CVE-2026-21859 is a medium-severity (CVSS 5.8) Server-Side Request Forgery (SSRF) vulnerability in github.com/axllent/mailpit. A fix is available for github.com/axllent/mailpit — see the affected versions and patch details below.
Mailpit Proxy Endpoint is Vulnerable to Server-Side Request Forgery (SSRF)
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-21859.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-21859 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,156 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
github.com/axllent/mailpitReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
Summary
A Server-Side Request Forgery (SSRF) vulnerability exists in Mailpit's /proxy endpoint that allows attackers to make requests to internal network resources.
Description
The /proxy endpoint allows requests to internal network resources. While it validates http:// and https:// schemes, it does not block internal IP addresses, allowing attackers to access internal services and APIs.
Proof of Concept
Basic SSRF Request
GET /proxy?url=http://127.0.0.1:8025/api/v1/info
This returns internal API data including database path and runtime statistics.
Impact Assessment
1. Internal Network Scanning
Attacker can probe and discover internal services on the network.
2. Information Disclosure
Access to internal API data, database paths, and runtime statistics.
3. Email Content Access
Ability to read all captured emails via internal API endpoints.
4. Cloud Metadata Access
If deployed in cloud environments (AWS/GCP/Azure), potential access to instance metadata services (e.g., http://169.254.169.254/).
Attack Scenarios
Scenario 1: Development Environment Exposure
If Mailpit is accidentally exposed to the internet, attackers can leverage SSRF to access internal development resources and services.
Scenario 2: Container Escape Information
In containerized deployments, SSRF can reveal container metadata and internal service configurations.
Scenario 3: Lateral Movement
In corporate networks, SSRF can be used to discover and interact with internal services, facilitating lateral movement.
Mitigating Factors
This vulnerability is limited to HTTP GET requests with minimal headers. Additionally, Mailpit's web UI & API should be protected by basic authentication when exposed to the internet, which prevents access to the proxy endpoint.
References
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/axllent/mailpit | all versions | 1.28.1go get github.com/axllent/mailpit@v1.28.1 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/axllent/mailpit, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/axllent/mailpit to 1.28.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-21859 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-21859 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-21859. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
How to detect CVE-2026-21859
A community-maintained Nuclei template exists for this CVE. You can scan for it directly:
nuclei -id cve-2026-21859 -u https://target- Template
- Mailpit < 1.28.3 - Server-Side Request Forgery
- Severity
- high
- Impact
- Attackers can access internal network services and APIs, potentially exposing sensitive internal resources.
- Remediation
- Update to version 1.28.1 or later.
Template by ProjectDiscovery nuclei-templates (omarkurt), MIT licensed. View the full template. Scan only systems you are authorised to test.
Frequently Asked Questions
Is CVE-2026-21859 in your dependencies?
O3 Security finds CVE-2026-21859 across Go dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.