How severe is CVE-2026-23829?

CVE-2026-23829 has a CVSS score of 5.3/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2026-23829?

CVE-2026-23829 affects the following packages: github.com/axllent/mailpit (Go). Ecosystems affected: Go.

How do I fix CVE-2026-23829?

Update github.com/axllent/mailpit to 1.28.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-23829 is resolved across your whole dependency graph.

How do I detect CVE-2026-23829 in my Go dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/axllent/mailpit. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.

How do I mitigate CVE-2026-23829 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

How does O3 Security protect against CVE-2026-23829?

O3 pinpoints whether CVE-2026-23829 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Is CVE-2026-23829 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-23829 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-23829?

CVE-2026-23829 has an EPSS (Exploit Prediction Scoring System) score of 1.4%, placing it in the 70th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-23829?

CVE-2026-23829 is classified as CWE-93 (CWE-93), CWE-150 (CWE-150). These weakness types describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-23829 published, and has it been updated?

CVE-2026-23829 was published on January 18, 2026 and was last updated on April 10, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹 Go

CVE-2026-23829

MEDIUM

Mailpit has SMTP Header Injection via Regex Bypass

Also known asGHSA-54wq-72mp-cq7cGO-2026-4333

Published

Jan 18, 2026

Updated

Apr 10, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

EPSS Exploitation Probability

via FIRST.org ↗

1.4%probability of exploitation in next 30 days

Lower Risk70th percentile-0.15%

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

Blast Radius

1 pkg affected

🐹github.com/axllent/mailpit

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

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.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/axllent/mailpit`	all versions	1.28.3

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/axllent/mailpit. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update github.com/axllent/mailpit to 1.28.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-23829 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-23829 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Tailored to CVE-2026-23829. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

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.

O3 Security · Impact-Aware SCA

Is CVE-2026-23829 in your dependencies?

O3 detects CVE-2026-23829 across Go dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

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