{"id":"CVE-2026-54911","aliases":["GHSA-3j69-69wj-xqx2","PYSEC-2026-2294"],"title":"UltraJSON: Malformed/Truncated UTF-8 Accepted and Silently Rewritten in ujson.dumps()","summary":"UltraJSON: Malformed/Truncated UTF-8 Accepted and Silently Rewritten in ujson.dumps()","severity":"medium","cvss":6.5,"cvssVector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N","vendor":"ujson","product":"ujson","ecosystem":"pip","affected":["ujson < 5.13.0"],"patched":["ujson 5.13.0"],"published":"2026-06-19","updated":"2026-09-10","sourceUpdated":"2026-09-10T03:50:55.310105853Z","source":"OSV","sourceUrl":"https://osv.dev/vulnerability/GHSA-3j69-69wj-xqx2","references":[{"url":"https://github.com/ultrajson/ultrajson/security/advisories/GHSA-3j69-69wj-xqx2"},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-54911"},{"url":"https://github.com/ultrajson/ultrajson/commit/169eaf36b1116fece5034ee79a7a0ef3f6deedcf"},{"url":"https://github.com/pypa/advisory-database/tree/main/vulns/ujson/PYSEC-2026-2294.yaml"},{"url":"https://github.com/ultrajson/ultrajson"},{"url":"https://github.com/ultrajson/ultrajson/releases/tag/5.13.0"},{"url":"https://github.com/advisories/GHSA-3j69-69wj-xqx2"}],"tags":["osv","pip","ghsa"],"epss":0.00371,"epssPercentile":0.31045,"cwe":["CWE-20"],"ingestedAt":"2026-06-22T13:35:24.395Z","slug":"CVE-2026-54911","body":"## Overview\n\n### 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.\n\n## Affected packages\n\n- `ujson < 5.13.0`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `ujson 5.13.0`","depth":"sunlit","depthScore":36,"depthScoreParts":{"impact":35.8,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[]}