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 18.8th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog; a public proof-of-concept is referenced.
Threat & Defense at a Glance
Threat & Defense Details
Likely Mitigating ControlsAI
Per-CVE control mapping for this CVE has not run yet; the list below is derived from the weakness types (CWEs) cited in the NVD entry.
Penetration testing attempts server-side requests to internal resources, identifying SSRF weaknesses for remediation.
Outbound connections to external resources can be monitored and limited at the boundary, reducing SSRF impact.
Validates server-side URLs and resource references to block SSRF attempts.
Detects server-side request forgery through monitoring of unexpected outbound connections.
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
SSRF in public-facing Mailpit API endpoint directly enables remote exploitation of the application without auth.
NVD Description
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.
Deeper analysisAI
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).
Details
- CWE(s)