diff --git a/dummy-metric b/dummy-metric
index 3e761d3..284bcdb 100755
--- a/dummy-metric
+++ b/dummy-metric
@@ -4,3 +4,5 @@ echo "Dummy"
 echo "95"
 echo "normal"
 awk "BEGIN{srand(); r=rand(); print r * 100}"
+
+exit 0
\ No newline at end of file
diff --git a/kiwi_simple_metrics/metrics/external.py b/kiwi_simple_metrics/metrics/external.py
index 7759ed4..9dd18d3 100644
--- a/kiwi_simple_metrics/metrics/external.py
+++ b/kiwi_simple_metrics/metrics/external.py
@@ -1,3 +1,5 @@
+import os
+import subprocess
 from typing import Iterator
 
 from ..settings import SETTINGS
@@ -5,16 +7,112 @@ from ._report import Report, ReportData
 
 
 def _hwdata() -> Iterator[ReportData]:
-    yield ReportData(
-        name="Foo",
-        value=69.42,
-        threshold=80,
-        inverted=False,
-        format=SETTINGS.cpu.report,
+    def parse_output(exe: os.PathLike) -> ReportData:
+        try:
+            # check exe is executable
+            assert os.access(exe, os.X_OK)
+
+            # run exe
+            # => TimeoutExpired: execution took too long
+            execution = subprocess.run(
+                args=[exe],
+                stdout=subprocess.PIPE,
+                timeout=SETTINGS.external.timeout,
+            )
+
+            # look at the first four output lines
+            # => UnicodeDecodeError: output is not decodable
+            output = execution.stdout.decode().split("\n")[:4]
+
+            # extract and check name (fail if empty)
+            assert (name := "".join(
+                char
+                for char in output[0]
+                if char.isprintable()
+            )[:100]) != ""
+
+        except (AssertionError,
+                subprocess.TimeoutExpired,
+                UnicodeDecodeError,
+                IndexError):
+            return ReportData.from_settings(
+                name=os.path.basename(exe)[:100],
+                value=100,
+                settings=SETTINGS.external,
+            )
+
+        try:
+            # check exit status
+            assert execution.returncode == 0
+
+            # check output length
+            assert len(output) == 4
+
+            # extract threshold and value
+            threshold = float(output[1])
+            value = float(output[3])
+
+            # extract and check inversion
+            assert (inverted := output[2].strip().lower()) in (
+                "normal", "inverted",
+            )
+
+        except (ValueError, AssertionError):
+            return ReportData.from_settings(
+                name=name,
+                value=100,
+                settings=SETTINGS.external,
+            )
+
+        # success
+        return ReportData(
+            name=name,
+            value=value,
+            threshold=threshold,
+            inverted=inverted == "inverted",
+            format=SETTINGS.external.report,
+        )
+
+    yield from (
+        parse_output(exe)
+        for exe in SETTINGS.external.executables
     )
 
 
 def external() -> Report | None:
+    """
+    External Metric
+    =====
+
+    This metric's values are defined external executables (e.g. shell scripts).
+    Any executable with suitable output can be used as a value for this metric.
+
+    To comply, the executable's output must be UTF-8 decodable and start with
+    four consecutive lines holding the following information:
+
+    1. value name
+    2. percent threshold
+    3. the string "normal" or "inverted", without quotes
+    4. percent current value
+
+    The executable may produce additional output, which will be ignored.
+    Percentages may be floating point numbers and must use a decimal point "."
+    as a separator in that case.
+    The output is evaluated once execution finishes with exit status 0.
+    A report is generated for each executable. Its value name is stripped of
+    non-printable characters and limited to a length of 100.
+
+    Non-compliance will be reported as failed values, i.e. normal values with a
+    threshold of 0% and a value of 100%, in these cases:
+
+    - non-executable files and executables outputting non-UTF8:
+        reported as the files' basename
+    - executables with generally noncompliant outputs:
+        reported as the first line of output
+    - failure to parse any of the threshold, inversion or current value
+    - otherwise compliant executables with non-zero exit status
+    """
+
     return Report.aggregate(
         settings=SETTINGS.external,
         get_data=_hwdata,
diff --git a/kiwi_simple_metrics/settings/metric.py b/kiwi_simple_metrics/settings/metric.py
index 14a3722..a0de329 100644
--- a/kiwi_simple_metrics/settings/metric.py
+++ b/kiwi_simple_metrics/settings/metric.py
@@ -82,39 +82,11 @@ class DiskMS(MetricSettings):
 
 
 class ExternalMS(MetricSettings):
-    """
-    External Metric
-    =====
-
-    This metric's values are defined external executables (e.g. bash scripts).
-    Any executable with suitable output can be used as a value for this metric.
-
-    To comply, the executable's output must start with four consecutive lines,
-    holding the following information:
-
-    1. value name (max. 100 characters)
-    2. percent threshold
-    3. the string "normal" or "inverted", without quotes
-    4. percent current value
-
-    Percentages may be floating point numbers and must use a decimal point "."
-    as a separator in that case.
-
-    Non-compliance will be reported as failed values as follows:
-
-    - non-executable files are reported as the files' basename
-    - executables with generally noncompliant outputs are reported as
-      the first line of their output truncated to 100 chars
-    - failure to parse the threshold or inversion results in
-      an upper threshold of 0%
-    - failure to parse the current value results in
-      an upper threshold of 0% and a value of 100%
-    - compliant executables with non-zero exit status are still
-      reported as a failed value
-    """
-
-    name: str = "External Metrics"
+    name: str = "External Metric"
     threshold: float = 0
 
     # path to executable files
     executables: list[FilePath] = Field(default_factory=list)
+
+    # wait at most this many seconds for each executable
+    timeout: int = 60