Summary
Oj: Use-After-Free in Oj::Parser arrayclass/hashclass GC Marking
Oj::Parser in usual mode does not mark array_class and hash_class references during garbage collection. If GC runs after the class is assigned but before a parse, the class object is reclaimed, leaving the parser holding a dangling VALUE. The subsequent parse call dereferences the freed object, producing a segfault.
Version
- Software: oj gem
- Affected: all versions with
ext/oj/usual.c/ext/oj/parser.c - Latest tested: 3.17.1 (confirmed present)
Details
The parser_mark function in ext/oj/parser.c is registered as the GC mark callback for the parser's TypedData. If array_class (stored as d->array_class in the Usual struct) is not passed to rb_gc_mark, the GC does not know it is referenced and may collect it.
When close_array_class (usual.c:405) later calls rb_funcallv on the collected class VALUE, it accesses freed memory, crashing at RIP: 0x7f... / 0x0000000000000000.
Crash output:
array_class finalized
about to parse
[BUG] Segmentation fault at 0x0000000000000000
close_array_class+0x194 /ext/oj/usual.c:405
parse+0x17b3 /ext/oj/parser.c:715
parser_parse+0x10b /ext/oj/parser.c:1408
RIP: 0x7fd1b46d68b7 RBP: 0x0000000000000000
Reproduce
require 'oj'
p = Oj::Parser.new(:usual,
array_class: (ac = Class.new { def <<(_x); end }))
ObjectSpace.define_finalizer(ac, proc { warn 'array_class finalized' })
ac = nil
GC.start(full_mark: true, immediate_sweep: true) # collect the class
p.parse('[1]') # segfault
Impact
Memory is accessed after it has been freed, leading to undefined behavior in native code. Typical impact: memory corruption, crash, or potential code execution.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Already deployed Kodem?
See it in your environmentNew to Kodem? Get a demo →Remediation advice
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2026-54901? CVE-2026-54901 is a high-severity use after free vulnerability in oj (rubygems), affecting versions < 3.17.2. It is fixed in 3.17.3. Memory is accessed after it has been freed, leading to undefined behavior in native code.
- Which versions of oj are affected by CVE-2026-54901? oj (rubygems) versions < 3.17.2 is affected.
- Is there a fix for CVE-2026-54901? Yes. CVE-2026-54901 is fixed in 3.17.3. Upgrade to this version or later.
- Is CVE-2026-54901 exploitable, and should I be worried? Whether CVE-2026-54901 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-54901 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-54901? Upgrade
ojto 3.17.3 or later.