jsmn: use size_t for token offsets and harden against overflow

Use size_t for token start/end offsets instead of int, replace the -1
sentinel with SIZE_MAX, and add a defensive guard against offset
wraparound. This prevents overflow when parsing very large JSON inputs.

This addresses issue #282.

Credit to @ylwango613 for repporting.

3 files changed, 59 insertions, 25 deletions

diff --git a/src/jplist.c b/src/jplist.c
index 9a40844..2c88756 100644
--- a/src/jplist.c
+++ b/src/jplist.c
@@ -719,8 +719,8 @@ static plist_t parse_array(const char* js, jsmntok_info_t* ti, int* index, uint3
         return NULL;
     }
     plist_t arr = plist_new_array();
-    int num_tokens = ti->tokens[*index].size;
-    int num;
+    size_t num_tokens = ti->tokens[*index].size;
+    size_t num;
     int j = (*index)+1;
     for (num = 0; num < num_tokens; num++) {
         if (j >= ti->count) {
@@ -770,8 +770,8 @@ static plist_t parse_object(const char* js, jsmntok_info_t* ti, int* index, uint
         ti->err = PLIST_ERR_MAX_NESTING;
         return NULL;
     }
-    int num_tokens = ti->tokens[*index].size;
-    int num;
+    size_t num_tokens = ti->tokens[*index].size;
+    size_t num;
     int j = (*index)+1;
     if (num_tokens % 2 != 0) {
         PLIST_JSON_ERR("%s: number of children must be even\n", __func__);
@@ -844,14 +844,15 @@ plist_err_t plist_from_json(const char *json, uint32_t length, plist_t * plist)
 
     jsmn_parser parser;
     jsmn_init(&parser);
-    int maxtoks = 256;
-    int curtoks = 0;
+    unsigned int maxtoks = 256;
+    unsigned int curtoks = 0;
     int r = 0;
     jsmntok_t *tokens = NULL;
 
     do {
         jsmntok_t* newtokens = (jsmntok_t*)realloc(tokens, sizeof(jsmntok_t)*maxtoks);
         if (!newtokens) {
+            free(tokens);
             PLIST_JSON_ERR("%s: Out of memory\n", __func__);
             return PLIST_ERR_NO_MEM;
         }
@@ -861,8 +862,14 @@ plist_err_t plist_from_json(const char *json, uint32_t length, plist_t * plist)
 
         r = jsmn_parse(&parser, json, length, tokens, maxtoks);
         if (r == JSMN_ERROR_NOMEM) {
+            if (maxtoks > (unsigned int)INT_MAX - 16) {
+                free(tokens);
+                return PLIST_ERR_NO_MEM;
+            }
             maxtoks+=16;
             continue;
+        } else if (r < 0) {
+            break;
         }
     } while (r == JSMN_ERROR_NOMEM);
 
@@ -879,6 +886,10 @@ plist_err_t plist_from_json(const char *json, uint32_t length, plist_t * plist)
             PLIST_JSON_ERR("%s: Incomplete JSON, more bytes expected\n", __func__);
             free(tokens);
             return PLIST_ERR_PARSE;
+        case JSMN_ERROR_LIMIT:
+            PLIST_JSON_ERR("%s: Input data too large\n", __func__);
+            free(tokens);
+            return PLIST_ERR_PARSE;
         default:
             break;
     }
diff --git a/src/jsmn.c b/src/jsmn.c
index f190312..889b8d7 100644
--- a/src/jsmn.c
+++ b/src/jsmn.c
@@ -3,6 +3,8 @@
  * Simple JSON parser
  *
  * Copyright (c) 2010 Serge A. Zaitsev
+ * Updated to use size_t for token offsets and harden against overflows.
+ *   (Nikias Bassen, January 2026)
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy
  * of this software and associated documentation files (the "Software"), to deal
@@ -24,20 +26,25 @@
  */
 
 #include <stdlib.h>
+#include <stdint.h>
+#include <limits.h>
+#include <assert.h>
 
