Cisco Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense Software for Firepower 3100 and 4200 Series TLS 1.3 Cipher Denial of Service Vulnerability

🚨SEVERITY: HIGH — CVSS 7.7Security Advisory

TL;DR 📌

  • A vulnerability in the TLS 1.3 implementation for a specific cipher for Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software for Cisco Firepower 3100 and 4200 Series devices could allow an authenticated, remote attacker to consume resources that are associated with incoming TLS 1.3 connections, which eventually…
  • Highest CVSS: 7.7 (High).
  • Check the advisory for fixed releases — remediation detail is in the vendor link below.
  • CVEs: CVE-2025-20127.

What it is

CVE-2025-20127 affects Cisco Secure Firewall ASA Software and Secure FTD Software running on Firepower 3100 and 4200 Series appliances. The flaw sits in the TLS 1.3 handling of a specific cipher, TLS_CHACHA20_POLY1305_SHA256.

Exploitation requires the device to have an SSL listen socket configured and TLS_CHACHA20_POLY1305_SHA256 enabled as an available TLS 1.3 cipher — this is not the default configuration. An authenticated, remote attacker who can reach that listen socket sends a large number of TLS 1.3 connections using this cipher, which exhausts Security Context Block (SCB) handles. These handles are used for all incoming encrypted connections — SSL, VPN, TLS, and IKE alike.

Once exhausted, the device stops accepting any new encrypted connections, affecting both data traffic and user-management traffic. There is no self-recovery: the device must be reloaded to clear the condition. Cisco Secure Firewall Management Center (FMC) Software is confirmed not affected.

What to do

  • Check exposure: run show asp table socket | include SSL to confirm an SSL listening socket exists, then show running-config all ssl | include TLS_CHACHA20_POLY1305_SHA256 (or, where tlsv1.3 high/medium/low is set, show ssl ciphers <level> | include v1.3) to confirm the cipher is actually enabled.
  • If vulnerable, apply the workaround immediately: remove the cipher with no ssl cipher tlsv1.3 custom <cipher list>. Test this in your environment first, as Cisco notes it may affect functionality depending on deployment.
  • Watch for indicators of compromise: repeating error:1424A044:SSL routines:write_state_machine:internal error@libssl_ext_hndshk_accel.c:87 entries in show ssl errors, or a rapidly incrementing HANDLE_ALLOC_FAILED counter in show counters | include HANDLE_ALLOC_FAILED. Either indicates SCB exhaustion; the device will need a reload to restore connectivity.
  • Move to fixed software as advised by Cisco. The advisory does not list specific fixed release numbers here — use Cisco’s Software Checker with your current release (for example, entering an ASA release such as 9.20.3.4 or an FTD release such as 7.4.2 as prompted) to identify the appropriate upgrade path for your platform.
  • CVSS 3.1 base score is 7.7 (AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:H), reflecting low attack complexity but a requirement for low-privileged authentication. Cisco PSIRT is not aware of public exploitation.

For leadership 🧭

Executive summary. Firewalls running ASA or FTD on Firepower 3100 and 4200 hardware with TLS_CHACHA20_POLY1305_SHA256 enabled can be driven into a state where no new SSL, TLS, VPN or IKE connections are accepted, requiring a manual reload to recover. This isn’t the default configuration and needs low-privilege authentication to trigger, so check exposure this week rather than treating it as an emergency.

Why it matters:

  • An authenticated attacker who can reach the SSL listen socket exhausts Security Context Block handles, which underpin all encrypted connections on the device — SSL, VPN, TLS and IKE alike, not just the affected cipher’s traffic.
  • Once exhausted there is no self-recovery: the appliance must be physically reloaded, meaning an outage of VPN access and management traffic until someone intervenes.
  • Only devices with TLS_CHACHA20_POLY1305_SHA256 explicitly enabled (directly or via tlsv1.3 high/medium/low cipher settings) are exposed — this is a non-default setting, so the fix is often a quick configuration check rather than an emergency patch.
  • Firewall Management Center is confirmed unaffected, so the exposure is limited to the ASA/FTD data-plane devices themselves on the 3100 and 4200 Series.

Now / Next / Later:

  • Now: Run show asp table socket | include SSL and show running-config all ssl | include TLS_CHACHA20_POLY1305_SHA256 (or show ssl ciphers <level> | include v1.3 if using tlsv1.3 high/medium/low) on each Firepower 3100/4200 ASA or FTD device to confirm whether the cipher is enabled.
  • Next: Where the cipher is enabled, remove it with no ssl cipher tlsv1.3 custom <cipher list> in a change window after testing for impact on legitimate TLS 1.3 clients, then schedule the upgrade to Cisco’s fixed release identified via the Software Checker for your platform.
  • Later: Add the HANDLE_ALLOC_FAILED counter and the write_state_machine SSL error string to routine monitoring on Firepower 3100/4200 devices, and review TLS cipher configuration standards across the ASA/FTD estate so unnecessary ciphers aren’t left enabled by default.

Source