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CVE-2026-45820 101arrowz CVE debrief

CVE-2026-45820 is a denial-of-service vulnerability in fflate through version 0.8.2. The vulnerability is caused by an infinite loop in the unzipSync() function. This occurs when a crafted ZIP archive with a central directory entry declaring compressed_size=0xFFFFFFFF (ZIP64 sentinel) but missing the required ZIP64 extra field tag 0x0001 is processed, leading to out-of-bounds reads that return undefined, which coerces to 0, keeping the loop condition permanently true. This vulnerability could potentially be exploited to cause a denial of service. Developers and administrators should be aware of this vulnerability and take steps to mitigate it.

Vendor
101arrowz
Product
fflate
CVSS
MEDIUM 6.6
CISA KEV
Not listed in stored evidence
Original CVE published
2026-07-22
Original CVE updated
2026-07-22
Advisory published
2026-07-22
Advisory updated
2026-07-22

Who should care

Developers and administrators using fflate version 0.8.2 or earlier should be aware of this vulnerability. This vulnerability could potentially be exploited to cause a denial of service. Affected operators, platforms, and security teams should review and address this vulnerability.

Technical summary

The vulnerability exists in the unzipSync() function of the fflate library. When processing a specially crafted ZIP archive, the function enters an infinite loop due to out-of-bounds reads returning undefined, which is coerced to 0. This causes the loop condition to remain permanently true. The crafted ZIP archive must have a central directory entry with compressed_size set to 0xFFFFFFFF (the ZIP64 sentinel) but lack the required ZIP64 extra field tag 0x0001. This issue can lead to a denial-of-service condition.

Defensive priority

Medium priority should be given to updating fflate to a version beyond 0.8.2, as this vulnerability has a CVSS score of 6.6 and is classified as MEDIUM severity.

Recommended defensive actions

  • Update fflate to a version beyond 0.8.2.
  • Review and monitor for suspicious ZIP archive processing.
  • Implement compensating controls to detect and prevent potential exploitation.
  • Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up.
  • Review the supplied official advisory or CVE record to validate affected scope, severity, and vendor guidance.
  • Plan vendor-supported updates or mitigations through normal change control where exposure is confirmed.
  • Check relevant monitoring, detection, and logs for exposed assets that need extra review.

Evidence notes

The CVE record was published on 2026-07-22T08:16:23.280Z and has not been modified since then. The NVD entry is currently 6.6 (Medium). Evidence is limited to public sources and may not reflect the full scope or impact of this vulnerability. Defenders should verify affected deployments and review official advisories for specific guidance.

Official resources

AI-assisted PatchSiren debrief based on the supplied source corpus. The CVE record was published on 2026-07-22T08:16:23.280Z and has not been modified since then.