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CVE-2026-45820

fflate unzipSync can enter an infinite loop when parsing malformed ZIP64 archives

6.6 MediumFix available

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.

Attack profile

How easy it is to exploit

Attack vector
Network
Complexity
Low
Privileges needed
None
User interaction
None

What an attacker can affect

Confidentiality
None
Integrity
None
Availability
High

Exploit likelihood

0.28%Unchanged since Jul 22, 2026
90%50%10%1%

What each source reports

CVE List Score used 6.6

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.

Record
CVE-2026-45820
CVSS 4.0
6.6 MediumCVSS: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
Weakness
CWE-835, CWE-400
Updated
2026-11-02
EUVDEUVD-2026-47616 6.6

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.

Record
EUVD-2026-47616
CVSS 4.0
6.6 MediumCVSS: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
Updated
2026-11-02
GitHub AdvisoriesGHSA-px8p-9vwx-vf98 6.6

fflate unzipSync can enter an infinite loop when parsing malformed ZIP64 archives

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.

Record
GHSA-px8p-9vwx-vf98
CVSS 4.0
6.6 MediumCVSS: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
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Weakness
CWE-400
Updated
2026-11-02
NVD 6.6

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.

Record
CVE-2026-45820
CVSS 4.0
6.6 MediumCVSS: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/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:N/AU:Y/R:U/V:D/RE:M/U:Amber
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Weakness
CWE-400, CWE-835
Updated
2026-11-02
OSV 6.6

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.

Record
CVE-2026-45820
CVSS 4.0
6.6 MediumCVSS: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
Weakness
CWE-400, CWE-835
Updated
2026-11-02
OSVGHSA-px8p-9vwx-vf98 6.6

fflate unzipSync can enter an infinite loop when parsing malformed ZIP64 archives

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.

Record
GHSA-px8p-9vwx-vf98
CVSS 4.0
6.6 MediumCVSS: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
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Weakness
CWE-400
Updated
2026-11-02
OSVCLSA-2026-1789897715 No score

TuxCare security update for fflate (1 CVE)

TuxCare rebuilt fflate to address 1 CVE. Fixed in 0.8.2-tuxcare.1.

Record
CLSA-2026-1789897715
Updated
2026-11-02
OSVCLSA-2026-1790161290 No score

TuxCare security update for fflate (1 CVE)

TuxCare rebuilt fflate to address 1 CVE. Fixed in 0.8.1-tuxcare.1.

Record
CLSA-2026-1790161290
Updated
2026-11-02

Affected products

Affected packages

Affected packages
PackageAffectedFixed
fflate
npm
0.4.5 up to 0.4.9
0.4.9
fflate
npm
0.5.0 up to 0.5.4
0.5.4
fflate
npm
0.6.0 up to 0.6.11
0.6.11
fflate
npm
0.7.0 up to 0.7.5
0.7.5
fflate
npm
0.8.0 up to 0.8.3
0.8.3
fflate
TuxCare:npm
0.8.2-tuxcare.0 up to 0.8.2-tuxcare.1, plus listed versions
0.8.2
0.8.2-tuxcare.1
fflate
TuxCare:npm
0.8.1-tuxcare.0 up to 0.8.1-tuxcare.1, plus listed versions
0.8.1
0.8.1-tuxcare.1

History

History
RecordedChangedSources
first indexedCVE List, EUVD, GitHub Advisories, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2026-45820 is a medium vulnerability (CVSS 6.6) in 101arrowz Fflate, published July 22, 2026. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.28%, the 20th percentile. Fixed in fflate 0.4.9.

References

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