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Fencer Intel
medium 5.8

CVE-2026-21859 json

Mailpit Proxy Endpoint is Vulnerable to Server-Side Request Forgery (SSRF)

published 10mo ago modified 4mo ago also GHSA-8v65-47jx-7mfr , GO-2026-4284 , EUVD-2026-1038

cvss
5.8
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.8%
EPSS 0.8% chance of exploitation in 30 days, percentile 57%
public exploits
0
none known, plus a Nuclei check

Summary

Mailpit is an email testing tool and API for developers. Versions 1.28.0 and below have a Server-Side Request Forgery (SSRF) vulnerability in the /proxy endpoint, allowing attackers to make requests to internal network resources. The /proxy endpoint validates http:// and https:// schemes, but it does not block internal IP addresses, enabling attackers to access internal services and APIs. This vulnerability is limited to HTTP GET requests with minimal headers. The issue is fixed in version 1.28.1.

Patch / fix

  • go github.com/axllent/mailpit 1.28.1
  • axllent mailpit 1.28.1
  • govulncheck-vulndb openSUSE Leap 15.6 0.0.20260114T191543-150000.1.137.1

suse advisories SUSE-SU-2026:0142-1

  • govulncheck-vulndb openSUSE Leap 16.0 0.0.20260723T184607-160000.1.1

opensuse advisories openSUSE-SU-2026:21483-1

Is CVE-2026-21859 in your dependencies? Fencer shows which of your repositories are affected as soon as an advisory lands, and opens the fix as a pull request.

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CISA SSVC

Exploitation
Proof of concept is public
Automatable
yes
Technical impact
partial
Assessed
10mo ago

Exploit prediction

0.81% chance of exploitation in the next 30 days, higher than 57% of all scored CVEs (FIRST EPSS, 2026-11-09).

Exploits and checks

  • Nuclei template Mailpit < 1.28.3 - Server-Side Request Forgery nuclei detection template
  • github.com https://github.com/axllent/mailpit/security/advisories/GHSA-8v65-47jx-7mfr referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 3.1 5.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:N/A:N
NVD 3.1 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
GitHub 3.1 5.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:N/A:N

Affected packages

ecosystem package affected fixed in
go github.com/axllent/mailpit
<1.28.1
1.28.1

Affected products

  • axllent mailpit

Classification

Weakness
  • CWE-918 Server-Side Request Forgery

References

Sources

Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.