Cisco has confirmed active exploitation of CVE-2026-76519, a critical (CVSS 9.8) relative path traversal (CWE-23) in the Advanced Malware Protection (AMP) File Reputation handling of Cisco Secure Email Gateway (SEG, formerly ESA). CISA added it to the Known Exploited Vulnerabilities catalog on October 14, 2026, with a federal remediation deadline of October 17.

What happened

Cisco PSIRT says it became aware of exploitation in October 2026. An unauthenticated remote attacker can write arbitrary files to the appliance’s filesystem and, from there, execute commands as root. The trigger is an inbound email with a crafted attachment, so the attacker needs no credentials and no access to the management interface. The appliance only has to accept and scan the message.

Technical details

The bug sits in how attachment metadata is handled when File Reputation queries run through the AMP content scanning pipeline. A traversal sequence escapes the intended working directory, giving a write primitive anywhere the scanning process can reach. Because the scanner runs with elevated privileges, the primitive converts directly into root code execution, for example by dropping a file into a path that is later executed or loaded.

Exposure depends on configuration. Reported conditions:

  • The device is a physical or virtual Secure Email Gateway on a vulnerable AsyncOS release.
  • File Reputation (AMP) is enabled and assigned to a mail policy.
  • Content Scanner Tools is older than 25.3.0.5525.

Mail gateways sit on the network edge and process untrusted input by design, which makes this a high-value target. Compromise gives an attacker a view of mail flow, a pivot into internal networks, and a trusted sender for follow-on phishing.

Impact

Any internet-facing SEG with AMP File Reputation active is exposed to unauthenticated root compromise. Cisco has published indicators of compromise. A root-level attacker can tamper with or delete on-device logs, so a clean local log is not evidence of a clean appliance. Public proof-of-concept code had not been reported at the time of writing, but exploitation is already under way, and the delivery vector is simple enough that reproduction is likely.

Mitigation

Cisco lists no workarounds. Do the following now:

  1. Update Content Scanner Tools to 25.3.0.5525 or later. Reporting indicates this applies without a full software upgrade or restart.
  2. Upgrade AsyncOS. Reported fixed releases are 15.5.5-014 and 16.5.0-780. Sources disagree on whether a fixed AsyncOS 16.0 build exists, so check the advisory for your train and plan a migration if none is listed.
  3. Hunt off-box. Check firewall, proxy, and NetFlow logs for unexpected outbound connections or file transfers from the gateway, and review mail logs for unusual attachment-bearing messages. Do not rely on appliance logs alone.
  4. Apply Cisco’s published IOCs and treat any match as a full compromise: isolate, rebuild from known-good media, and rotate credentials and secrets stored on the device (LDAP bind accounts, API keys, certificates).
  5. Federal agencies must remediate under the KEV deadline of October 17.

References