diff --git a/models/issues/issue_search.go b/models/issues/issue_search.go
index e05b1101c2..ebf1f52538 100644
--- a/models/issues/issue_search.go
+++ b/models/issues/issue_search.go
@@ -220,7 +220,7 @@ func applyCustomFieldCondition(sess *xorm.Session, opts *IssuesOptions) {
 	// custom_field_value row for every specified field (AND semantics).
 	for fieldID, value := range opts.CustomFieldFilters {
 		subQuery := builder.Select("entity_id").From("custom_field_value").Where(
-			builder.Eq{"field_id": fieldID, "value": value},
+			builder.Eq{"field_id": fieldID, "value": value, "entity_type": "issue"},
 		)
 		sess.And(builder.In("issue.id", subQuery))
 	}
diff --git a/routers/api/v1/repo/issue.go b/routers/api/v1/repo/issue.go
index 80f3abd8f4..4fe5f7709e 100644
--- a/routers/api/v1/repo/issue.go
+++ b/routers/api/v1/repo/issue.go
@@ -289,16 +289,9 @@ func SearchIssues(ctx *context.APIContext) {
 		}
 	}
 
-	// Parse custom field filters from cf_{fieldID}={value} query params.
-	cfFilters := make(map[int64]string)
-	for key, values := range ctx.Req.URL.Query() {
-		if after, ok := strings.CutPrefix(key, "cf_"); ok && len(values) > 0 && values[0] != "" {
-			if fieldID, parseErr := strconv.ParseInt(after, 10, 64); parseErr == nil {
-				cfFilters[fieldID] = values[0]
-			}
-		}
-	}
-	if len(cfFilters) > 0 {
+	if cfFilters, cfErr := parseAPICustomFieldFilters(ctx); cfErr != nil {
+		return
+	} else if len(cfFilters) > 0 {
 		searchOpt.CustomFieldFilters = cfFilters
 	}
 
@@ -530,16 +523,9 @@ func ListIssues(ctx *context.APIContext) {
 		searchOpt.MentionID = optional.Some(mentionedByID)
 	}
 
-	// Parse custom field filters from cf_{fieldID}={value} query params.
-	cfFilters := make(map[int64]string)
-	for key, values := range ctx.Req.URL.Query() {
-		if after, ok := strings.CutPrefix(key, "cf_"); ok && len(values) > 0 && values[0] != "" {
-			if fieldID, parseErr := strconv.ParseInt(after, 10, 64); parseErr == nil {
-				cfFilters[fieldID] = values[0]
-			}
-		}
-	}
-	if len(cfFilters) > 0 {
+	if cfFilters, cfErr := parseAPICustomFieldFilters(ctx); cfErr != nil {
+		return
+	} else if len(cfFilters) > 0 {
 		searchOpt.CustomFieldFilters = cfFilters
 	}
 
@@ -579,6 +565,25 @@ func getUserIDForFilter(ctx *context.APIContext, queryName string) int64 {
 	return user.ID
 }
 
+// parseAPICustomFieldFilters extracts cf_{fieldID}=value query parameters.
+// Returns an error (and writes a 400 response) if a field ID is non-numeric or non-positive.
+func parseAPICustomFieldFilters(ctx *context.APIContext) (map[int64]string, error) {
+	filters := make(map[int64]string)
+	for key, values := range ctx.Req.URL.Query() {
+		after, ok := strings.CutPrefix(key, "cf_")
+		if !ok || len(values) == 0 || values[0] == "" {
+			continue
+		}
+		fieldID, err := strconv.ParseInt(after, 10, 64)
+		if err != nil || fieldID <= 0 {
+			ctx.APIError(http.StatusBadRequest, fmt.Sprintf("invalid custom field filter: cf_%s must use a positive numeric field ID", after))
+			return nil, fmt.Errorf("invalid cf_ param")
+		}
+		filters[fieldID] = values[0]
+	}
+	return filters, nil
+}
+
 // GetIssue get an issue of a repository
 func GetIssue(ctx *context.APIContext) {
 	// swagger:operation GET /repos/{owner}/{repo}/issues/{index} issue issueGetIssue
diff --git a/routers/web/repo/issue_list.go b/routers/web/repo/issue_list.go
index a1650878c9..4552ea610a 100644
--- a/routers/web/repo/issue_list.go
+++ b/routers/web/repo/issue_list.go
@@ -525,19 +525,14 @@ func prepareIssueFilterAndList(ctx *context.Context, milestoneID int64, projectI
 	prepareIssueFilterExclusiveOrderScopes(ctx, preparedLabelFilter.AllLabels)
 
 	// Parse custom field filters from query params (cf_{fieldID}={value}).
-	customFieldFilters := make(map[int64]string)
-	for key, values := range ctx.Req.URL.Query() {
-		if after, ok := strings.CutPrefix(key, "cf_"); ok && len(values) > 0 && values[0] != "" {
-			if fieldID, err := strconv.ParseInt(after, 10, 64); err == nil {
-				customFieldFilters[fieldID] = values[0]
-			}
-		}
-	}
+	customFieldFilters := parseCustomFieldQueryParams(ctx.Req.URL.Query())
 
 	// Load custom field definitions for the filter UI.
+	// If this fails, clear filters so users don't get invisible filtering.
 	customFieldDefs, cfErr := issues_model.GetCustomFieldsByOwner(ctx, repo.OwnerID, issues_model.CustomFieldScopeIssue)
 	if cfErr != nil {
 		log.Error("prepareIssueFilterAndList: GetCustomFieldsByOwner: %v", cfErr)
+		customFieldFilters = make(map[int64]string)
 	}
 	ctx.Data["CustomFieldDefs"] = customFieldDefs
 	ctx.Data["CustomFieldFilters"] = customFieldFilters
@@ -551,7 +546,9 @@ func prepareIssueFilterAndList(ctx *context.Context, milestoneID int64, projectI
 	for _, f := range customFieldDefs {
 		if f.Options != "" {
 			var opts []string
-			if err := json.Unmarshal([]byte(f.Options), &opts); err == nil {
+			if err := json.Unmarshal([]byte(f.Options), &opts); err != nil {
+				log.Error("prepareIssueFilterAndList: invalid options JSON for field %d (%s): %v", f.ID, f.Name, err)
+			} else {
 				fieldOptions[f.ID] = opts
 			}
 		}
@@ -810,3 +807,17 @@ func Issues(ctx *context.Context) {
 
 	ctx.HTML(http.StatusOK, tplIssues)
 }
+
+// parseCustomFieldQueryParams extracts cf_{fieldID}=value query parameters.
+// Non-numeric or non-positive field IDs are silently skipped.
+func parseCustomFieldQueryParams(query url.Values) map[int64]string {
+	filters := make(map[int64]string)
+	for key, values := range query {
+		if after, ok := strings.CutPrefix(key, "cf_"); ok && len(values) > 0 && values[0] != "" {
+			if fieldID, err := strconv.ParseInt(after, 10, 64); err == nil && fieldID > 0 {
+				filters[fieldID] = values[0]
+			}
+		}
+	}
+	return filters
+}