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CVE-2026-54911

Status: AnalyzedMedium (6.5)—

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.

CVSS

Exploitation probability (EPSS)

EPSS (Exploit Prediction Scoring System, FIRST) estimates how likely a vulnerability is to be exploited in the wild within 30 days. It complements CVSS (impact) and CISA KEV (confirmed exploitation).

Affected technologies (1)

Ultrajson Project — Ultrajson

CWEs

References

Raw JSON (NVD)

Show
{
  "id": "CVE-2026-54911",
  "cveTags": [],
  "metrics": {
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          "options": [
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              "exploitation": "none"
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              "technicalImpact": "partial"
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          ],
          "version": "2.0.3",
          "timestamp": "2026-06-23T14:19:57.855154Z"
        }
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          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N",
          "integrityImpact": "LOW",
          "userInteraction": "NONE",
          "attackComplexity": "LOW",
          "availabilityImpact": "NONE",
          "privilegesRequired": "NONE",
          "confidentialityImpact": "LOW"
        },
        "impactScore": 2.5,
        "exploitabilityScore": 3.9
      }
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  "affected": [
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      "source": "security-advisories@github.com",
      "affectedData": [
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          "versions": [
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  "published": "2026-06-22T22:16:50.397",
  "references": [
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      "lang": "en",
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    }
  ],
  "lastModified": "2026-06-26T20:10:54.850",
  "configurations": [
    {
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