Summary "Oj::Parser#parse" is vulnerable to a heap use-after-free when a SAJ/SAJ2 callback mutates the input JSON string during parsing. The C engine holds a raw "const byte *" pointer into the Ruby string's internal buffer. If a callback (e.g. "hash_start") resizes the string — for example by calling "String#replace" with a longer value — Ruby reallocates the string buffer and frees the old one. The C parser's pointer is left dangling; the next character read at "parser.c:607" is a use-after-free. Version - Software: oj gem - Affected: all versions with "ext/oj/parser.c" - Latest tested: 3.17.1 (confirmed present) Details "ext/oj/parser.c", "parser_parse" → "parse": static VALUE parser_parse(VALUE self, VALUE json) { const byte *ptr = (const byte *)StringValuePtr(json); // raw pointer into Ruby string // ... parse(p, ptr); // ptr used throughout; any realloc frees the backing buffer } // parser.c:607 static void parse(ojParser p, const byte *json) { const byte *b = json; // ... for (; '\0' != *b; b++) { // ← UAF: reads freed memory after callback resizes json Ruby's "String#replace" (or "<<", "gsub!", etc.) can trigger a reallocation of the string's internal buffer if the new content is larger than the embedded capacity, freeing the old buffer that "ptr" still points to. ASAN report: ==372273==ERROR: AddressSanitizer: heap-use-after-free on address 0x51900008ed81 READ of size 1 at 0x51900008ed81 thread T0 #0 parse /ext/oj/parser.c:607 #1 parser_parse /ext/oj/parser.c:1408 0x51900008ed81 is located 1 bytes inside of 1023-byte region [0x51900008ed80, 0x51900008f17f) freed by thread T0 here: #0 free #1 ruby_sized_xfree (libruby-3.3.so.3.3) Shadow bytes: [fd]fd fd fd fd fd ... (entire region freed) Reproduce require 'oj' class Mutator def initialize(json) = (@json = json; @done = false) def hash_start(key) return if @done; @done = true @json.replace('x' * 1_000_000) # triggers String realloc, frees original buffer end def hash_end(key); end def array_start(key); end def array_end(key); end def add_value(value, key); end end json = '{"a":1,"pad":"' + ('A' * 1000) + '","z":2}' parser = Oj::Parser.new(:saj) parser.handler = Mutator.new(json) parser.parse(json)