CVE-2025-20339: Cisco SD-WAN ACL Bypass Vulnerability

CVE-2025-20339 Overview

CVE-2025-20339 is an access control list (ACL) bypass vulnerability in Cisco SD-WAN vEdge Software. The flaw exists in how the software processes IPv4 packets against configured ACLs. An unauthenticated, remote attacker can send crafted IPv4 traffic to an interface on an affected device and bypass ACL enforcement.

The root cause is improper enforcement of the implicit deny all rule at the end of a configured ACL. Traffic that should be dropped by the default deny is permitted through the interface. The vulnerability is classified under [CWE-284: Improper Access Control].

Critical Impact
Unauthenticated remote attackers can bypass configured ACLs on Cisco SD-WAN vEdge devices, exposing internal network segments to unauthorized IPv4 traffic.

Affected Products

Cisco SD-WAN vEdge Software
Cisco SD-WAN vEdge routers running affected versions
Deployments relying on IPv4 ACLs for interface-level filtering

Discovery Timeline

2025-09-24 - CVE-2025-20339 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-20339

Vulnerability Analysis

The vulnerability resides in the IPv4 packet processing path of Cisco SD-WAN vEdge Software. When an administrator configures an ACL on an interface, the platform is expected to evaluate each rule in sequence and drop any traffic that does not match a permit rule. This behavior is enforced by the implicit deny all that terminates every ACL.

In affected versions, the implicit deny is not applied correctly. Packets that fall through the configured rules are forwarded rather than dropped. This defeats the security assumption that unmatched traffic is silently discarded at the ACL boundary.

The impact is scoped to integrity of network segmentation. Attackers cannot directly read confidential data or crash the device through this flaw, but they can reach services and network segments that were intended to be filtered.

Root Cause

The flaw stems from incorrect logic in the ACL evaluation engine for IPv4 packets. The implicit terminating deny rule is either skipped or evaluated as an implicit permit. This is an access control weakness ([CWE-284]) rather than a memory safety issue.

Attack Vector

Exploitation requires no authentication and no user interaction. An attacker sends unauthorized IPv4 traffic toward an interface on the affected vEdge device that has an ACL applied. Traffic that should match the implicit deny is instead forwarded to its destination. The attack is remote and network-based, and the scope changes because the bypass affects resources beyond the vulnerable component itself.

No public proof-of-concept exploit and no reports of exploitation in the wild are associated with this CVE. Consult the Cisco Security Advisory for vendor-supplied technical details.

Detection Methods for CVE-2025-20339

Indicators of Compromise

Unexpected IPv4 flows reaching internal hosts or services that should be blocked by an interface ACL on a vEdge device
NetFlow or IPFIX records showing source-destination pairs that violate the documented ACL policy
Connection attempts to management or restricted subnets originating from untrusted zones

Detection Strategies

Compare configured ACL policy against observed traffic flows using NetFlow, IPFIX, or sFlow exported from vEdge devices
Deploy passive taps or span sessions downstream of the ACL to identify traffic that should have been dropped
Correlate vManage configuration snapshots with firewall and IDS logs to identify policy drift or bypass

Monitoring Recommendations

Enable and centralize flow telemetry from all vEdge interfaces where ACLs are enforced
Alert on any traffic reaching restricted network segments from sources outside the permitted set
Track Cisco PSIRT advisories and correlate vEdge software versions in inventory against fixed releases

How to Mitigate CVE-2025-20339

Immediate Actions Required

Inventory all Cisco SD-WAN vEdge devices and identify software versions in use
Apply the fixed software release listed in the Cisco Security Advisory as soon as change windows allow
Audit interface ACLs and confirm that critical filtering does not rely solely on the implicit deny

Patch Information

Cisco has published fixed software releases in the Cisco Security Advisory cisco-sa-defaultacl-pSJk9nVF. Administrators should consult the advisory for the specific fixed versions applicable to their deployed vEdge platforms and upgrade paths.

Workarounds

Add an explicit terminating deny ip any any rule to every affected ACL rather than relying on the implicit deny
Layer additional filtering upstream or downstream of the vEdge device using a firewall that enforces the intended policy
Restrict administrative and management interface exposure until patched software is deployed

bash

# Example: explicit terminating deny for an IPv4 ACL on affected vEdge devices
# Replace <acl-name> and permit rules with your policy
policy access-list <acl-name>
  sequence 10
    match source-ip 10.0.0.0/24
    action accept
  sequence 9999
    match source-ip 0.0.0.0/0
    action drop