diff --git a/.github/workflows/ci-cd.yml b/.github/workflows/ci-cd.yml
index ee8a930c..b7420349 100644
--- a/.github/workflows/ci-cd.yml
+++ b/.github/workflows/ci-cd.yml
@@ -13,7 +13,7 @@ on:
 env:
   JDK_JAVA_OPTIONS: -XX:+PrintCommandLineFlags -Xss10M # JDK_JAVA_OPTIONS is _the_ env. variable to use for modern Java
   JVM_OPTS: -XX:+PrintCommandLineFlags -Xss10M # for Java 8 only (sadly, it is not modern enough for JDK_JAVA_OPTIONS)
-  scala_212_version: "2.12.20"
+  scala_212_version: "2.12.21"
   scala_213_version: "2.13.16"
   scala_3_version: "3.3.6"
   SonatypeUrl: "https://finos.sonatype.app/platform/"
@@ -38,7 +38,7 @@ jobs:
       fail-fast: false
       matrix:
         java: ["11", "17"]
-        scala: ["2.12.20", "2.13.16", "3.3.6"]
+        scala: ["2.12.21", "2.13.16", "3.3.6"]
         node-version: ["20.x"]
     steps:
       - name: Checkout current branch
@@ -98,7 +98,7 @@ jobs:
       fail-fast: false
       matrix:
         java: ["17"] # Note there is no need ro actually run this for multiple JVM versions for JS
-        scala: ["2.12.20", "2.13.16", "3.3.6"]
+        scala: ["2.12.21", "2.13.16", "3.3.6"]
     steps:
       - name: Checkout current branch
         uses: actions/checkout@v4
@@ -135,7 +135,7 @@ jobs:
       fail-fast: false
       matrix:
         java: ["11"] # Note there is no need ro actually run this for multiple JVM versions for native
-        scala: ["2.12.20", "2.13.16", "3.3.6"]
+        scala: ["2.12.21", "2.13.16", "3.3.6"]
     steps:
       - name: Checkout current branch
         uses: actions/checkout@v4
diff --git a/mill-build/src/millbuild/deps.scala b/mill-build/src/millbuild/deps.scala
index 610734f8..0379c7bf 100644
--- a/mill-build/src/millbuild/deps.scala
+++ b/mill-build/src/millbuild/deps.scala
@@ -202,7 +202,7 @@ object Deps {
     }
 
     case object slf4j {
-      val `slf4j-api` = ivy"org.slf4j:slf4j-api:${Versions.slf4j}"
+      val `slf4j-api`    = ivy"org.slf4j:slf4j-api:${Versions.slf4j}"
       val `slf4j-simple` = ivy"org.slf4j:slf4j-simple:${Versions.slf4j}"
     }
 
@@ -282,7 +282,7 @@ object Versions {
 object ScalaVersions {
   import DevMode._
   val all      = if (devMode) Seq(scala3x) else Seq(scala213, scala3x)
-  def scala212 = "2.12.20"
+  def scala212 = "2.12.21"
   def scala213 = "2.13.16"
   def scala3x  = "3.3.6"
 
diff --git a/mill-build/src/millbuild/settings/ScalaSettings.scala b/mill-build/src/millbuild/settings/ScalaSettings.scala
index 1651d6f0..7710def1 100644
--- a/mill-build/src/millbuild/settings/ScalaSettings.scala
+++ b/mill-build/src/millbuild/settings/ScalaSettings.scala
@@ -20,7 +20,7 @@ object ScalaSettings {
   lazy val defaultVersion                                    = defaultScala3xVersion
   implicit val rw: upickle.default.ReadWriter[ScalaSettings] = upickle.default.macroRW
 
-  val defaultScala212Version = "2.12.20"
+  val defaultScala212Version = "2.12.21"
   val defaultScala213Version = "2.13.16"
   val defaultScala3xVersion  = "3.3.6"
   val defaultCrossScalaVersions: List[String] =