Unfurl's unbounded zlib decompression allows decompression bomb DoS

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

8.7 · 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 · score source

EPSS

• CVE-2026-40036: 0.51% estimated exploitation activity in the next 30 days; percentile 43.46 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary The compressed data parser uses zlib.decompress() without a maximum output size. A small, highly compressed payload can expand to a very large output, causing memory exhaustion and denial of service. Details - unfurl/parsers/parsecompressed.py calls zlib.decompress(decoded) with no size limit. - Inputs are accepted from URL components that match base64 patterns. - Highly compressible payloads can expand orders of magnitude larger than their compressed size. PoC 1. Generate a payload with securitypoc/pocdecompressionbomb.py --generate-only. 2. The script creates a base64-encoded zlib payload embedded in a URL. 3. Submitting the URL to /json/visjs can cause the server to allocate large amounts of memory. 4. The script includes a --test mode but warns it can crash the service. PoC Script Impact A remote, unauthenticated attacker can cause high memory usage and potentially crash the service. The impact depends on deployment limits (process memory, URL length limits, and request…

Published by the source

29 Jan 2026, 15:31 UTC

Last modified by the source

7 Jul 2026, 17:57 UTC

Source last checked

26 Oct 2026, 13:40 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-h5qv-qjv4-pc5m

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-h5qv-qjv4-pc5m

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# Patch Next advisory brief: CVE-2026-40036

Advisory: Unfurl's unbounded zlib decompression allows decompression bomb DoS
IDs: CVE-2026-40036, GHSA-h5qv-qjv4-pc5m
Record: https://patchnext.com/advisories/cve-2026-40036 (revision 3, source checked 26 Oct 2026, 13:40 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: dfir-unfurl 20260405. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI dfir-unfurl: affected < 20260405; fixed in 20260405

## Suggested inspection (read-only)
- `python -m pip show dfir-unfurl`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on dfir-unfurl.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-h5qv-qjv4-pc5m (OSV record GHSA-h5qv-qjv4-pc5m lists affected ranges: PyPI dfir-unfurl >=0 <20260405 (fixed))
- Verified: https://github.com/advisories/GHSA-h5qv-qjv4-pc5m (GitHub-reviewed advisory GHSA-h5qv-qjv4-pc5m lists affected ranges: PyPI dfir-unfurl >=0 <20260405)

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