fix: validate AES-CBC ciphertext block alignment in Decrypt

.github/workflows/lint.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go-version: ["1.25.x"]
+        go-version: ["1.26.x"]
 
     steps:
       - uses: actions/checkout@v5

.github/workflows/test.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        go-version: ["1.25.x"]
+        go-version: ["1.26.x"]
 
     steps:
       - uses: actions/checkout@v5

aescbc/aescbc.go

@@ -28,6 +28,11 @@ func Decrypt(key []byte, ciphertext []byte) ([]byte, error) {
 
 	iv := ciphertext[:aes.BlockSize]
 	ciphertext = ciphertext[aes.BlockSize:]
+
+	if len(ciphertext) == 0 || len(ciphertext)%aes.BlockSize != 0 {
+		return nil, fmt.Errorf("ciphertext after IV must be a non-zero multiple of %d bytes but was %d", aes.BlockSize, len(ciphertext))
+	}
+
 	decrypter := cipher.NewCBCDecrypter(block, iv)
 
 	plaintextData := make([]byte, len(ciphertext))

aescbc/aescbc_test.go

@@ -30,6 +30,39 @@ func TestDecrypt(t *testing.T) {
 	assert.Equal(t, []byte("Hello world"), dec)
 }
 
+func TestDecryptBlockSizeValidation(t *testing.T) {
+	key := []byte("12345678901234567890123456789012") // 32 bytes
+
+	t.Run("IV only, no data", func(t *testing.T) {
+		ciphertext := make([]byte, 16) // exactly aes.BlockSize
+		_, err := aescbc.Decrypt(key, ciphertext)
+		assert.ErrorContains(t, err, "ciphertext after IV must be a non-zero multiple of 16 bytes but was 0")
+	})
+
+	t.Run("non-block-aligned after IV", func(t *testing.T) {
+		ciphertext := make([]byte, 17) // aes.BlockSize + 1
+		_, err := aescbc.Decrypt(key, ciphertext)
+		assert.ErrorContains(t, err, "ciphertext after IV must be a non-zero multiple of 16 bytes but was 1")
+	})
+
+	t.Run("block-aligned after IV does not panic", func(t *testing.T) {
+		ciphertext := make([]byte, 32) // aes.BlockSize + aes.BlockSize
+		assert.NotPanics(t, func() {
+			// Will fail on padding validation, but must not panic
+			_, _ = aescbc.Decrypt(key, ciphertext)
+		})
+	})
+
+	t.Run("round-trip", func(t *testing.T) {
+		plaintext := []byte("security audit fix")
+		enc, err := aescbc.Encrypt(rand.Reader, key, plaintext)
+		require.NoError(t, err)
+		dec, err := aescbc.Decrypt(key, enc)
+		require.NoError(t, err)
+		assert.Equal(t, plaintext, dec)
+	})
+}
+
 func FuzzEncryptAndDecrypt(f *testing.F) {
 	f.Add(uint64(0), uint64(0), uint64(0), uint64(0), []byte("hello"))
 	f.Fuzz(func(t *testing.T, u0, u1, u2, u3 uint64, plaintext []byte) {