CVE-2026-54902: Oj: Use-After-Free in Oj::Parser SAJ Long Key Callback

Summary

Oj::Parser in SAJ mode does not protect cached object keys (≥ 35 bytes) from garbage collection. A Ruby callback that triggers GC inside hashend can cause the key string to be reclaimed while the C parser still holds a pointer to it. The subsequent access to the freed string VALUE results in a segfault, confirmed by an RIP pointing to address 0x4242 (a canary-style pattern suggesting control over the freed memory's content).

Version

- Software: oj gem - Affected: all versions with ext/oj/saj2.c / ext/oj/parser.c - Latest tested: 3.17.1 (confirmed present)

Details

Short keys (≤ 34 bytes) are stored inline on the C stack and are safe. Long keys (≥ 35 bytes) are stored as heap-allocated Ruby String objects passed to rbfuncall as the key argument. Between the key being resolved and the callback completing, a GC triggered inside the callback (e.g. GC.start) can collect the key String, leaving a dangling VALUE.

Crash output: longkeytrigger [BUG] Segmentation fault at 0x0000000000004242 closeobject+0x260 /ext/oj/usual.c:405 (calls rbfuncall with freed key) parse+0x11ff /ext/oj/parser.c:693 parserparse+0x145 /ext/oj/parser.c:1408

RIP: 0x7fd1b46d68b7 RDI: 0x0000000000004242 (freed key VALUE) R12: 0x0000000000004242

The freed VALUE 0x4242 shows the attacker-controlled content of the key string was loaded as a pointer — a classic use-after-free indicator.

Reproduce

ruby require 'oj'

class H < Oj::Saj def addvalue(value, key) GC.start(fullmark: true, immediatesweep: true) if key == 'x' end def hashstart(key); end def hashend(key); end end

p = Oj::Parser.new(:saj) p.handler = H.new p.parse('{"' + 'A' 35 + '":{"x":1}}') # long outer key, GC fires on inner key