# CVE-2026-54911: UltraJSON: Malformed/Truncated UTF-8 Accepted and Silently Rewritten in ujson.dumps()

> Attackers can bypass input validation and corrupt data integrity by exploiting silent UTF-8 rewriting in UltraJSON when reject_bytes is disabled.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 22 Jun 2026
- **Updated:** 23 Jun 2026
- **Also known as:** DEBIAN-CVE-2026-54911, ECHO-0346-5369-6592, GHSA-3j69-69wj-xqx2, MINI-89q8-3p3r-9gwc, PYSEC-2026-2294, ROOT-APP-PYPI-CVE-2026-54911, UBUNTU-CVE-2026-54911

## What is CVE-2026-54911?

CVE-2026-54911 is a medium-severity vulnerability in ultrajson, published on 22 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-20 (Improper Input Validation)

## Which versions are affected by CVE-2026-54911?

3 open source packages ship an affected version, including ujson. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `ujson` | PyPI | All versions | 5.13.0 |
| `ujson` | PyPI | All versions | 5.13.0 |
| `ujson` | PyPI | All versions | 5.13.0 |

Products: Ultrajson Project Ultrajson.

## How do I fix CVE-2026-54911?

Upgrade ujson to 5.13.0 or later.

## Is CVE-2026-54911 being exploited?

There is no known exploitation of CVE-2026-54911 in the wild and no public exploit code.

## Other vulnerabilities in ultrajson

- [CVE-2026-44660](https://insights.sbomify.com/CVE-2026-44660/): UltraJSON: Memory Leak in ujson.dump() on Write Failure (high 8.7)
- [CVE-2026-32874](https://insights.sbomify.com/CVE-2026-32874/): UltraJSON has a Memory Leak parsing large integers allows DoS (high 7.5)
- [CVE-2026-32875](https://insights.sbomify.com/CVE-2026-32875/): UltraJSON has an integer overflow handling large indent leads to buffer overflow or infinite loop (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-54911 and fix it if it is affected.

What it is: CVE-2026-54911 is a medium-severity vulnerability in ultrajson, published on 22 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-54911 in the wild and no public exploit code.

Affected packages and fixed versions:
- ujson (PyPI): affected all versions; fixed in 5.13.0
- ujson (PyPI): affected all versions; fixed in 5.13.0
- ujson (PyPI): affected all versions; fixed in 5.13.0

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-54911/ (Markdown: https://insights.sbomify.com/CVE-2026-54911/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-54911
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-54911
- OSV: https://osv.dev/vulnerability/CVE-2026-54911

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-54911/
Summaries are generated and may contain mistakes; the official description above is the record.