# CVE-2026-124016: postal-mime < 4.2.1 Algorithmic Complexity DoS via processLine

Status: published. Published: 2026-11-10. Updated: 2026-11-10.

postal-mime before 4.2.1 contains an algorithmic complexity denial of service vulnerability that allows unauthenticated remote attackers to cause excessive CPU consumption in the PostalMime.processLine() function by crafting deeply nested multipart email messages with boundary lines sharing long prefixes with open boundaries. Attackers can deliver a malicious email with nesting up to the default maxNestingDepth of 256, causing per-line parsing cost to scale as O(depth x shared prefix length) and blocking the Node.js event loop for seconds per message.

## Remediation

- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: VulnCheck) — https://github.com/postalsys/postal-mime/releases/tag/v4.2.1

## Affected products and versions

- postalsys / postal-mime (postal-mime): ≥ 0; < 4.2.1 (affected, semver). Source: CNA: VulnCheck

## Severity and prioritization

- CVSS 4.0: 6.9 (medium); CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N; source: CNA: VulnCheck

- CVSS 3.1: 5.3 (medium); CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L; source: CNA: VulnCheck

- CISA KEV: No listing in collected sources

- EPSS: Not available

- SSVC: exploitation poc; automatable yes; technical impact partial; source ADP: CISA-ADP

- Weaknesses: CWE-407

## Exploit evidence

- exploit: https://github.com/postalsys/postal-mime/security/advisories/GHSA-q8wm-54mj-r36p (ADP: CISA-ADP)

## References

- https://github.com/postalsys/postal-mime/security/advisories/GHSA-q8wm-54mj-r36p (CNA: VulnCheck; vendor-advisory)

- https://github.com/postalsys/postal-mime/releases/tag/v4.2.1 (CNA: VulnCheck; release-notes, patch)

- https://www.vulncheck.com/advisories/postal-mime-algorithmic-complexity-dos-via-processline (CNA: VulnCheck; third-party-advisory)

- https://github.com/postalsys/postal-mime/security/advisories/GHSA-q8wm-54mj-r36p (ADP: CISA-ADP; exploit)

## Provenance

- CNA: VulnCheck: https://www.cve.org/CVERecord?id=CVE-2026-124016; retrieved 2026-11-10; source updated 2026-11-10

Revision: 1ee895c9862d0e9fffbf63f5775c153c20f9e2ef6a90068599c5c47cd785ece9

Canonical: https://y2.dev/cve/CVE-2026-124016

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