How severe is CVE-2020-26256?

CVE-2020-26256 has a CVSS score of 6.5/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2020-26256?

CVE-2020-26256 affects the following packages: fast-csv (npm), @fast-csv/parse (npm). Ecosystems affected: npm.

How do I fix CVE-2020-26256?

Update fast-csv to 4.3.6 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-26256 is resolved across your whole dependency graph.

How do I detect CVE-2020-26256 in my npm dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for fast-csv, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2020-26256 if there is no patch (or I can't update yet)?

Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Is CVE-2020-26256 actively exploited in the wild?

Yes. There are 1 known exploit references for CVE-2020-26256, including 1 in-the-wild exploitation observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2020-26256?

CVE-2020-26256 has an EPSS (Exploit Prediction Scoring System) score of 1.5%, placing it in the 74th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2020-26256?

CVE-2020-26256 is classified as Uncontrolled Resource Consumption (CWE-400). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2020-26256 published, and has it been updated?

CVE-2020-26256 was published on December 8, 2020 and was last updated on July 9, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

📦

📦 npm

Not in CISA KEV

MEDIUM severity

CVE-2020-26256 — fast-csv

MEDIUMFix: C2FO/fast-csv@4bbd39f

CVE-2020-26256 is a medium-severity (CVSS 6.5) Uncontrolled Resource Consumption vulnerability in fast-csv. 1 public exploit reference exists, so weaponization risk is real. A fix is available for fast-csv — see the affected versions and patch details below.

Denial of service in fast-csv

Also known asGHSA-8cv5-p934-3hwp

Published

Dec 8, 2020

Updated

Jul 9, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

1 known

Exploitation data as of Oct 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

1.5%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs74th percentile — riskier than 74% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2020-26256 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

2 pkgs affected

How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.

963other npm packages depend on this — each one inherits the vulnerability until it's patched upstream

fast-csvnpm

19.3Mdownloads / week

@fast-csv/parsenpm

19.6Mdownloads / week

Description

Impact

Possible ReDoS (Regular Expression Denial of Service) when using ignoreEmpty option when parsing.

Patches

This has been patched in v4.3.6

Workarounds

You will only be affected by this if you use the ignoreEmpty parsing option. If you do use this option it is recommended that you upgrade to the latest version v4.3.6

References

This vulnerability was found using a CodeQL query which identified EMPTY_ROW_REGEXP regular expression as vulnerable. Link to query run.

For more information

If you have any questions or comments about this advisory:

Open an issue in fast-csv

Affected Packages

2 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
📦npm	`fast-csv`	all versions	4.3.6`npm install fast-csv@4.3.6`
📦npm	`@fast-csv/parse`	all versions	4.3.6`npm install @fast-csv/parse@4.3.6`

Affected Products

1 product · 1 configurations

Application

fast-csvc2fo

< 4.3.6

range

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

Fast-csv is an npm package for parsing and formatting CSVs or any other …

lgtm.comNVDDec 2020

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for fast-csv, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update fast-csv to 4.3.6 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-26256 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Frequently Asked Questions

### Impact Possible ReDoS (Regular Expression Denial of Service) when using `ignoreEmpty` option when parsing. ### Patches This has been patched in `v4.3.6` ### Workarounds You will only be affected by this if you use the `ignoreEmpty` parsing option. If you do use this option it is recommended that you upgrade to the latest version `v4.3.6` ### References This vulnerability was found using a [CodeQL](https://securitylab.github.com/tools/codeql) query which identified `EMPTY_ROW_REGEXP` regular expression as vulnerable. [Link to query run](https://lgtm.com/query/8609731774537641779/). ##