diff --git a/subprojects/groovy-json/src/main/java/groovy/json/JsonLexer.java b/subprojects/groovy-json/src/main/java/groovy/json/JsonLexer.java
index 4ec3fa5d2be..11b60e429ae 100644
--- a/subprojects/groovy-json/src/main/java/groovy/json/JsonLexer.java
+++ b/subprojects/groovy-json/src/main/java/groovy/json/JsonLexer.java
@@ -45,6 +45,10 @@ public class JsonLexer implements Iterator<JsonToken> {
     private static final char UPPER_E  = 'E';
     private static final char ZERO     = '0';
     private static final char NINE     = '9';
+    /** The characters which may follow a backslash in a JSON string, other than {@code u}. */
+    private static final String SIMPLE_ESCAPES = "\"\\/bfnrt";
+    /** The digits a {@code \\u} escape may carry; deliberately ASCII only. */
+    private static final String HEX_DIGITS = "0123456789abcdefABCDEF";
 
     private final LineColumnReader reader;
 
@@ -107,18 +111,19 @@ public JsonToken nextToken() {
                 StringBuilder currentContent = new StringBuilder("\"");
                 // consume the first double quote starting the string
                 reader.read();
-                boolean isEscaped = false;
                 for (;;) {
                     int read = reader.read();
                     if (read == -1) return null;
 
-                    isEscaped = (!isEscaped && currentContent.charAt(currentContent.length() - 1) == '\\');
-
                     char charRead = (char) read;
                     currentContent.append(charRead);
 
-                    if (charRead == '"' && !isEscaped &&
-                            possibleTokenType.matching(currentContent.toString())) {
+                    if (charRead == '\\') {
+                        // Consume and check the escape sequence where it is read. Validating
+                        // here keeps the scan linear in the length of the token, and reports
+                        // the position of the offending escape rather than of the token end.
+                        readEscapeSequence(currentContent, possibleTokenType);
+                    } else if (charRead == '"') {
                         token.setEndLine(reader.getLine());
                         token.setEndColumn(reader.getColumn());
                         token.setText(unescape(currentContent.toString()));
@@ -163,6 +168,37 @@ public JsonToken nextToken() {
         }
     }
 
+    /**
+     * Reads the remainder of an escape sequence whose backslash has already been consumed,
+     * appending it to the token content and rejecting anything RFC 8259 does not allow to
+     * follow a backslash.
+     *
+     * @param currentContent the token content read so far, ending in the backslash
+     * @param type the token type being matched, for the error message
+     * @throws IOException if reading fails
+     */
+    private void readEscapeSequence(StringBuilder currentContent, JsonTokenType type) throws IOException {
+        int escapeRead = reader.read();
+        if (escapeRead == -1) return; // unterminated; the caller stops at end of input
+        char escapeChar = (char) escapeRead;
+        currentContent.append(escapeChar);
+
+        if (escapeChar == 'u') {
+            for (int i = 0; i < 4; i += 1) {
+                int hexRead = reader.read();
+                if (hexRead == -1) return;
+                char hexChar = (char) hexRead;
+                currentContent.append(hexChar);
+                // Character.digit would accept non-ASCII digits, which JSON does not.
+                if (HEX_DIGITS.indexOf(hexChar) == -1) {
+                    throwJsonException(currentContent.toString(), type);
+                }
+            }
+        } else if (SIMPLE_ESCAPES.indexOf(escapeChar) == -1) {
+            throwJsonException(currentContent.toString(), type);
+        }
+    }
+
     private void throwJsonException(String content, JsonTokenType type) {
         throw new JsonException(
                 "Lexing failed on line: " +
diff --git a/subprojects/groovy-json/src/test/groovy/groovy/json/JsonSlurperMalformedStringTest.groovy b/subprojects/groovy-json/src/test/groovy/groovy/json/JsonSlurperMalformedStringTest.groovy
index 5dc44817b46..56f6811a7a2 100644
--- a/subprojects/groovy-json/src/test/groovy/groovy/json/JsonSlurperMalformedStringTest.groovy
+++ b/subprojects/groovy-json/src/test/groovy/groovy/json/JsonSlurperMalformedStringTest.groovy
@@ -133,6 +133,50 @@ class JsonSlurperMalformedStringTest {
         assertValidUnicodeEscapes('JsonSlurperClassic') { String doc -> slurper.parseText(doc).k }
     }
 
+    /**
+     * GROOVY-12272: the lexer used to re-validate the whole accumulated token at every unescaped
+     * quote, so a string holding an invalid escape followed by many quotes cost O(n^2). The
+     * escape is now checked where it is read, which both bounds the work and reports the
+     * position of the offending escape rather than of the end of the document.
+     */
+    @Test
+    void testInvalidEscapeFollowedByManyQuotesStaysLinear() {
+        def document = { int quotes -> '{"k":"\\q' + ('"' * quotes) + '"}' }
+
+        // Warm up so the measurement is not dominated by class loading and JIT.
+        3.times { parseIgnoringFailure(document(2000)) }
+
+        long small = timeParse(document(4000))
+        long large = timeParse(document(16000))
+
+        // Four times the input. Quadratic cost would be about sixteen times the work; allow a
+        // wide margin so the test is about the growth curve, not the absolute speed.
+        assert large < Math.max(small, 5) * 8,
+                "parse time grew from ${small}ms to ${large}ms for a 4x larger document"
+    }
+
+    @Test
+    void testInvalidEscapeIsReportedWhereItOccurs() {
+        def e = shouldFail(JsonException) {
+            new JsonSlurperClassic().parseText('{"k":"a\\qb"}')
+        }
+        // The report names the escape, not the whole remaining document.
+        assert e.message.contains('\\q')
+    }
+
+    private static void parseIgnoringFailure(String text) {
+        try {
+            new JsonSlurperClassic().parseText(text)
+        } catch (JsonException ignored) {
+        }
+    }
+
+    private static long timeParse(String text) {
+        long start = System.nanoTime()
+        parseIgnoringFailure(text)
+        (System.nanoTime() - start) / 1_000_000L
+    }
+
     private static void assertRejected(String context, String description, Closure parse) {
         def thrown = null
         try {