Raw vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:N/A:NSummary
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 32th 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 map to AC-4 (Information Flow Enforcement) and SI-10 (Information Input Validation) — see the control section below for these in your framework.
Deeper analysis AI-assisted summary
Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.
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).
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-3296
Vulnerability Data
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 Techniques
CVEs Like This One
Affected Assets
Mitigating Controls
Control response
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V1.3.6V1.5.3V5.3.2V10.4.7
Mitigating Controls (NIST 800-53 r5) AI
Information flow enforcement can restrict which destinations the server is allowed to contact on behalf of users.
Input validation directly stops untrusted URLs from being accepted and fetched without destination checks.
Boundary protection limits the network reach of server-initiated requests even if SSRF occurs.
Mitigating Controls (NIST CSF 2.0) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.
Secure development practices directly include input validation and destination allow-listing that prevent SSRF.
Runtime monitoring of web applications and services can detect anomalous outbound requests indicative of SSRF.
Vulnerability identification processes can discover and record SSRF flaws in web applications.
Network segmentation and egress controls can limit the damage from successful SSRF requests.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.
Operational threat data describing SSRF campaigns can be used to tighten outbound-request allow-lists and detection rules before attackers exploit them.