# CVE-2021-45958: UltraJSON vulnerable to Out-of-bounds Write

Source: https://junglewise.ai/threats/cve-2021-45958-ultrajson-vulnerable-to-out-of-bounds-write  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2022-01-01  
License: CC BY 4.0 with a link back

CVE-2021-45958 · Severity: low · CVSS 3.1 · Published 2022-01-01

UltraJSON (aka ujson) through 5.1.0 has a stack-based buffer overflow in Buffer_AppendIndentUnchecked (called from encode). Exploitation can, for example, use a large amount of indentation.

Technologies: [ujson (PyPI)](https://junglewise.ai/threats/technologies/ujson). Vendors: [PyPI](https://junglewise.ai/threats/vendors/pypi).

## Quick answers

**Is CVE-2021-45958 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2021-45958 being exploited in the wild.

**How severe is CVE-2021-45958?** Low severity, with a CVSS score of 3.1, published 1 January 2022.

**Which products and versions are affected?** ujson (PyPI) ECOSYSTEM:introduced=1.34,fixed=5.2.0, ECOSYSTEM:introduced=4.0.2,fixed=5.1.0.

**Is there a fix for CVE-2021-45958?** Yes. ujson (PyPI): upgrade to 5.1.0, 5.2.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**Am I affected by CVE-2021-45958?** Check whether you run ujson (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- ujson (PyPI): upgrade to 5.1.0, 5.2.0 or later

  ```sh
  pip install "ujson>=5.2.0"
  ```


## Affected products

- ujson (PyPI) ECOSYSTEM:introduced=1.34,fixed=5.2.0, ECOSYSTEM:introduced=4.0.2,fixed=5.1.0

## Identifiers

- CVE-2021-45958
- GHSA-fh56-85cw-5pq6
- PYSEC-2022-25

## Related threats

- [PYSEC-2026-1057 - Incorrect handling of invalid surrogate pair characters](https://junglewise.ai/threats/cve-2022-31116-incorrect-handling-of-invalid-surrogate-pair-characters)
- [PYSEC-2026-1056 - Potential double free of buffer during string decoding](https://junglewise.ai/threats/cve-2022-31117-potential-double-free-of-buffer-during-string-decoding)
- [UltraJSON improper input validation in ujson.dumps UTF-8 handling](https://junglewise.ai/threats/cve-2026-54911-ultrajson-improper-input-validation-in-ujson-dumps-utf-8-handling)
- [UltraJSON memory leak in ujson.dump on write failure](https://junglewise.ai/threats/cve-2026-44660-ultrajson-memory-leak-in-ujson-dump-on-write-failure)
- [UltraJSON integer overflow in indent handling](https://junglewise.ai/threats/cve-2026-32875-ultrajson-integer-overflow-in-indent-handling)