Cisco IOS XE Software TLS Memory Exhaustion Vulnerability (CVE-2026-20004)

CVE-2026-20004 is a critical vulnerability affecting the TLS library within Cisco IOS XE Software. An unauthenticated attacker positioned adjacent to a vulnerable device can exploit this flaw to exhaust the device's available memory. This vulnerability arises from improper management of memory resources during the establishment of TLS connections. The attack can be initiated in multiple ways, including but not limited to, repeated attempts at Extensible Authentication Protocol (EAP) authentication when local EAP is active. Another attack vector involves a machine-in-the-middle (MitM) approach, where TLS connections are reset between the affected Cisco device and other network entities. Successful exploitation leads to complete memory exhaustion, forcing an unexpected device reload and a subsequent denial-of-service (DoS) condition. This can impact network availability and critical services relying on the affected Cisco device.

Attack Chain

1. Attacker establishes network adjacency to the target Cisco IOS XE device.
2. Attacker initiates a series of TLS connection requests to the target device.
3. If local EAP is enabled, the attacker repeatedly attempts EAP authentication, triggering memory allocation on each attempt.
4. Alternatively, the attacker performs a Machine-in-the-Middle (MitM) attack to intercept and reset existing TLS connections between the target device and other devices.
5. The target device improperly manages memory resources during each TLS connection setup (either through EAP or MitM reset), leading to increasing memory consumption.
6. The attacker continues to send TLS connection requests or reset existing connections, further exhausting the device's memory.
7. The available memory on the affected device is completely depleted due to the continuous allocation.
8. The device experiences an unexpected reload, resulting in a denial-of-service condition, disrupting network services.

Impact

Successful exploitation of CVE-2026-20004 can lead to a complete denial of service on the affected Cisco IOS XE device. This can disrupt network operations, causing downtime and impacting critical services. The severity is amplified as it can be triggered by an unauthenticated, adjacent attacker. The number of potential victims depends on the number of Cisco IOS XE devices running vulnerable software versions with exposed TLS services. The impact extends to any organization relying on these devices for network connectivity and service delivery.

Recommendation

• Upgrade Cisco IOS XE Software to a version that addresses CVE-2026-20004 as soon as possible, as documented in the Cisco security advisory.
• Monitor network traffic for suspicious TLS connection patterns, particularly repeated connection attempts from the same adjacent source (using network_connection logs and creating custom rules).
• Implement network segmentation to limit the exposure of Cisco IOS XE devices to adjacent networks where potential attackers may reside.
• Consider disabling local EAP if it is not required, to mitigate the risk of exploitation via repeated EAP authentication attempts.