CVE-2026-23845
Published: 19 January 2026
Summary
CVE-2026-23845 is a medium-severity SSRF (CWE-918) vulnerability in Axllent Mailpit. Its CVSS base score is 5.8 (Medium).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 7.0th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog; a public proof-of-concept is referenced.
The strongest mitigations our analysis identified are NIST 800-53 SI-10 (Information Input Validation) and AC-4 (Information Flow Enforcement).
Deeper analysis
CVE-2026-23845 is a Server-Side Request Forgery (SSRF) vulnerability, classified under CWE-918, affecting Mailpit, an email testing tool and API for developers. Versions prior to 1.28.3 are vulnerable through the HTML Check feature at the `/api/v1/message/{ID}/html-check` endpoint, which analyzes HTML emails for compatibility. Specifically, the `inlineRemoteCSS()` function automatically downloads CSS files referenced in external `<link rel="stylesheet" href="...">` tags to inline them, without validation of the URLs. The vulnerability has a CVSS v3.1 base score of 5.8 (AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:N/A:N), indicating network accessibility, low attack complexity, no privileges or user interaction required, with changed scope and low confidentiality impact.
An attacker can exploit this vulnerability remotely without authentication by sending a malicious HTML email containing a crafted external CSS link to a Mailpit instance, then triggering the HTML Check endpoint on that message ID. This causes the server to make unauthorized requests to arbitrary URLs specified in the CSS href, potentially allowing access to internal network resources or metadata from external services. The impact is limited to low confidentiality, enabling attackers to retrieve limited sensitive information, such as internal service responses, depending on network configuration.
Mitigation is available in Mailpit version 1.28.3, which addresses the issue in the `inlineRemoteCSS()` function. Security practitioners should upgrade to this version or later. Relevant advisories and fixes are detailed in the GitHub security advisory (GHSA-6jxm-fv7w-rw5j), the release notes for v1.28.3, and the fixing commit (1679a0aba592ebc8487a996d37fea8318c984dfe).
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-3296
Vulnerability details
Mailpit is an email testing tool and API for developers. Versions prior to 1.28.3 are vulnerable to Server-Side Request Forgery (SSRF) via HTML Check CSS Download. The HTML Check feature (`/api/v1/message/{ID}/html-check`) is designed to analyze HTML emails for compatibility. During…
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this process, the `inlineRemoteCSS()` function automatically downloads CSS files from external `<link rel="stylesheet" href="...">` tags to inline them for testing. Version 1.28.3 fixes the issue.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
SSRF in public-facing Mailpit API endpoint directly enables remote exploitation of the application without auth.
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Requires validation of URLs extracted from email HTML before the inlineRemoteCSS() function performs outbound fetches, directly blocking the unvalidated href values that enable SSRF.
Enforces boundary controls on outbound connections from the Mailpit server, limiting the ability of the /api/v1/message/{ID}/html-check endpoint to reach arbitrary internal or external targets.
Enforces information flow policies that restrict the server-initiated requests triggered by external <link> tags in processed email content.