{"id":"CVE-2026-54911","aliases":["GHSA-3j69-69wj-xqx2","PYSEC-2026-2294"],"url":"https://o3.security/vulnerability/CVE-2026-54911","summary":"UltraJSON: Malformed/Truncated UTF-8 Accepted and Silently Rewritten in ujson.dumps()","details":"### Summary\n`ujson.dumps()` (or `ujson.dump()` or `ujson.encode()`) have a `reject_bytes=False` option. When set, they may accept malformed or truncated UTF-8 byte sequences, silently rewriting them into different Unicode characters instead of rejecting them. This leads to input validation bypass and data integrity issues.\n\n### Details\n\nThe expected behavior is that for `x` being any bytes string, `x == ujson.loads(ujson.dumps(x, reject_bytes=False)).encode(errors=\"surrogatepass\")` should always either be true or `ujson.dumps()` will throw an exception. In reality, some strings which should've been errors are silently rewritten as other strings:\n\n* Invalid continuation bytes are replaced with valid ones: `b'\\xcf\\x13'` -> `b'\\xcf\\x93'`\n* Unterminated sequence completes the sequence: `b'\\xc3'` -> `b'\\xc3\\x80'`\n* ... or leads to reading past the end of string: `b'\\xf0\\x90\\x94'` -> `b\"\\xf0\\x90\\x94\\x80inxcontrib'\"`\n\n### Impact\n\nAn application relying on reject_bytes=False for UTF-8 handling may experience:\n\n- Data integrity issues\n- Experience validation bypass if said validation occurs before serialisation\n\n### Remediation\n\nThe missing/broken UTF-8 validation checks were added/fixed in https://github.com/ultrajson/ultrajson/commit/169eaf36b1116fece5034ee79a7a0ef3f6deedcf. We recommend upgrading to [UltraJSON 5.13.0](https://github.com/ultrajson/ultrajson/releases/tag/5.13.0).\n\n### Workarounds\n\nDecoding bytes to strings in Python before passing them to `ujson.dumps()` avoids this issue.","published":"2026-06-22T20:53:07.019Z","modified":"2026-10-28T11:09:56.652670414Z","cvss":{"score":6.5,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N"},"epss":{"score":0.00371,"percentile":0.30599,"asOf":"2026-09-12"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"PyPI","name":"ujson","fixedVersion":"5.13.0"}],"fix":{"url":"https://github.com/ultrajson/ultrajson/commit/169eaf36b1116fece5034ee79a7a0ef3f6deedcf","label":"ultrajson/ultrajson@169eaf3"},"references":[{"type":"WEB","url":"https://github.com/ultrajson/ultrajson/releases/tag/5.13.0"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/54xxx/CVE-2026-54911.json"},{"type":"ADVISORY","url":"https://github.com/ultrajson/ultrajson/security/advisories/GHSA-3j69-69wj-xqx2"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-54911"},{"type":"FIX","url":"https://github.com/ultrajson/ultrajson/commit/169eaf36b1116fece5034ee79a7a0ef3f6deedcf"},{"type":"WEB","url":"https://github.com/pypa/advisory-database/tree/main/vulns/ujson/PYSEC-2026-2294.yaml"},{"type":"PACKAGE","url":"https://github.com/ultrajson/ultrajson"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-28T11:09:56.652670414Z"}}