Oj: Use-After-Free in Oj::Parser Symbol Key Cache Toggle

Summary

Disabling symbol_keys on a reused Oj::Parser instance triggers a heap use-after-free. When symbol_keys is toggled from true to false, opt_symbol_keys_set frees the internal key cache (cache_free) but does not clear the pointer. The next parse call reads from the freed cache via cache_intern, producing a use-after-free.

Version

• Software: oj gem
• Affected: all versions with ext/oj/usual.c
• Latest tested: 3.17.1 (confirmed present)

Details

ext/oj/usual.c, opt_symbol_keys_set:

// usual.c:1043–1051
if (symbol_keys) {
    d->key_cache = cache_create(...);   // allocate
} else {
    cache_free(d->key_cache);           // free — but d->key_cache pointer not NULLed
}

On the next parse call, cache_key → cache_intern reads from d->key_cache which now points to freed memory.

ASAN report:

==145265==ERROR: AddressSanitizer: heap-use-after-free on address 0x50b00001a318
READ of size 8 at 0x50b00001a318 thread T0
    #0 cache_intern            /ext/oj/cache.c:328
    #1 cache_key               /ext/oj/usual.c:161
    #2 close_object            /ext/oj/usual.c:285
    #3 parse                   /ext/oj/parser.c:693
    #4 parser_parse            /ext/oj/parser.c:1408
freed by thread T0 here:
    #0 free
    #1 cache_free              /ext/oj/cache.c:277
    #2 opt_symbol_keys_set     /ext/oj/usual.c:1051
    #3 option                  /ext/oj/usual.c:1111
    #4 parser_missing          /ext/oj/parser.c:1362
0x50b00001a318 is 40 bytes inside freed 112-byte region [fd]fd fd fd fd fd fd fd

Reproduce

require 'oj'
p = Oj::Parser.new(:usual, symbol_keys: true)
p.symbol_keys = false     # frees cache without nulling pointer
p.parse('{"attacker":1}') # UAF: reads freed cache