diff --git a/internal/actions/severity.go b/internal/actions/severity.go
index 6dd404cd7..f6855063b 100644
--- a/internal/actions/severity.go
+++ b/internal/actions/severity.go
@@ -46,6 +46,7 @@ func (a *severityFn) Init(r plugintypes.RuleMetadata, data string) error {
 		return err
 	}
 	r.(*corazawaf.Rule).Severity_ = sev
+	r.(*corazawaf.Rule).HasSeverity_ = true
 	return nil
 }
 
diff --git a/internal/corazarules/rule.go b/internal/corazarules/rule.go
index 09822481e..b0f11278d 100644
--- a/internal/corazarules/rule.go
+++ b/internal/corazarules/rule.go
@@ -20,14 +20,17 @@ type RuleMetadata struct {
 	Line_     int
 	Rev_      string
 	Severity_ types.RuleSeverity
-	Version_  string
-	Tags_     []string
-	Maturity_ int
-	Accuracy_ int
-	Operator_ string
-	Phase_    types.RulePhase
-	Raw_      string
-	SecMark_  string
+	// HasSeverity_ reports whether the rule explicitly set a severity action.
+	// This distinguishes an unset value from RuleSeverityEmergency (0).
+	HasSeverity_ bool
+	Version_     string
+	Tags_        []string
+	Maturity_    int
+	Accuracy_    int
+	Operator_    string
+	Phase_       types.RulePhase
+	Raw_         string
+	SecMark_     string
 	// Contains the Id of the parent rule if you are inside
 	// a chain. Otherwise, it will be 0
 	ParentID_ int
diff --git a/internal/corazawaf/transaction.go b/internal/corazawaf/transaction.go
index d868b51cf..af0daf452 100644
--- a/internal/corazawaf/transaction.go
+++ b/internal/corazawaf/transaction.go
@@ -555,10 +555,19 @@ func (tx *Transaction) MatchRule(r *Rule, mds []types.MatchData) {
 	}
 
 	// set highest_severity
-	hs := tx.variables.highestSeverity
-	maxSeverity, _ := types.ParseRuleSeverity(hs.Get())
-	if r.Severity_ > maxSeverity {
-		hs.Set(strconv.Itoa(r.Severity_.Int()))
+	// Only update when severity was explicitly set via the severity action.
+	// This mirrors ModSecurity v3 behavior (m_severity > -1 check in transaction.cc).
+	if r.HasSeverity_ {
+		hs := tx.variables.highestSeverity
+		currentVal := defaultHighestSeverity
+		if v := hs.Get(); v != "" {
+			if parsed, err := strconv.Atoi(v); err == nil {
+				currentVal = parsed
+			}
+		}
+		if r.Severity_.Int() < currentVal {
+			hs.Set(strconv.Itoa(r.Severity_.Int()))
+		}
 	}
 
 	mr := &corazarules.MatchedRule{
diff --git a/internal/corazawaf/waf.go b/internal/corazawaf/waf.go
index 54e28eaac..f72f07889 100644
--- a/internal/corazawaf/waf.go
+++ b/internal/corazawaf/waf.go
@@ -32,6 +32,14 @@ var wafIDCounter atomic.Uint64
 const (
 	// DefaultRequestBodyJsonDepthLimit is the default limit for the depth of JSON objects in the request body
 	DefaultRequestBodyJsonDepthLimit = 1024
+
+	// defaultHighestSeverity is the default value for HIGHEST_SEVERITY when no rules
+	// with severity have been matched. Value 255 aligns with ModSecurity behavior:
+	// - ModSec v2: apache2/msc_util.c highest_severity initialized to 255
+	//   https://github.com/owasp-modsecurity/ModSecurity/blob/v2/master/apache2/msc_util.c
+	// - ModSec v3: src/transaction.cc m_highestSeverity initialized to 255
+	//   https://github.com/owasp-modsecurity/ModSecurity/blob/v3/master/src/transaction.cc
+	defaultHighestSeverity = 255
 )
 
