CVE-2026-76442: Cisco Secure Email Input Validation Vulnerability

CVE-2026-76442 Overview

Cisco disclosed CVE-2026-76442 as part of an internal security review of Cisco Secure Email Gateway (ESA) and Cisco Secure Email and Web Manager. The vulnerability stems from improper validation of specified quantity in input, classified under CWE-1284. A remote, unauthenticated attacker can exploit the flaw over the network to cause an availability impact on affected appliances. No confidentiality or integrity impact is reported. Cisco identified the issue during proactive engineering review rather than in response to in-the-wild exploitation.

Critical Impact
Unauthenticated network attackers can trigger a denial-of-service condition on Cisco Secure Email Gateway and Secure Email and Web Manager appliances without user interaction.

Affected Products

Cisco Secure Email Gateway (ESA)
Cisco Secure Email and Web Manager (SEMA)
Refer to the Cisco Security Advisory for specific fixed release versions

Discovery Timeline

2026-09-14 - CVE-2026-76442 published to NVD
2026-09-16 - Last updated in NVD database

Technical Details for CVE-2026-76442

Vulnerability Analysis

CVE-2026-76442 is an improper input validation issue (CWE-1284) in the input processing paths of Cisco Secure Email Gateway and Cisco Secure Email and Web Manager. The affected components fail to enforce constraints on quantity-related input fields, such as counts, sizes, or repetition indicators supplied by network clients. When a malformed value is supplied, downstream processing consumes excessive resources or reaches an unrecoverable state.

The result is an availability impact on the appliance. Because the vulnerable code path is reachable over the network without authentication or user interaction, any attacker capable of sending traffic to the exposed service can attempt exploitation. Cisco discovered the issue internally during a hardening review, and no exploitation has been reported.

Root Cause

The root cause lies in missing or insufficient validation of quantity fields in attacker-controlled input. Under CWE-1284, such flaws occur when software processes a value that specifies count, length, or size without confirming it falls within safe operational bounds. The affected subsystem accepts these values and passes them to logic that assumes valid ranges.

Attack Vector

Exploitation requires only network access to the vulnerable service on the appliance. No credentials and no user interaction are needed. An attacker sends a crafted request containing an out-of-range or unexpected quantity value, which triggers the resource condition on the target. Because Cisco has not published a public proof of concept, the specific protocol field and message format are documented only in the vendor advisory. Administrators should consult the Cisco Security Advisory for detailed technical guidance.

Detection Methods for CVE-2026-76442

Indicators of Compromise

Unexpected service restarts, process crashes, or watchdog-triggered reboots on Cisco ESA or SEMA appliances.
Sudden spikes in CPU, memory, or connection-table utilization coinciding with inbound traffic bursts.
Log entries showing malformed protocol messages or oversized field values rejected by internal validators.

Detection Strategies

Monitor appliance health telemetry and SNMP counters for availability regressions correlated with external traffic sources.
Inspect perimeter logs for repeated connections from single sources targeting mail-processing ports immediately before service degradation.
Correlate Cisco appliance syslog output with network flow data to identify anomalous request patterns preceding outages.

Monitoring Recommendations

Forward Cisco ESA and SEMA syslog and SNMP traps to a centralized SIEM for correlation with network telemetry.
Alert on service restart events and process termination messages originating from mail-handling daemons.
Baseline normal inbound SMTP and management-plane traffic volumes so anomalous surges trigger investigation.

How to Mitigate CVE-2026-76442

Immediate Actions Required

Review the Cisco Security Advisory and identify affected AsyncOS versions in your environment.
Schedule upgrades to the fixed software releases published by Cisco for ESA and SEMA appliances.
Restrict management-plane and administrative interfaces to trusted networks only.

Patch Information

Cisco has released hardened software versions for Cisco Secure Email Gateway and Cisco Secure Email and Web Manager that address CVE-2026-76442 along with other internally discovered issues. Consult the vendor advisory for the specific fixed AsyncOS train applicable to each deployed appliance and follow Cisco's standard upgrade procedures.

Workarounds

No official workaround is listed in the Cisco advisory; upgrading to a fixed release is the recommended remediation.
Where immediate patching is not feasible, place upstream rate limiting and access control lists in front of the appliance to reduce exposure.
Limit inbound connectivity to the mail-processing service to known upstream mail transfer agents where operationally possible.

bash

# Configuration example: restrict administrative access to trusted subnets
# (illustrative CLI syntax; consult Cisco documentation for exact commands)
sshconfig
accesscontrol
  add 10.0.0.0/24 allow
  add 0.0.0.0/0 deny
commit