CVE-2026-54911 is a medium-severity improper input validation vulnerability in ujson (pip), affecting versions <= 5.12.1. It is fixed in 5.13.0.
Summary ujson.dumps() (or ujson.dump() or ujson.encode()) have a rejectbytes=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, rejectbytes=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.
The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.
CVE-2026-54911 has a CVSS score of 6.5 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment.
A fixed version is available (5.13.0). Upgrading removes the vulnerable code path.
pip
ujson (<= 5.12.1)ujson → 5.13.0 (pip)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 instead of chasing every advisory.
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Upgrade ujson to 5.13.0 or later to resolve this vulnerability.
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
CVE-2026-54911 is a medium-severity improper input validation vulnerability in ujson (pip), affecting versions <= 5.12.1. It is fixed in 5.13.0. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
CVE-2026-54911 has a CVSS score of 6.5 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
ujson (pip) versions <= 5.12.1 is affected.
Yes. CVE-2026-54911 is fixed in 5.13.0. Upgrade to this version or later.
Whether CVE-2026-54911 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
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.
Upgrade ujson to 5.13.0 or later.