UltraJSON is a fast JSON encoder and decoder written in pure C with bindings for Python 3.7+. Prior to 5.13.0, 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. This vulnerability is fixed in 5.13.0.
PUBLISHED5.2CWE-20
UltraJSON: Malformed/Truncated UTF-8 Accepted and Silently Rewritten in ujson.dumps()
Problem type
Affected products
ultrajson
ultrajson
< 5.13.0 - AFFECTED
References
https://github.com/ultrajson/ultrajson/security/advisories/GHSA-3j69-69wj-xqx2
https://github.com/ultrajson/ultrajson/security/advisories/GHSA-3j69-69wj-xqx2
https://github.com/ultrajson/ultrajson/commit/169eaf36b1116fece5034ee79a7a0ef3f6deedcf
https://github.com/ultrajson/ultrajson/commit/169eaf36b1116fece5034ee79a7a0ef3f6deedcf
https://github.com/ultrajson/ultrajson/releases/tag/5.13.0
https://github.com/ultrajson/ultrajson/releases/tag/5.13.0
GitHub Security Advisories
GHSA-3j69-69wj-xqx2
UltraJSON: Malformed/Truncated UTF-8 Accepted and Silently Rewritten in ujson.dumps()
https://github.com/advisories/GHSA-3j69-69wj-xqx2Summary
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.
Details
The 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:
- Invalid continuation bytes are replaced with valid ones:
b'\xcf\x13'->b'\xcf\x93' - Unterminated sequence completes the sequence:
b'\xc3'->b'\xc3\x80' - ... or leads to reading past the end of string:
b'\xf0\x90\x94'->b"\xf0\x90\x94\x80inxcontrib'"
Impact
An application relying on reject_bytes=False for UTF-8 handling may experience:
- Data integrity issues
- Experience validation bypass if said validation occurs before serialisation
Remediation
The 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.
Workarounds
Decoding bytes to strings in Python before passing them to ujson.dumps() avoids this issue.
github.com
https://github.com/ultrajson/ultrajson/security/advisories/GHSA-3j69-69wj-xqx2
github.com
https://github.com/ultrajson/ultrajson/commit/169eaf36b1116fece5034ee79a7a0ef3f6deedcf
github.com
https://github.com/ultrajson/ultrajson/releases/tag/5.13.0
github.com
https://github.com/advisories/GHSA-3j69-69wj-xqx2
JSON source
https://cveawg.mitre.org/api/cve/CVE-2026-54911Click to expand
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