CVE-2026-21859: Mailpit Email Testing Tool SSRF Vulnerability

CVE-2026-21859 Overview

CVE-2026-21859 is a Server-Side Request Forgery (SSRF) vulnerability affecting Mailpit, an email testing tool and API for developers. Versions 1.28.0 and below contain a security flaw in the /proxy endpoint that allows attackers to make requests to internal network resources. While the endpoint validates http:// and https:// schemes, it fails to block internal IP addresses, enabling attackers to access internal services and APIs that should be restricted from external access.

Critical Impact
Attackers can leverage the unprotected proxy endpoint to scan internal networks, access sensitive internal services, and potentially exfiltrate confidential data from systems that would otherwise be unreachable from external networks.

Affected Products

Mailpit versions 1.28.0 and earlier
Mailpit /proxy endpoint implementations
Development and staging environments running vulnerable Mailpit instances

Discovery Timeline

2026-01-08 - CVE CVE-2026-21859 published to NVD
2026-01-08 - Last updated in NVD database

Technical Details for CVE-2026-21859

Vulnerability Analysis

This SSRF vulnerability (CWE-918) exists in Mailpit's /proxy endpoint, which is designed to fetch external resources for email rendering functionality. The core issue is insufficient validation of destination URLs—while the endpoint correctly validates that URLs use HTTP or HTTPS schemes, it does not implement IP address filtering to prevent requests to internal network addresses such as 127.0.0.1, 10.0.0.0/8, 172.16.0.0/12, or 192.168.0.0/16.

The vulnerability is constrained to HTTP GET requests with minimal headers, which limits the attack surface compared to full-featured SSRF vulnerabilities. However, this is still sufficient for attackers to enumerate internal services, access metadata endpoints in cloud environments, and potentially retrieve sensitive configuration data.

Root Cause

The root cause is missing input validation for internal IP addresses in the /proxy endpoint's URL handling logic. The endpoint implements scheme validation to ensure only HTTP and HTTPS protocols are used, but lacks the corresponding validation to block requests to private IP address ranges and localhost. This oversight allows external users to leverage the Mailpit server as a proxy to reach internal network resources.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker can craft malicious requests to the /proxy endpoint with URLs targeting internal services. Common attack scenarios include:

Accessing cloud metadata endpoints (e.g., http://169.254.169.254/latest/meta-data/)
Scanning internal networks for open ports and services
Retrieving configuration from internal APIs
Bypassing firewall rules that restrict direct external access

The fix in version 1.28.1 restricts the screenshot proxy to only support asset links contained within the email message itself, preventing arbitrary URL proxying.

text

 	methods: {
 		initScreenshot() {
 			this.loading = 1;
+			const baseUrl = `${location.protocol}//${location.host}/`;
+			// absolute proxy URL
+			const proxy = new URL(this.resolve("/proxy"), baseUrl).href;
+			const urlRegex = /(url\(('|")?(https?:\/\/[^)'"]+)('|")?\))/gim;
+
 			// remove base tag, if set
 			let h = this.message.HTML.replace(/<base .*>/im, "");
-			const proxy = this.resolve("/proxy");
 
 			// Outlook hacks - else screenshot returns blank image
 			h = h.replace(/<html [^>]+>/gim, "<html>"); // remove html attributes
 			h = h.replace(/<o:p><\/o:p>/gm, ""); // remove empty `<o:p></o:p>` tags
 			h = h.replace(/<o:/gm, "<"); // replace `<o:p>` tags with `<p>`
 			h = h.replace(/<\/o:/gm, "</"); // replace `</o:p>` tags with `</p>`
 
-			// update any inline `url(...)` absolute links
-			const urlRegex = /(url\(('|")?(https?:\/\/[^)'"]+)('|")?\))/gim;
-			h = h.replaceAll(urlRegex, (match, p1, p2, p3) => {
-				if (typeof p2 === "string") {
-					return `url(${p2}${proxy}?url=` + encodeURIComponent(this.decodeEntities(p3)) + `${p2})`;
-				}
-				return `url(${proxy}?url=` + encodeURIComponent(this.decodeEntities(p3)) + `)`;
-			});
-
 			// create temporary document to manipulate
 			const doc = document.implementation.createHTMLDocument();
 			doc.open();

Source: GitHub Commit Details

Detection Methods for CVE-2026-21859

Indicators of Compromise

Unusual outbound requests from Mailpit server to internal IP ranges (10.x.x.x, 172.16.x.x, 192.168.x.x, 127.0.0.1)
High volume of requests to the /proxy endpoint from external sources
Requests to cloud metadata endpoints (e.g., 169.254.169.254) originating from Mailpit servers
Access logs showing proxy requests to non-standard ports on internal hosts

Detection Strategies

Monitor web server access logs for /proxy endpoint requests containing internal IP addresses or localhost
Implement network-level monitoring to detect egress traffic from Mailpit servers to internal network segments
Configure alerting for requests targeting cloud metadata endpoints from application servers
Review firewall logs for unexpected outbound connections originating from Mailpit instances

Monitoring Recommendations

Enable verbose logging on Mailpit instances to capture all proxy requests with full URL details
Deploy network segmentation monitoring to detect cross-segment traffic from development tool servers
Implement web application firewall rules to inspect and block suspicious proxy requests
Set up automated scanning to identify Mailpit instances running versions below 1.28.1

How to Mitigate CVE-2026-21859

Immediate Actions Required

Upgrade Mailpit to version 1.28.1 or later immediately
Restrict network access to Mailpit instances to only authorized users and networks
Review access logs for evidence of exploitation attempts against the /proxy endpoint
Implement network-level controls to prevent Mailpit servers from accessing internal services

Patch Information

The vulnerability is fixed in Mailpit version 1.28.1. The patch restricts the screenshot proxy functionality to only support asset links that are contained within email messages, preventing arbitrary URL proxying. The fix can be reviewed in the GitHub Commit Details. Additional details are available in the GitHub Security Advisory GHSA-8v65-47jx-7mfr.

Workarounds

Place Mailpit behind a reverse proxy that filters requests to the /proxy endpoint
Implement firewall rules to prevent outbound connections from Mailpit to internal network ranges
Restrict access to Mailpit instances using authentication and IP allowlisting
Deploy network segmentation to isolate development tools from production infrastructure

bash

# Example: iptables rules to block Mailpit from accessing internal networks
iptables -A OUTPUT -s <mailpit-server-ip> -d 10.0.0.0/8 -j DROP
iptables -A OUTPUT -s <mailpit-server-ip> -d 172.16.0.0/12 -j DROP
iptables -A OUTPUT -s <mailpit-server-ip> -d 192.168.0.0/16 -j DROP
iptables -A OUTPUT -s <mailpit-server-ip> -d 127.0.0.0/8 -j DROP
iptables -A OUTPUT -s <mailpit-server-ip> -d 169.254.169.254 -j DROP