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.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
- Exploited
- No known exploitation
- Fix
- Fixed in 4.2.1
- Who is exposed
- Not reported
Do first: Upgrade to a fixed version: 4.2.1.
- Last modified
- Whether it needs action now or is a reference record, by fixed rules, with why. Last checked is when cve.now last re-read it.What do these mean? →
- Reference
- Last checked
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 25/100 · provisional
- How many real-world setups can hit the bug, from the vendor's advisory and CISA data. It does not know your setup.What do these mean? →
- Unclear
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
- Exploitation: none
- Automatable: no
- Technical impact: partial
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.
Read the original 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.
cve.now Index
- Exploitation0/40No signal
- Exposure17/25No login or user action needed, reachable over the network without a login · Unknown: Advisories don't say whether default setups are affected
- Reach?/20Unknown. No tracked technology or package matched
- Severity8.2/10CVSS 8.2
- No patch0/5No signal
What would change the score
- Would score about 70 if added to CISA KEV
- Would score about 65 if CISA's SSVC rates exploitation active
- Would score about 30 if advisories confirm default setups are exposed
What to do
Upgrade to a fixed version: 4.2.1.
Check your systems with an agent
Paste this into a coding or ops agent. It checks your systems read-only and reports a table per asset.
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.
Timeline
- CVE published
- First seen by cve.now
- CISA SSVC assessment
Exploitation: none · Automatable: no · Technical impact: partial
- Record last updated
Affected versions
- postalsys postal-mime
fixed in 4.2.1
Evidence 7
Vendor advisory · 1
Establishes: Affected and fixed versions.
- Advisory
CVE record · 2
Establishes: The identifier and description.
- Record
- Record
Other links · 4
- Patch
- Repository
- Repository
- Link
Corrections
None so far.
Something wrong on this page? Report an error. Every fix is listed here.
Questions about CVE-2026-125756
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: cve.now feed data
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: vulncheck.com, github.com
Which versions are affected by CVE-2026-125756?
postal-mime: before 4.2.1.Source: cve.now feed data
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: vulncheck.com, github.com
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: vulncheck.com, github.com