Overview
CVE-2025-11936 describes a denial-of-service (DoS) vulnerability in wolfSSL version 5.8.2. This vulnerability stems from improper input validation during the parsing of TLS 1.3 KeyShareEntry values within the ClientHello message. A remote, unauthenticated attacker can exploit this flaw by sending a specially crafted ClientHello message containing duplicate KeyShareEntry values for the same supported group. This can lead to excessive CPU and memory consumption on the server, resulting in a denial of service.
Technical Details
The vulnerability resides in the TLS 1.3 handshake process, specifically within the handling of the KeyShare extension in the ClientHello message. The KeyShare extension allows the client to propose key shares for various supported groups. wolfSSL v5.8.2 fails to adequately validate the KeyShareEntry values within this extension. An attacker can send a ClientHello message containing multiple KeyShareEntry values associated with the same supported group. The lack of proper duplicate detection causes the wolfSSL server to process these redundant entries, consuming excessive CPU and memory resources. This sustained resource consumption can render the server unresponsive, effectively causing a denial of service.
CVSS Analysis
Currently, the CVSS score for CVE-2025-11936 is not available (N/A). However, given that it’s a denial-of-service vulnerability exploitable remotely without authentication, a high severity rating would be expected if a complete CVSS score was calculated. The lack of authentication and ease of remote exploitation significantly increase the potential risk.
Possible Impact
The exploitation of CVE-2025-11936 can have a significant impact on affected systems. The primary consequence is a denial of service, preventing legitimate users from accessing the affected service. This can lead to:
- Website downtime.
- Disruption of critical services relying on wolfSSL.
- Potential data loss or corruption in scenarios where the DoS interrupts ongoing operations.
- Reputational damage to the organization using the vulnerable wolfSSL version.
Mitigation or Patch Steps
The primary mitigation strategy is to upgrade to a patched version of wolfSSL. According to the provided reference, the fix is available. It is highly recommended to upgrade wolfSSL to the latest version or a version that includes the fix for Pull Request #9117. This pull request contains the necessary input validation improvements to prevent the processing of duplicate KeyShareEntry values.
If immediate patching is not feasible, consider implementing temporary workarounds such as:
- Rate limiting connections to the affected service.
- Deploying a Web Application Firewall (WAF) with rules to detect and block ClientHello messages containing suspicious KeyShareEntry patterns.
- Monitoring CPU and memory usage on servers running wolfSSL and implementing automated restart procedures if resource consumption exceeds predefined thresholds.
These workarounds are not substitutes for patching but can provide temporary protection while a proper upgrade is planned and executed.
