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
Impact
Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside. Typical impact: access to internal metadata services, internal APIs, or cloud credentials.
CVE-2026-21859 has a CVSS score of 5.8 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.28.1); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2026-21859? CVE-2026-21859 is a medium-severity server-side request forgery (SSRF) vulnerability in github.com/axllent/mailpit (go), affecting versions <= 1.28.0. It is fixed in 1.28.1. Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside.
- How severe is CVE-2026-21859? CVE-2026-21859 has a CVSS score of 5.8 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of github.com/axllent/mailpit are affected by CVE-2026-21859? github.com/axllent/mailpit (go) versions <= 1.28.0 is affected.
- Is there a fix for CVE-2026-21859? Yes. CVE-2026-21859 is fixed in 1.28.1. Upgrade to this version or later.
- Is CVE-2026-21859 exploitable, and should I be worried? Whether CVE-2026-21859 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-21859 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-21859? Upgrade
github.com/axllent/mailpitto 1.28.1 or later.