# CVE-2026-23829: Mailpit has SMTP Header Injection via Regex Bypass > Attackers can inject arbitrary SMTP headers or corrupt existing ones in Mailpit by exploiting a regex bypass in address validation. - **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N`) - **Exploited in the wild:** Not known - **Login needed:** Not needed - **Attackable:** Over the internet - **Victim action needed:** Not needed - **Fix available:** Yes - **Published:** 18 Jan 2026 - **Updated:** 20 Jan 2026 - **Also known as:** GHSA-54wq-72mp-cq7c, GO-2026-4333 ## What is CVE-2026-23829? CVE-2026-23829 is a medium-severity vulnerability in axllent mailpit, published on 18 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim. ### Official description Mailpit is an email testing tool and API for developers. Prior to version 1.28.3, Mailpit's SMTP server is vulnerable to Header Injection due to an insufficient Regular Expression used to validate `RCPT TO` and `MAIL FROM` addresses. An attacker can inject arbitrary SMTP headers (or corrupt existing ones) by including carriage return characters (`\r`) in the email address. This header injection occurs because the regex intended to filter control characters fails to exclude `\r` and `\n` when used inside a character class. Version 1.28.3 fixes this issue. Weakness: CWE-93 (Improper Neutralization of CRLF Sequences), CWE-150 (Improper Neutralization of Escape, Meta, or Control Sequences) ## Which versions are affected by CVE-2026-23829? 3 open source packages ship an affected version, including github.com/axllent/mailpit. It affects 1 product. | Package | Ecosystem | Affected | Fixed in | | --- | --- | --- | --- | | `github.com/axllent/mailpit` | Go | All versions | 1.28.3 | | `github.com/axllent/mailpit` | Go | All versions | 1.28.3 | | `github.com/axllent/mailpit` | Go | All versions | 1.28.3 | Products: Axllent Mailpit. ## How do I fix CVE-2026-23829? Upgrade github.com/axllent/mailpit to 1.28.3 or later. ## Is CVE-2026-23829 being exploited? It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-23829 exists. Public exploit code: - FROM HEADER INJECTION TO FULL RCE DOMINANCE The Original Flaw (CVE-2026-23829) · CVSS: 6.1/10 (Medium) - Header injection via regex bypass · Impact: Email spoofing, limited header manipulation · Reality: Boring, basic, barely worth weaponizing CTT Enhancement Revelation (github_poc): https://github.com/SimoesCTT/CTT-Mailpit-RCE-v1.0---Temporal-Resonance-Mail-Server-Takeover - It's a CRLF injection in Mailpit's SMTP server — a classic data corruption vulnerability. But under CTT, it's not just a bug. It's a phase reconstruction opportunity. (github_poc): https://github.com/SimoesCTT/-CVE-2026-23829-CTT-Mailpit-phase-reconstruction- - Mailpit < 1.28.2 - SMTP CRLF Injection (nuclei) ## Other vulnerabilities in axllent mailpit - [CVE-2026-45713](https://insights.sbomify.com/CVE-2026-45713/): Mailpit: Unauthenticated remote memory-exhaustion DoS via unlimited SMTP DATA and /api/v1/send body sizes (high 7.5) - [CVE-2026-22689](https://insights.sbomify.com/CVE-2026-22689/): Mailpit is vulnerable to Cross-Site WebSocket Hijacking (CSWSH) allowing unauthenticated access to emails (medium 6.5) - [CVE-2026-45711](https://insights.sbomify.com/CVE-2026-45711/): Mailpit: Path traversal & arbitrary file write in mailpit dump --http via attacker-controlled message IDs (medium 5.9) - [CVE-2026-45712](https://insights.sbomify.com/CVE-2026-45712/): Mailpit: Concurrent map read & write in proxy CSS rewriter - remote unauth crash (fatal error: concurrent map read and map write) (medium 5.9) - [CVE-2026-45709](https://insights.sbomify.com/CVE-2026-45709/): Mailpit has an incomplete fix for GHSA-6jxm: HTML check still permits SSRF to private/loopback/IMDS via missing IP-filter dialer (medium 5.8) ## Fix prompt for AI coding agents ```text Check this codebase for CVE-2026-23829 and fix it if it is affected. What it is: CVE-2026-23829 is a medium-severity vulnerability in axllent mailpit, published on 18 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim. Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-23829 exists. Affected packages and fixed versions: - github.com/axllent/mailpit (Go): affected all versions; fixed in 1.28.3 - github.com/axllent/mailpit (Go): affected all versions; fixed in 1.28.3 - github.com/axllent/mailpit (Go): affected all versions; fixed in 1.28.3 Task: 1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies. 2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it. 3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead. 4. Run the tests and summarise what changed and why. 5. If nothing in the project is affected, say so and change nothing. Reference: https://insights.sbomify.com/CVE-2026-23829/ (Markdown: https://insights.sbomify.com/CVE-2026-23829/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2026-23829 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-23829 - OSV: https://osv.dev/vulnerability/CVE-2026-23829 --- From sbomify Insights: https://insights.sbomify.com/CVE-2026-23829/ Summaries are generated and may contain mistakes; the official description above is the record.