 // WAF instance is used to store configurations and rules
@@ -259,7 +267,7 @@ func (w *WAF) newTransaction(opts Options) *Transaction {
 	tx.variables.reqbodyProcessorError.Set("0")
 	tx.variables.requestBodyLength.Set("0")
 	tx.variables.duration.Set("0")
-	tx.variables.highestSeverity.Set("0")
+	tx.variables.highestSeverity.Set(strconv.Itoa(defaultHighestSeverity))
 	tx.variables.uniqueID.Set(tx.id)
 	tx.setTimeVariables()
 
diff --git a/testing/engine/rulemetadata.go b/testing/engine/rulemetadata.go
index 8ba4f6d3d..34bf211c9 100644
--- a/testing/engine/rulemetadata.go
+++ b/testing/engine/rulemetadata.go
@@ -33,3 +33,88 @@ SecAction "id:1, log, severity:5"
 SecRule HIGHEST_SEVERITY "@eq 5" "id:2, log"
 `,
 })
+
+var _ = profile.RegisterProfile(profile.Profile{
+	Meta: profile.Meta{
+		Author:      "majiayu000",
+		Description: "Test HIGHEST_SEVERITY defaults to 255 when no rules with severity fire",
+		Enabled:     true,
+		Name:        "rulemetadata_default_severity.yaml",
+	},
+	Tests: []profile.Test{
+		{
+			Title: "highest_severity_default",
+			Stages: []profile.Stage{
+				{
+					Stage: profile.SubStage{
+						Output: profile.ExpectedOutput{
+							TriggeredRules: []int{1},
+						},
+					},
+				},
+			},
+		},
+	},
+	Rules: `
+SecRule HIGHEST_SEVERITY "@eq 255" "id:1, log"
+`,
+})
+
+// Regression: a rule without severity must not poison HIGHEST_SEVERITY.
+// Without the HasSeverity_ guard, the no-severity rule's zero value (0) would
+// win over a later explicit severity:5, leaving HIGHEST_SEVERITY stuck at 0.
+var _ = profile.RegisterProfile(profile.Profile{
+	Meta: profile.Meta{
+		Author:      "majiayu000",
+		Description: "Test that rules without severity do not poison HIGHEST_SEVERITY",
+		Enabled:     true,
+		Name:        "rulemetadata_no_severity_poison.yaml",
+	},
+	Tests: []profile.Test{
+		{
+			Title: "no_severity_then_explicit",
+			Stages: []profile.Stage{
+				{
+					Stage: profile.SubStage{
+						Output: profile.ExpectedOutput{
+							TriggeredRules: []int{1, 2, 3},
+						},
+					},
+				},
+			},
+		},
+	},
+	Rules: `
+SecAction "id:1, log"
+SecAction "id:2, log, severity:5"
+SecRule HIGHEST_SEVERITY "@eq 5" "id:3, log"
+`,
+})
+
+var _ = profile.RegisterProfile(profile.Profile{
+	Meta: profile.Meta{
+		Author:      "majiayu000",
+		Description: "Test HIGHEST_SEVERITY with multiple severities keeps the lowest number",
+		Enabled:     true,
+		Name:        "rulemetadata_highest_severity.yaml",
+	},
+	Tests: []profile.Test{
+		{
+			Title: "highest_severity_multiple",
+			Stages: []profile.Stage{
+				{
+					Stage: profile.SubStage{
+						Output: profile.ExpectedOutput{
+							TriggeredRules: []int{1, 2, 3},
+						},
+					},
+				},
+			},
+		},
+	},
+	Rules: `
+SecAction "id:1, log, severity:5"
+SecAction "id:2, log, severity:2"
+SecRule HIGHEST_SEVERITY "@eq 2" "id:3, log"
+`,
+})