CVE-2026-54911— UltraJSON: Malformed/Truncated UTF-8 Accepted and Silently Rewritten in ujson.dumps()
Quick assessment
Affected
ultrajson ultrajson
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.
ultrajson ultrajson是ultrajson的JSON解码库。 UltraJSON 5.12.1之前版本存在输入验证错误漏洞，该漏洞源于ujson.dumps()的reject_bytes=False选项接受畸形或截断的UTF-8字节序列，导致输入验证绕过和数据完整性问题。
Possible ATT&CK Techniques 1 AI
T1564.008 · Email Hiding Rules
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I. Basic Information for CVE-2026-54911
Vulnerability Information
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Vulnerability Title
UltraJSON: Malformed/Truncated UTF-8 Accepted and Silently Rewritten in ujson.dumps()
Source: CVE Program / CVE List V5
Vulnerability Description
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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
输入验证不恰当
Source: CVE Program / CVE List V5
Vulnerability Title
UltraJSON 输入验证错误漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
ultrajson ultrajson是ultrajson的JSON解码库。 UltraJSON 5.12.1之前版本存在输入验证错误漏洞，该漏洞源于ujson.dumps()的reject_bytes=False选项接受畸形或截断的UTF-8字节序列，导致输入验证绕过和数据完整性问题。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)
Affected Products
II. Public POCs for CVE-2026-54911
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Patches & Fixes for CVE-2026-54911 (1)
Vendor Advisories for CVE-2026-54911 (1)
Other References for CVE-2026-54911 (1)
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🔗 Release 5.13.0 · ultrajson/ultrajson · GitHub x_refsource_MISC
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V. Comments for CVE-2026-54911
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