CVE-2026-21859
MediumExploitedPublic exploitMailpit Proxy Endpoint has Server-Side Request Forgery (SSRF) vulnerability
Published Jan 6, 2026 · Updated Aug 12, 2026 · Also known as GHSA-8v65-47jx-7mfr, GO-2026-4284
Key Facts
Patch immediately- CVE-2026-21859 is a medium-severity vulnerability (CVSS 5.3 v3.1) affecting Mailpit (Axllent), github.com/axllent/mailpit (go).
- It is exploited in the wild, listed in Industry KEV feed since Feb 11, 2026. Public exploit code is available.
- EPSS estimates a 0.8% probability of exploitation in the next 30 days.
- Fixed versions are available, including Mailpit 1.28.1, github.com/axllent/mailpit 1.28.1.
- Brandefense priority: patch immediately, because it is listed as exploited in the wild.
CVSS v3.1
5.3
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
EPSS (30-day exploit likelihood)
0.8%
Higher than 56.2% of all scored CVEs
Exploited in the wild
Yes
1 catalog listing
Public exploit code
Available
Proof-of-concept or weaponized module
Get the full CVE-2026-21859 brief as a PDF
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Description
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.
Known-Exploited Catalog Listings
| Catalog | Added | Remediation due | Ransomware use |
|---|---|---|---|
| Industry KEV feed | Feb 11, 2026 | n/a | Not reported |
Required action: Apply remediations or mitigations per vendor instructions or discontinue use of the product if remediation or mitigations are unavailable.
Affected Products
3 affected entries in total| Product | Affected versions | Fixed in |
|---|---|---|
| MailpitAxllent | >= < 1.28.1 | 1.28.1 |
| github.com/axllent/mailpitgo | See advisory | 1.28.1 |
Patched Releases
Mailpit
1.28.1
github.com/axllent/mailpit
1.28.1
Timeline
Publicly disclosed
Jan 6, 2026
Added to Industry KEV feed
Feb 11, 2026
Last updated
Aug 12, 2026
Weakness Type
- CWE-918 Server-Side Request Forgery (SSRF)
References
- advisoryGHSA advisory
- advisoryhttps://github.com/CVEProject/cvelistV5/tree/main/cves/2026/21xxx/CVE-2026-21859.json
- advisoryhttps://nvd.nist.gov/vuln/detail/CVE-2026-21859
- exploitVulnCheck exploitation report
- exploitVulnCheck exploitation report
FAQ
Common Questions.
Mailpit is an email testing tool and API for developers.
Affected: Mailpit (Axllent) versions >= < 1.28.1; github.com/axllent/mailpit (go). The full list is in the CVE brief.
It is exploited in the wild, listed in Industry KEV feed since Feb 11, 2026.
No, it is not currently listed in the CISA Known Exploited Vulnerabilities catalog.
Public exploit code is available.
Fixed versions are available, including Mailpit 1.28.1, github.com/axllent/mailpit 1.28.1.
Brandefense priority: patch immediately, because it is listed as exploited in the wild.
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