CVE-2026-39244 cthackers CVE debrief

Who should care

Developers and administrators who use adm-zip in their applications should be aware of this vulnerability and take steps to mitigate it. This includes updating to version 0.5.18 or later of adm-zip, and being cautious when accepting untrusted ZIP files. The vulnerability can cause an immediate process crash, and defenders should review compensating controls for exposed systems while remediation is scheduled and verified.

Technical summary

The vulnerability is caused by the lack of validation of the uncompressed size header field in the ZIP central directory header. The size value is read directly from the binary header without bounds checking, allowing an attacker to craft a ZIP file that declares a large uncompressed size. This can cause a memory allocation amplification ratio of over 33 million to 1, leading to an immediate process crash. The allocation occurs before CRC validation, so the malicious payload cannot be rejected early. The vulnerability affects all extraction and read methods: readFile(), readAsText(), extractEntryTo(), extractAllTo(), extractAllToAsync(), test(), and entry.getData().

Defensive priority

High

Recommended defensive actions

• Update to version 0.5.18 or later of adm-zip
• Be cautious when accepting untrusted ZIP files
• Validate the uncompressed size header field in ZIP files
• Implement bounds checking for memory allocation
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented

Evidence notes

The CVE record was published on 2026-07-10T17:16:57.123Z and has not been modified since then. The NVD entry is currently Deferred. There is limited information available about the vulnerability, and defenders should verify the affected scope and severity with the vendor. The vulnerability occurs in adm-zip before version 0.5.18, and it is recommended to update to the latest version. However, the CVE record does not provide detailed information about the vulnerability, and defenders should be cautious when accepting untrusted ZIP files.

Official resources