CVE-2025-11936 Overview
CVE-2025-11936 is an improper input validation vulnerability [CWE-20] in the TLS 1.3 KeyShareEntry parsing logic of wolfSSL version 5.8.2. A remote unauthenticated attacker can trigger a denial-of-service condition by sending a crafted ClientHello message containing duplicate KeyShareEntry values for the same supported group. Processing the malformed message forces excessive CPU and memory consumption on the target server. The flaw affects wolfSSL deployments across multiple platforms and requires no authentication or user interaction to exploit.
Critical Impact
Remote unauthenticated attackers can exhaust server CPU and memory resources by sending a single crafted TLS 1.3 ClientHello, degrading availability of any service that terminates TLS with wolfSSL 5.8.2.
Affected Products
- wolfSSL v5.8.2
- Applications and appliances embedding wolfSSL 5.8.2 for TLS 1.3 termination
- Multi-platform deployments using the affected library version
Discovery Timeline
- 2025-11-21 - CVE-2025-11936 published to NVD
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2025-11936
Vulnerability Analysis
The vulnerability resides in the TLS 1.3 KeyShareEntry parsing routine inside wolfSSL 5.8.2. TLS 1.3 clients advertise cryptographic key shares in the key_share extension of the ClientHello message. Each entry pairs a named group (such as x25519 or secp256r1) with an ephemeral public key. RFC 8446 requires each supported group to appear at most once in the extension.
wolfSSL does not enforce this uniqueness constraint during parsing. When an attacker submits a ClientHello containing repeated KeyShareEntry structures for the same group, the server processes every duplicate entry. Each entry drives additional allocation and cryptographic work during handshake setup. The resulting CPU and memory pressure produces a denial-of-service condition on the TLS listener.
Root Cause
The root cause is missing input validation on the key_share extension contents. The parser accepts semantically invalid handshake input that a compliant implementation would reject. This falls under [CWE-20] Improper Input Validation and manifests as an algorithmic resource-exhaustion issue at the protocol layer.
Attack Vector
Exploitation is network-based and requires no credentials. An attacker only needs the ability to open a TCP connection to the TLS port and complete the initial handshake exchange far enough to deliver a crafted ClientHello. A single connection is sufficient to trigger elevated resource usage, and repeated connections amplify the impact into a full denial-of-service against the listening service.
No verified proof-of-concept code is publicly available at the time of writing. Technical remediation details are documented in the wolfSSL Pull Request #9117.
Detection Methods for CVE-2025-11936
Indicators of Compromise
- TLS 1.3 ClientHello messages containing two or more KeyShareEntry values referencing the same named group
- Sustained high CPU utilization tied to the wolfSSL TLS handshake process without a matching increase in completed sessions
- Elevated memory allocation during handshake processing followed by aborted or stalled sessions from the same source addresses
Detection Strategies
- Inspect TLS ClientHello packets at network sensors and alert when duplicate named groups appear in the key_share extension
- Correlate handshake failure rates and connection latency spikes with source IPs generating disproportionate ClientHello volume
- Enable verbose TLS logging on wolfSSL-based services to capture malformed handshake attempts for offline analysis
Monitoring Recommendations
- Baseline CPU and memory usage on TLS-terminating services and alert on deviations that coincide with handshake activity
- Track per-source connection rates against the TLS listener and rate-limit clients that fail to complete handshakes
- Forward TLS handshake telemetry into a central data lake for behavioral correlation across affected hosts
How to Mitigate CVE-2025-11936
Immediate Actions Required
- Inventory all systems, appliances, and embedded products running wolfSSL 5.8.2 with TLS 1.3 enabled
- Apply the upstream fix from wolfSSL Pull Request #9117 or upgrade to a fixed release once available
- Restrict network exposure of wolfSSL-based TLS services to trusted networks where feasible until patching is complete
Patch Information
The fix is tracked in the upstream repository via wolfSSL Pull Request #9117. Rebuild and redeploy any downstream products that statically link or bundle the wolfSSL library after upgrading to a patched version. Verify that firmware images for embedded devices are refreshed, since wolfSSL is frequently shipped inside IoT and appliance builds.
Workarounds
- Place a compliant TLS reverse proxy in front of wolfSSL-based endpoints to normalize or reject malformed ClientHello messages
- Enforce connection rate limits and concurrent handshake caps at the network edge to blunt resource exhaustion
- Deploy Web Application Firewall or IPS signatures that drop ClientHello messages containing duplicate KeyShareEntry groups
# Example: rate-limit new TLS connections per source with iptables
iptables -A INPUT -p tcp --dport 443 -m conntrack --ctstate NEW \
-m recent --set --name TLSNEW
iptables -A INPUT -p tcp --dport 443 -m conntrack --ctstate NEW \
-m recent --update --seconds 10 --hitcount 20 --name TLSNEW -j DROP
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
