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fix: truncate baggage/tag values to prevent metrics cardinality explosion #861

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Sajan Ghimire (SajanGhimire1) wants to merge 3 commits into
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fix: truncate baggage/tag values to prevent metrics cardinality explosion #861

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6f53cec
fix: truncate baggage/tag values to prevent metrics cardinality explo…
SajanGhimire1 May 26, 2026
e678382
fix: address Copilot review feedback
SajanGhimire1 May 26, 2026
8f18a42
Merge branch 'main' into fix/baggage-cardinality-dos
neilr81 May 28, 2026
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22 changes: 19 additions & 3 deletions src/Activities/Internal/ActivityMetricsSender.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -23,6 +23,13 @@ internal sealed class ActivityMetricsSender : IActivitiesEventSender, IDisposabl
private readonly HashSet<string> m_customTagObjectsDimension;
private readonly bool m_isSetParentNameAsDimensionEnabled;

/// <summary>
/// Maximum length for metric tag values to prevent
/// cardinality explosion via externally controlled
/// baggage or tag values.
/// </summary>
private const int MaxMetricTagValueLength = 256;

public ActivityMetricsSender(
IExecutionContext executionContext,
IHostEnvironment hostEnvironment,
Expand Down Expand Up @@ -68,7 +75,11 @@ public void SendActivityMetric(Activity activity)
string? baggageItem = activity.GetBaggageItem(dimension);
if (!string.IsNullOrWhiteSpace(baggageItem))
{
tagList.Add(dimension, baggageItem);
// Truncate to prevent metrics cardinality explosion
// via externally controlled W3C baggage header values
tagList.Add(dimension, baggageItem.Length > MaxMetricTagValueLength
? baggageItem[..MaxMetricTagValueLength]
: baggageItem);
}
}

Expand All @@ -77,7 +88,12 @@ public void SendActivityMetric(Activity activity)
object? tagItem = activity.GetTagItem(dimension);
if (tagItem != null)
{
tagList.Add(dimension, tagItem);
// Truncate string representation to prevent
// cardinality explosion via high-cardinality tag values
string tagValue = tagItem.ToString() ?? string.Empty;
tagList.Add(dimension, tagValue.Length > MaxMetricTagValueLength
? tagValue[..MaxMetricTagValueLength]
: tagValue);
}
}

Expand All @@ -95,4 +111,4 @@ public void SendActivityMetric(Activity activity)

public void Dispose() => m_meter.Dispose();
}
}
}