MediumPublic exploit

SSRF in Mailpit /proxy endpoint

IdentifiersCVE-2026-21859CWE-918· Server-Side Request Forgery (SSRF)

Mailpit, an email testing tool and developer API, is vulnerable to server-side request forgery (SSRF) in versions 1.28.0 and earlier. The flaw is in the /proxy endpoint, which permits attacker-supplied URLs using the http:// and https:// schemes but does not prevent requests to internal, loopback, private, or link-local addresses. As a result, an attacker who can reach the Mailpit web UI or API can cause the server to issue outbound HTTP GET requests to internal network services and APIs that are not otherwise externally reachable. Reported exposure includes access to internal APIs and cloud instance metadata services such as 169.254.169.254. The available information indicates the endpoint is limited to HTTP GET requests with minimal headers.

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ANALYST BRIEF

Impact, mitigation & remediation

What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.

Impact

What an attacker gets, and what they’ve been doing with it.

Successful exploitation allows an attacker to use the Mailpit server as a proxy to reach internal network resources, enabling internal service enumeration, interaction with internal HTTP endpoints, and disclosure of data exposed by those services. Depending on deployment, this may include internal API responses, runtime or diagnostic information, database paths, captured email contents exposed through internal APIs, and cloud instance metadata in AWS, GCP, or Azure environments. While the issue is constrained to HTTP GET requests with minimal headers, it can still materially expand attacker visibility into otherwise non-public services and facilitate follow-on attacks.

Mitigation

If you can’t patch tonight, do this now.

Until patched, do not expose the Mailpit web UI or API to untrusted networks. Require authentication, such as basic auth, if external exposure is unavoidable. Apply SSRF defenses by blocking requests from the application and/or network egress layer to loopback, private, link-local, and cloud metadata addresses, including 169.254.169.254. Additional network segmentation and outbound filtering should be used to prevent the Mailpit process from reaching sensitive internal services.

Remediation

Patch, then assume compromise.

Upgrade Mailpit to version 1.28.1 or later, which fixes the /proxy endpoint SSRF issue. If Mailpit is packaged or vendored via github.com/axllent/mailpit, ensure the dependency is updated to 1.28.1+ and redeployed anywhere the vulnerable endpoint is exposed.

PUBLIC EXPLOITS

Exploits

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VALID 0 / 0 TOTALView more in app

No public exploit code observed for this vulnerability.

EXPOSURE SURFACE

Affected products & vendors

Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.

VendorProductType

AxllentMailpitapplication

Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.

ACTIVITY FEED

Recent activity

13 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.

13 SOURCESView more in app

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Mar 3, 2026

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Unknown

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Exposure mapping

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Threat actor evidence

Every observed campaign linking this CVE to a named adversary.

Associated malware

Malware families riding this exploit, with evidence and IOCs.

Detection signatures

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Vendor-by-vendor mapping

Cross-references every affected SKU, including bundled OEM variants.

Social activity11