CVE-2025-1991 is a high-severity vulnerability affecting IBM Informix Dynamic Server versions 12.10, 14.10, and 15.0. The vulnerability stems from an integer underflow (CWE-191) occurring during the processing of network packets by the server. An integer underflow happens when an arithmetic operation causes a value to wrap around below its minimum representable value, potentially leading to unexpected behavior or memory corruption. In this case, the underflow can be triggered remotely without any authentication or user interaction, allowing an attacker to cause a denial of service (DoS) condition. The vulnerability is exploitable over the network (AV:N), requires low attack complexity (AC:L), and no privileges (PR:N) or user interaction (UI:N), making it relatively easy to exploit. The impact is limited to availability, with no confidentiality or integrity compromise reported. Although no known exploits are currently observed in the wild, the vulnerability's characteristics and the widespread use of IBM Informix Dynamic Server in enterprise environments make it a significant risk. The lack of available patches at the time of publication increases the urgency for organizations to implement mitigations and monitor for potential exploitation attempts.

For European organizations using IBM Informix Dynamic Server, this vulnerability poses a risk of service disruption due to denial of service attacks. Informix is commonly used in sectors such as finance, manufacturing, telecommunications, and government, where database availability is critical. A successful DoS attack could lead to downtime, impacting business operations, customer service, and potentially causing financial losses. Additionally, service outages could affect compliance with data availability requirements under regulations like GDPR, especially if the downtime impacts access to personal data or critical services. Given the remote and unauthenticated nature of the exploit, attackers could target exposed Informix servers directly from the internet or internal networks, increasing the threat surface. The absence of known exploits currently provides a window for proactive defense, but organizations should not delay remediation efforts.

1. Network Segmentation and Access Controls: Restrict network access to Informix Dynamic Server instances by implementing strict firewall rules and network segmentation. Only trusted hosts and applications should be allowed to communicate with the database server on required ports. 2. Intrusion Detection and Prevention: Deploy IDS/IPS solutions with signatures or anomaly detection capabilities tuned to detect unusual packet patterns or malformed packets targeting Informix servers. 3. Monitoring and Logging: Enable detailed logging on Informix servers and network devices to detect unusual connection attempts or service disruptions indicative of exploitation attempts. 4. Patch Management: Monitor IBM security advisories closely for patches addressing CVE-2025-1991 and apply them promptly once available. 5. Rate Limiting and Connection Throttling: Implement rate limiting on network devices to mitigate the impact of potential DoS attempts exploiting this vulnerability. 6. Application Layer Gateways or Proxies: Use application-aware proxies to validate and sanitize incoming packets before they reach the Informix server, reducing the risk of malformed packet exploitation. 7. Incident Response Preparedness: Develop and test incident response plans specifically for database service outages to minimize downtime and impact.