# CVE-2018-1000656: Flask is vulnerable to Denial of Service via incorrect encoding of JSON data

Source: https://junglewise.ai/threats/cve-2018-1000656-flask-is-vulnerable-to-denial-of-service-via-incorrect-encoding  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2018-08-20  
License: CC BY 4.0 with a link back

CVE-2018-1000656 · Severity: low · CVSS 3.1 · Published 2018-08-20

The Pallets Project flask version Before 0.12.3 contains a CWE-20: Improper Input Validation vulnerability in flask that can result in Large amount of memory usage possibly leading to denial of service. This attack appear to be exploitable via Attacker provides JSON data in incorrect encoding. This vulnerability appears to have been fixed in 0.12.3. NOTE: this may overlap CVE-2019-1010083.

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

## Quick answers

**Is CVE-2018-1000656 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2018-1000656 being exploited in the wild.

**How severe is CVE-2018-1000656?** Low severity, with a CVSS score of 3.1, published 20 August 2018.

**Which products and versions are affected?** flask (PyPI) ECOSYSTEM:introduced=0,fixed=0.12.3.

**Is there a fix for CVE-2018-1000656?** Yes. flask (PyPI): upgrade to 0.12.3 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**Am I affected by CVE-2018-1000656?** Check whether you run flask (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

- flask (PyPI): upgrade to 0.12.3 or later

  ```sh
  pip install "flask>=0.12.3"
  ```


## Affected products

- flask (PyPI) ECOSYSTEM:introduced=0,fixed=0.12.3

## Identifiers

- CVE-2018-1000656
- GHSA-562c-5r94-xh97
- PYSEC-2018-66

## Related threats

- [PYSEC-2026-1377 - Flask uses fallback key instead of current signing key](https://junglewise.ai/threats/cve-2025-47278-flask-uses-fallback-key-instead-of-current-signing-key)
- [Flask session does not add `Vary: Cookie` header when accessed in some ways](https://junglewise.ai/threats/cve-2026-27205-flask-session-does-not-add-vary-cookie-header-when-accessed-in)
- [Flask vulnerable to possible disclosure of permanent session cookie due to missing Vary: Cookie header](https://junglewise.ai/threats/cve-2023-30861-flask-vulnerable-to-possible-disclosure-of-permanent-session)
- [Pallets Project Flask is vulnerable to Denial of Service via Unexpected memory usage](https://junglewise.ai/threats/cve-2019-1010083-pallets-project-flask-is-vulnerable-to-denial-of-service-via)