 #include "jsmn.h"
 
+#define JSMN_POS_INVALID ((size_t)SIZE_MAX)
+
 /**
  * Allocates a fresh unused token from the token pull.
  */
 static jsmntok_t *jsmn_alloc_token(jsmn_parser *parser,
-                jsmntok_t *tokens, int num_tokens) {
+                jsmntok_t *tokens, unsigned int num_tokens) {
         jsmntok_t *tok;
-        if (parser->toknext >= num_tokens) {
+        if ((unsigned int)parser->toknext >= num_tokens) {
                 return NULL;
         }
         tok = &tokens[parser->toknext++];
-        tok->start = tok->end = -1;
+        tok->start = tok->end = JSMN_POS_INVALID;
         tok->size = 0;
 #ifdef JSMN_PARENT_LINKS
         tok->parent = -1;
@@ -49,7 +56,7 @@ static jsmntok_t *jsmn_alloc_token(jsmn_parser *parser,
  * Fills token type and boundaries.
  */
 static void jsmn_fill_token(jsmntok_t *token, jsmntype_t type,
-                            int start, int end) {
+                            size_t start, size_t end) {
         token->type = type;
         token->start = start;
         token->end = end;
@@ -60,9 +67,9 @@ static void jsmn_fill_token(jsmntok_t *token, jsmntype_t type,
  * Fills next available token with JSON primitive.
  */
 static jsmnerr_t jsmn_parse_primitive(jsmn_parser *parser, const char *js,
-                jsmntok_t *tokens, int num_tokens) {
+                jsmntok_t *tokens, unsigned int num_tokens) {
         jsmntok_t *token;
-        int start;
+        size_t start;
 
         start = parser->pos;
 
@@ -107,10 +114,10 @@ found:
  * Fills next token with JSON string.
  */
 static jsmnerr_t jsmn_parse_string(jsmn_parser *parser, const char *js,
-                jsmntok_t *tokens, int num_tokens) {
+                jsmntok_t *tokens, unsigned int num_tokens) {
         jsmntok_t *token;
 
-        int start = parser->pos;
+        size_t start = parser->pos;
 
         parser->pos++;
 
@@ -162,7 +169,7 @@ static jsmnerr_t jsmn_parse_string(jsmn_parser *parser, const char *js,
 /**
  * Parse JSON string and fill tokens.
  */
-jsmnerr_t jsmn_parse(jsmn_parser *parser, const char *js, unsigned int length, jsmntok_t *tokens,
+jsmnerr_t jsmn_parse(jsmn_parser *parser, const char *js, size_t length, jsmntok_t *tokens,
                 unsigned int num_tokens) {
         jsmnerr_t r;
         int i;
@@ -170,6 +177,13 @@ jsmnerr_t jsmn_parse(jsmn_parser *parser, const char *js, unsigned int length, j
 
         parser->end = length;
 
