HTTP Response Splitting Affecting s9y/serendipity package, versions <2.6.0
Threat Intelligence
The probability is the direct output of the EPSS model, and conveys an overall sense of the threat of exploitation in the wild. The percentile measures the EPSS probability relative to all known EPSS scores. Note: This data is updated daily, relying on the latest available EPSS model version. Check out the EPSS documentation for more details.
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Test your applications- Snyk IDSNYK-PHP-S9YSERENDIPITY-16109574
- published19 Apr 2026
- disclosed14 Apr 2026
- creditmabjr33
Introduced: 14 Apr 2026
NewCVE-2026-39971 (opens in a new tab)How to fix?
Upgrade s9y/serendipity to version 2.6.0 or higher.
Overview
Affected versions of this package are vulnerable to HTTP Response Splitting via the HTTP_HOST value being directly embedded into the Message-ID header during email generation. An attacker can inject arbitrary SMTP headers into outgoing emails by supplying a crafted Host header during actions that trigger email notifications, potentially enabling email spoofing, spam relay, and unauthorized BCC injection.