Suricata Quadratic Complexity Issue in SMTP URL Searching (CVE-2026-31934)

CVE-2026-31934 identifies a vulnerability affecting Suricata, a network IDS/IPS/NSM engine. Specifically, versions 8.0.0 up to but not including 8.0.4 are susceptible to a quadratic complexity issue during URL searching within MIME-encoded SMTP messages. This flaw can lead to substantial performance degradation as the complexity of the URL search increases quadratically with the input size. An attacker could potentially exploit this by sending crafted SMTP messages with complex MIME encoding and numerous URLs, causing the Suricata instance to consume excessive resources. The vulnerability has been addressed and patched in Suricata version 8.0.4. Defenders should prioritize upgrading to the patched version to mitigate potential performance impacts.

Attack Chain

1. Attacker crafts a malicious email containing MIME-encoded content.
2. The email includes a large number of URLs embedded within the MIME structure.
3. The attacker sends the crafted email through SMTP to a target network monitored by Suricata.
4. Suricata receives the SMTP traffic and begins to inspect the email content.
5. The Suricata engine attempts to identify URLs within the MIME-encoded message using an inefficient algorithm.
6. The complexity of the URL search increases quadratically with the number of URLs and the size of the MIME structure.
7. CPU utilization on the Suricata sensor increases significantly, leading to performance degradation.
8. The Suricata instance may become unresponsive, impacting its ability to perform real-time threat detection and potentially leading to a denial-of-service condition.

Impact

The primary impact of CVE-2026-31934 is a potential denial-of-service condition affecting Suricata instances. Exploitation of this vulnerability leads to excessive CPU consumption and performance degradation, hindering Suricata’s ability to effectively monitor network traffic. While the vulnerability does not directly compromise confidentiality or integrity, it can disrupt network security monitoring, potentially allowing malicious traffic to go undetected. The number of affected organizations depends on the adoption rate of vulnerable Suricata versions (8.0.0 to 8.0.3).

Recommendation

• Upgrade Suricata instances to version 8.0.4 or later to remediate the vulnerability as indicated by the vendor advisory (https://github.com/OISF/suricata/security/advisories/GHSA-hr89-h2pp-f3c8).
• Monitor CPU utilization on Suricata sensors; investigate any spikes in CPU usage associated with SMTP traffic inspection using process monitoring tools and correlating with network logs to identify potential exploitation attempts.
• Implement rate limiting on SMTP traffic to prevent attackers from overwhelming Suricata instances with crafted emails.