From fbb56daa279e65ad6905ec1b73d94f1eda7e291b Mon Sep 17 00:00:00 2001
From: Balbino Gamboa <balbinodaniel7@gmail.com>
Date: Fri, 21 Nov 2025 14:01:40 -0500
Subject: [PATCH] Fix: Enforce strict 4-digit limit on JSON Unicode escapes to
 prevent greedy parsing
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The JSON lexer was incorrectly parsing unicode escapes by consuming more
characters than the standard 4 hex digits specified in RFC 8259. This
could lead to incorrect parsing when valid hex characters followed a
unicode escape sequence.

Changes:
- Refactored appendHex() to strictly parse exactly 4 hex digits
- Replaced fromHexChar() with a fast O(1) lookup table (HEX_TABLE)
- Added explicit validation that fails on invalid hex digits
- Added test case to verify correct parsing of \u00f3a as 'óa'
---
 .../serialization/json/JsonParserTest.kt      |  8 ++++
 .../json/internal/lexer/AbstractJsonLexer.kt  | 48 ++++++++++++-------
 2 files changed, 40 insertions(+), 16 deletions(-)

diff --git a/formats/json-tests/commonTest/src/kotlinx/serialization/json/JsonParserTest.kt b/formats/json-tests/commonTest/src/kotlinx/serialization/json/JsonParserTest.kt
index 94f7052cd4..69e4c392ed 100644
--- a/formats/json-tests/commonTest/src/kotlinx/serialization/json/JsonParserTest.kt
+++ b/formats/json-tests/commonTest/src/kotlinx/serialization/json/JsonParserTest.kt
@@ -113,4 +113,12 @@ class JsonParserTest : JsonTestBase() {
         assertTrue { value.jsonPrimitive.isString }
         assertEquals("null", obj["k"]!!.jsonPrimitive.content)
     }
+
+    @Test
+    fun testUnicodeEscapeWithFollowingHex() {
+        // Test case for greedy parsing bug
+        val input = "\"\\u00f3a\""
+        val decoded = Json.decodeFromString<String>(input)
+        assertEquals("óa", decoded, "Should parse 'ó' then 'a', not try to consume 'a'")
+    }
 }
diff --git a/formats/json/commonMain/src/kotlinx/serialization/json/internal/lexer/AbstractJsonLexer.kt b/formats/json/commonMain/src/kotlinx/serialization/json/internal/lexer/AbstractJsonLexer.kt
index 5f570a95ec..85a2704470 100644
--- a/formats/json/commonMain/src/kotlinx/serialization/json/internal/lexer/AbstractJsonLexer.kt
+++ b/formats/json/commonMain/src/kotlinx/serialization/json/internal/lexer/AbstractJsonLexer.kt
@@ -148,6 +148,17 @@ internal abstract class AbstractJsonLexer {
 
     protected abstract val source: CharSequence
 
+    // Lookup table for fast hex digit validation and conversion
+    companion object {
+        private val HEX_TABLE = IntArray(128) { -1 }
+
+        init {
+            for (i in '0'..'9') HEX_TABLE[i.code] = i - '0'
+            for (i in 'a'..'f') HEX_TABLE[i.code] = i - 'a' + 10
+            for (i in 'A'..'F') HEX_TABLE[i.code] = i - 'A' + 10
+        }
+    }
+
     @JvmField
     internal var currentPosition: Int = 0 // position in source
 
@@ -498,6 +509,7 @@ internal abstract class AbstractJsonLexer {
     }
 
     private fun appendHex(source: CharSequence, startPos: Int): Int {
+        // Ensure we have at least 4 characters for the unicode sequence
         if (startPos + 4 >= source.length) {
             currentPosition = startPos
             ensureHaveChars()
@@ -505,12 +517,25 @@ internal abstract class AbstractJsonLexer {
                 fail("Unexpected EOF during unicode escape")
             return appendHex(source, currentPosition)
         }
-        escapedString.append(
-            ((fromHexChar(source, startPos) shl 12) +
-                    (fromHexChar(source, startPos + 1) shl 8) +
-                    (fromHexChar(source, startPos + 2) shl 4) +
-                    fromHexChar(source, startPos + 3)).toChar()
-        )
+
+        var value = 0
+        // Strict 4-iteration loop to prevent greedy parsing and comply with RFC 8259
+        for (i in 0..3) {
+            val char = source[startPos + i]
+            val code = char.code
+
+            // Fast O(1) lookup. Check range to avoid IndexOutOfBounds for non-ASCII chars
+            val digit = if (code < 128) HEX_TABLE[code] else -1
+
+            if (digit == -1) {
+                fail("Invalid Unicode escape sequence: expected hex digit, found '$char'")
+            }
+
+            // Accumulate result
+            value = (value shl 4) or digit
+        }
+
+        escapedString.append(value.toChar())
         return startPos + 4
     }
 
@@ -518,15 +543,6 @@ internal abstract class AbstractJsonLexer {
         if (!condition) fail(message(), position)
     }
 
-    private fun fromHexChar(source: CharSequence, currentPosition: Int): Int {
-        return when (val character = source[currentPosition]) {
-            in '0'..'9' -> character.code - '0'.code
-            in 'a'..'f' -> character.code - 'a'.code + 10
-            in 'A'..'F' -> character.code - 'A'.code + 10
-            else -> fail("Invalid toHexChar char '$character' in unicode escape")
-        }
-    }
-
     fun skipElement(allowLenientStrings: Boolean) {
         val tokenStack = mutableListOf<Byte>()
         var lastToken = peekNextToken()
@@ -759,4 +775,4 @@ internal abstract class AbstractJsonLexer {
             currentPosition = snapshot
         }
     }
-}
+}
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