diff --git a/xtraplatform-cql/src/main/java/de/ii/xtraplatform/cql/infra/CqlTextParser.java b/xtraplatform-cql/src/main/java/de/ii/xtraplatform/cql/infra/CqlTextParser.java
index 0f25822ec..278126b14 100644
--- a/xtraplatform-cql/src/main/java/de/ii/xtraplatform/cql/infra/CqlTextParser.java
+++ b/xtraplatform-cql/src/main/java/de/ii/xtraplatform/cql/infra/CqlTextParser.java
@@ -22,6 +22,12 @@
 
 public class CqlTextParser {
 
+  // Guard the recursive-descent parse (and the subsequent tree visit) against deeply nested or
+  // oversized input: both recurse with the nesting depth of the expression, so a filter with a few
+  // thousand nested parentheses would otherwise overflow the stack and crash the request thread.
+  private static final int MAX_LENGTH = 100_000;
+  private static final int MAX_NESTING_DEPTH = 250;
+
   private CqlParser.CqlFilterContext parseToTree(String cql) {
     CqlLexer lexer = new CqlLexer(CharStreams.fromString(cql));
     lexer.removeErrorListeners();
@@ -43,12 +49,57 @@ public Cql2Expression parse(String cql, EpsgCrs defaultCrs) throws CqlParseExcep
 
   @SuppressWarnings("PMD.AvoidUncheckedExceptionsInSignatures")
   public Cql2Expression parse(String cql, CqlTextVisitor visitor) throws CqlParseException {
+    checkComplexity(cql);
     try {
       CqlParser.CqlFilterContext cqlFilterContext = parseToTree(cql);
 
       return (Cql2Expression) visitor.visit(cqlFilterContext);
     } catch (ParseCancellationException e) {
       throw new CqlParseException(e.getMessage(), e);
+    } catch (StackOverflowError e) {
+      // Safety net in case a construct recurses past the stack despite the depth guard.
+      throw new CqlParseException("the filter expression is nested too deeply");
+    }
+  }
+
+  // Reject an over-long or too-deeply-nested filter before parsing. Nesting depth is measured by
+  // counting parentheses outside string literals (single-quoted, with '' as an escaped quote), so
+  // parentheses inside a literal value do not count toward the limit.
+  private static void checkComplexity(String cql) throws CqlParseException {
+    if (cql == null) {
+      return;
+    }
+    if (cql.length() > MAX_LENGTH) {
+      throw new CqlParseException(
+          "the filter expression exceeds the maximum length of " + MAX_LENGTH + " characters");
+    }
+    int depth = 0;
+    int maxDepth = 0;
+    boolean inString = false;
+    for (int i = 0; i < cql.length(); i++) {
+      char c = cql.charAt(i);
+      if (c == '\'') {
+        if (inString && i + 1 < cql.length() && cql.charAt(i + 1) == '\'') {
+          i++; // skip an escaped '' quote, staying inside the string
+        } else {
+          inString = !inString;
+        }
+      } else if (!inString) {
+        if (c == '(') {
+          depth++;
+          if (depth > maxDepth) {
+            maxDepth = depth;
+          }
+        } else if (c == ')' && depth > 0) {
+          depth--;
+        }
+      }
+    }
+    if (maxDepth > MAX_NESTING_DEPTH) {
+      throw new CqlParseException(
+          "the filter expression is nested too deeply (maximum "
+              + MAX_NESTING_DEPTH
+              + " levels of parentheses)");
     }
   }
 
diff --git a/xtraplatform-cql/src/test/groovy/de/ii/xtraplatform/cql/infra/CqlTextParserComplexitySpec.groovy b/xtraplatform-cql/src/test/groovy/de/ii/xtraplatform/cql/infra/CqlTextParserComplexitySpec.groovy
new file mode 100644
index 000000000..07945743a
--- /dev/null
+++ b/xtraplatform-cql/src/test/groovy/de/ii/xtraplatform/cql/infra/CqlTextParserComplexitySpec.groovy
@@ -0,0 +1,60 @@
+/*
+ * Copyright 2025 interactive instruments GmbH
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+package de.ii.xtraplatform.cql.infra
+
+import de.ii.xtraplatform.cql.domain.CqlParseException
+import de.ii.xtraplatform.crs.domain.OgcCrs
+import spock.lang.Specification
+
+class CqlTextParserComplexitySpec extends Specification {
+
+    CqlTextParser parser = new CqlTextParser()
+
+    def "a deeply nested filter is rejected with CqlParseException, not a StackOverflowError"() {
+        given:
+        String cql = ("(" * 300) + "foo = 1" + (")" * 300)
+
+        when:
+        parser.parse(cql, OgcCrs.CRS84)
+
+        then:
+        CqlParseException e = thrown()
+        e.message.toLowerCase().contains("nested too deeply")
+    }
+
+    def "an over-long filter is rejected"() {
+        given:
+        String cql = "a" * 100_001
+
+        when:
+        parser.parse(cql, OgcCrs.CRS84)
+
+        then:
+        CqlParseException e = thrown()
+        e.message.toLowerCase().contains("maximum length")
+    }
+
+    def "parentheses inside a string literal do not count toward the nesting limit"() {
+        given:
+        String cql = "foo = '" + ("(" * 300) + "'"
+
+        when:
+        parser.parse(cql, OgcCrs.CRS84)
+
+        then:
+        noExceptionThrown()
+    }
+
+    def "a normal filter parses"() {
+        when:
+        parser.parse("foo = 'bar'", OgcCrs.CRS84)
+
+        then:
+        noExceptionThrown()
+    }
+}