+        if (num_tokens >= INT_MAX) {
+                return JSMN_ERROR_LIMIT;
+        }
+        if (length > SIZE_MAX / 2) {
+                return JSMN_ERROR_LIMIT;
+        }
+
         for (; (parser->end > 0 && parser->pos < parser->end) && js[parser->pos] != '\0'; parser->pos++) {
                 char c;
                 jsmntype_t type;
@@ -198,10 +212,13 @@ jsmnerr_t jsmn_parse(jsmn_parser *parser, const char *js, unsigned int length, j
                                 }
                                 token = &tokens[parser->toknext - 1];
                                 for (;;) {
-                                        if (token->start != -1 && token->end == -1) {
+                                        if (token->start != JSMN_POS_INVALID && token->end == JSMN_POS_INVALID) {
                                                 if (token->type != type) {
                                                         return JSMN_ERROR_INVAL;
                                                 }
+                                                if (parser->pos == SIZE_MAX) {
+                                                        return JSMN_ERROR_INVAL;
+                                                }
                                                 token->end = parser->pos + 1;
                                                 parser->toksuper = token->parent;
                                                 break;
@@ -214,7 +231,7 @@ jsmnerr_t jsmn_parse(jsmn_parser *parser, const char *js, unsigned int length, j
 #else
                                 for (i = parser->toknext - 1; i >= 0; i--) {
                                         token = &tokens[i];
-                                        if (token->start != -1 && token->end == -1) {
+                                        if (token->start != JSMN_POS_INVALID && token->end == JSMN_POS_INVALID) {
                                                 if (token->type != type) {
                                                         return JSMN_ERROR_INVAL;
                                                 }
@@ -227,7 +244,7 @@ jsmnerr_t jsmn_parse(jsmn_parser *parser, const char *js, unsigned int length, j
                                 if (i == -1) return JSMN_ERROR_INVAL;
                                 for (; i >= 0; i--) {
                                         token = &tokens[i];
-                                        if (token->start != -1 && token->end == -1) {
+                                        if (token->start != JSMN_POS_INVALID && token->end == JSMN_POS_INVALID) {
                                                 parser->toksuper = i;
                                                 break;
                                         }
@@ -268,7 +285,7 @@ jsmnerr_t jsmn_parse(jsmn_parser *parser, const char *js, unsigned int length, j
 
         for (i = parser->toknext - 1; i >= 0; i--) {
                 /* Unmatched opened object or array */
-                if (tokens[i].start != -1 && tokens[i].end == -1) {
+                if (tokens[i].start != JSMN_POS_INVALID && tokens[i].end == JSMN_POS_INVALID) {
                         return JSMN_ERROR_PART;
                 }
         }
diff --git a/src/jsmn.h b/src/jsmn.h
index 380744d..629a0dd 100644
--- a/src/jsmn.h
+++ b/src/jsmn.h
@@ -3,6 +3,8 @@
  * Simple JSON parser (header file)
  *
  * Copyright (c) 2010 Serge A. Zaitsev
+ * Updated to use size_t for token offsets and harden against overflows.
+ *   (Nikias Bassen, January 2026)
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy
  * of this software and associated documentation files (the "Software"), to deal
@@ -25,6 +27,8 @@
 #ifndef __JSMN_H_
 #define __JSMN_H_
 
+#include <stddef.h>
+
 /**
  * JSON type identifier. Basic types are:
  *      o Object
@@ -46,6 +50,8 @@ typedef enum {
         JSMN_ERROR_INVAL = -2,
         /* The string is not a full JSON packet, more bytes expected */
         JSMN_ERROR_PART = -3,
+        /* Input exceeds implementation-defined limits */
+        JSMN_ERROR_LIMIT = -4,
         /* Everything was fine */
         JSMN_SUCCESS = 0
 } jsmnerr_t;
@@ -58,9 +64,9 @@ typedef enum {
  */
 typedef struct {
         jsmntype_t type;
-        int start;
-        int end;
-        int size;
+        size_t start;
+        size_t end;
+        size_t size;
 #ifdef JSMN_PARENT_LINKS
         int parent;
 #endif
@@ -71,8 +77,8 @@ typedef struct {
  * the string being parsed now and current position in that string
  */
 typedef struct {
-        unsigned int pos; /* offset in the JSON string */
-        unsigned int end; /* offset after last character of JSON string */
+        size_t pos; /* offset in the JSON string */
+        size_t end; /* offset after last character of JSON string */
         int toknext; /* next token to allocate */
         int toksuper; /* superior token node, e.g parent object or array */
 } jsmn_parser;
@@ -86,7 +92,7 @@ void jsmn_init(jsmn_parser *parser);
  * Run JSON parser. It parses a JSON data string into and array of tokens, each describing
  * a single JSON object.
  */
-jsmnerr_t jsmn_parse(jsmn_parser *parser, const char *js, unsigned int length,
+jsmnerr_t jsmn_parse(jsmn_parser *parser, const char *js, size_t length,
                 jsmntok_t *tokens, unsigned int num_tokens);
 
 #endif /* __JSMN_H_ */