How severe is CVE-2026-54911?

CVE-2026-54911 has a CVSS 3.x base score of 6.5, rated medium severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is low, integrity low, and availability none.

Is CVE-2026-54911 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (19th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-54911?

CVE-2026-54911 affects Ultrajson Project Ultrajson. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-54911?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.

Does CVE-2026-54911 have an EU (EUVD) identifier?

Yes. CVE-2026-54911 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-38362.

When was CVE-2026-54911 published?

CVE-2026-54911 was published on 2026-06-22 and last updated on 2026-06-26.

CVE-2026-54911

CVE-2026-54911 is a medium-severity vulnerability in Ultrajson Project Ultrajson with a CVSS 3.x base score of 6.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-20.

Key facts

Severity: Medium (CVSS 3.x base score 6.5)
EPSS exploit prediction: 0% (19th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-38362
Weakness: CWE-20
Affected product: Ultrajson Project Ultrajson
Published: 2026-06-22
Last modified: 2026-06-26

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.

Frequently asked questions

What is CVE-2026-54911?: 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.
How severe is CVE-2026-54911?: CVE-2026-54911 has a CVSS 3.x base score of 6.5, rated medium severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is low, integrity low, and availability none.
Is CVE-2026-54911 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (19th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-54911?: CVE-2026-54911 affects Ultrajson Project Ultrajson. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-54911?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
Does CVE-2026-54911 have an EU (EUVD) identifier?: Yes. CVE-2026-54911 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-38362.
When was CVE-2026-54911 published?: CVE-2026-54911 was published on 2026-06-22 and last updated on 2026-06-26.

References

Affected products (1)

cpe:2.3:a:ultrajson_project:ultrajson:*:*:*:*:*:python:*:*

More vulnerabilities in Ultrajson Project Ultrajson

CVE-2026-44660 — High (CVSS 7.5): UltraJSON is a fast JSON encoder and decoder written in pure C with bindings for Python 3.7+. Prior to 5.12.1, when…
CVE-2026-32875 — High (CVSS 7.5): UltraJSON is a fast JSON encoder and decoder written in pure C with bindings for Python 3.7+. Versions 5.10 through…
CVE-2026-32874 — High (CVSS 7.5): UltraJSON is a fast JSON encoder and decoder written in pure C with bindings for Python 3.7+. Versions 5.4.0 through…
CVE-2022-31116 — High (CVSS 7.5): UltraJSON is a fast JSON encoder and decoder written in pure C with bindings for Python 3.7+. Affected versions were…
CVE-2022-31117 — Medium (CVSS 5.9): UltraJSON is a fast JSON encoder and decoder written in pure C with bindings for Python 3.7+. In versions prior to…
CVE-2021-45958 — Medium (CVSS 5.5): UltraJSON (aka ujson) through 5.1.0 has a stack-based buffer overflow in Buffer_AppendIndentUnchecked (called from…

All CVEs affecting Ultrajson Project Ultrajson →

Other CWE-20 (Improper Input Validation) vulnerabilities

CVE-2026-48316 — Critical (CVSS 10.0): ColdFusion versions 2025.9, 2023.20 and earlier are affected by an Improper Input Validation vulnerability that could…
CVE-2026-48281 — Critical (CVSS 10.0): ColdFusion versions 2025.9, 2023.20 and earlier are affected by an Improper Input Validation vulnerability that could…
CVE-2026-48277 — Critical (CVSS 10.0): ColdFusion versions 2025.9, 2023.20 and earlier are affected by an Improper Input Validation vulnerability that could…
CVE-2026-48055 — Critical (CVSS 10.0): Streambert is a cross-platform Electron Desktop App to stream and download any video media. In versions 2.4.0 and…
CVE-2026-34910 — Critical (CVSS 10.0): A malicious actor with access to the network could exploit an Improper Input Validation vulnerability found in UniFi OS…
CVE-2026-33587 — Critical (CVSS 10.0): Lack of user input sanitisation in Open Notebook v1.8.3 allows the application user to execute Python code (and…

Browse all CWE-20 (Improper Input Validation) vulnerabilities →