"
+sed -i "s@free = \"$OLD_FREE_HASH\";@free = \"$NEW_FREE_HASH\";@" "$DRV_DIR/default.nix"
+echo "updating unfree hash"
+sed -i "s@unfree = \"$OLD_UNFREE_HASH\";@unfree = \"$NEW_UNFREE_HASH\";@" "$DRV_DIR/default.nix"
+
+echo "done"