CriticalCVE-2026-76519

    Cisco Secure Email Gateway Arbitrary File Write Vulnerability (CVE-2026-76519)

    Published Last modified In CISA KEV since Sources: CERT-EU, CVE.org, BSI, Cisco PSIRT

    What is affected, what to do

    A vulnerability in the Advanced Malware Protection (AMP) feature of Cisco Secure Email Gateway could allow an unauthenticated, remote attacker to write arbitrary files on the underlying operating system. This vulnerability is due to improper handling of email attachments when the file reputation service is enabled.

    The vulnerability is being actively exploited; treat it as a priority. Update Secure Email Gateway to version 15.5.5-014. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Cisco Secure Email eq 14.0.0-698..14.0.0-698
    • Cisco Secure Email eq 13.5.1-277..13.5.1-277
    • Cisco Secure Email eq 13.0.0-392..13.0.0-392
    • Cisco Secure Email eq 14.2.0-620..14.2.0-620
    • Cisco Secure Email eq 13.0.5-007..13.0.5-007
    • Cisco Secure Email eq 13.5.4-038..13.5.4-038
    • Cisco Secure Email eq 14.2.1-020..14.2.1-020
    • Cisco Secure Email eq 14.3.0-032..14.3.0-032
    • Cisco Secure Email eq 15.0.0-104..15.0.0-104
    • Cisco Secure Email eq 15.0.1-030..15.0.1-030
    • Cisco Secure Email eq 15.5.0-048..15.5.0-048
    • Cisco Secure Email eq 15.5.1-055..15.5.1-055
    • Cisco Secure Email eq 15.5.2-018..15.5.2-018
    • Cisco Secure Email eq 16.0.0-050..16.0.0-050
    • Cisco Secure Email eq 15.0.3-002..15.0.3-002
    • Cisco Secure Email eq 16.0.0-054..16.0.0-054
    • Cisco Secure Email eq 15.5.3-022..15.5.3-022
    • Cisco Secure Email eq 16.0.1-017..16.0.1-017
    • Cisco Secure Email eq 15.5.4-012..15.5.4-012
    • Cisco Secure Email eq 16.0.4-016..16.0.4-016
    • Cisco Secure Email eq 15.0.5-016..15.0.5-016
    • Cisco Secure Email eq 16.0.2-112..16.0.2-112
    • Cisco Secure Email eq 16.0.3-044..16.0.3-044

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    9.8

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: NVD, CVSS v3

    9.8

    CVSS v3

    Severity Critical on the scale from 0 to 10.

    Source: CERT-EU, CVE.org, BSI, Cisco PSIRT

    1.6 %

    EPSS score

    Moderate — model estimates 1-10% exploitation likelihood.

    Source: FIRST EPSS, percentile 75.0

    Show all metrics
    Severity
    Critical
    Priority score

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    224.01
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    actively exploited (KEV)Source: VulnCheck KEV (opens in a new tab)
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    9.8
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    9.8
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    1.6 %
    Moderate — model estimates 1-10% exploitation likelihood.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    75.0
    High — this CVE ranks above the median of all CVEs scored today (rank ≥ 36%).
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    Actively exploited
    Published
    In CISA KEV since

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-23

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This CVE is on KEV because it is reachable unauthenticated through the normal mail delivery path and yields root code execution on a trusted perimeter appliance — an attacker only has to send an email to any recipient behind the SEG. For European organisations running Cisco Secure Email Gateway as primary or secondary MTA this is a priority-1 patch, and under NIS2 the SEG qualifies as an essential component of the communication chain, so a compromise materially raises reporting obligations. Observed exploitation likelihood is in the upper mid-range — not wormable, but reliable enough that CISA set a remediation deadline. No documented ransomware use yet, but root on a mail gateway is an ideal pivot into both mail content and internal networks, so treat it as "critical, actively exploited" on your risk scorecard regardless.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Deploy the Cisco AsyncOS fix for Secure Email Gateway — pull the exact fixed release from the corresponding Cisco Security Advisory. Vulnerable builds include 13.0.0-392, 13.0.5-007, 13.5.1-277, 13.5.4-038, 14.0.0-698, 14.2.0-620, 14.2.1-020 and 14.3.0-032. Prioritise every appliance that accepts inbound SMTP from the internet.
      • If patching cannot happen within hours, disable the File Reputation Service (part of AMP) — this is the exact code path that parses the malicious attachment. Compensate by tightening AV/sandbox scanning on a downstream hop (secondary MTA, EDR on mailbox servers).
      • Lock down management access to the appliance (HTTPS 443, SSH 22, AsyncOS API 6080/6443) to admin jump hosts only. Does not stop SMTP-borne exploitation but limits lateral movement after a successful root compromise.
      • Run a compromise assessment on every SEG: diff cron, /etc/, /data/, web root and startup scripts against a known-good image or backup. On any deviation, isolate the appliance, export config, and rebuild from clean image — do not patch in place.
      • Rotate all secrets stored on the SEG: LDAP bind account, SMTP relay credentials, API tokens, TLS private keys, cluster secrets. Root-level code execution means every secret on the box must be treated as exfiltrated.

    2. Runbook step 2

      Mitigation layers

      • Network: place SEG appliances in a dedicated DMZ segment with default-deny egress — permit only Cisco update infrastructure, DNS, NTP and the defined downstream MTA. Blocks reverse shells and C2 pulls that typically follow an arbitrary-file-write primitive.
      • Mail flow: configure an upstream MTA or cloud mail hygiene tier to drop attachments whose MIME Content-Disposition: filename= contains traversal sequences (../, ..\, URL-encoded %2e%2e, embedded null bytes) before they reach the SEG.
      • IPS/NDR: Suricata/Snort signature on the SMTP DATA stream matching traversal patterns in MIME filename parameters. Example logic: content:"Content-Disposition"; content:"filename="; pcre:"/filename=\"?[^\"]*(\.\.[\/\\]|%2e%2e)/i".
      • Appliance hardening: reduce the AMP feature set to the minimum needed, tighten file analysis upload size limits, disable decoders for rarely used archive formats.
      • IAM: admin access to the SEG exclusively via MFA-protected PAM/jump host, disable local service accounts in favour of LDAP where possible. Reduces post-exploitation persistence options.
      • Backup/recovery: hold encrypted, offline config snapshots so that a post-incident rebuild is possible without reintroducing a persistently tampered configuration.

    3. Runbook step 3

      Detection rules

      • SEG mail logs (mail_logs): alert on attachments whose MIME filename contains ../, ..\, %2e%2e, backslash sequences or unusually long filenames (>200 chars). Sigma shape: logsource: cisco_esa / detection: filename|contains: ['../','..\\','%2e%2e'].
      • AMP / file-reputation processing logs (amp / amp_logs): correlate parser exceptions, crashes or abrupt service restarts — exploit attempts routinely throw an exception before the successful write.
      • Host telemetry (where CLI access or syslog export is available): auditd/syslog on unexpected writes outside the AsyncOS standard paths, especially into /etc/, /root/, /data/pub/configuration/, plus new cron or at jobs. KQL shape: Syslog | where ProcessName in~ ('amp','mailprocess') and SyslogMessage has_any ('open(','write(') and SyslogMessage has_any ('/etc/','/root/','/tmp/.').
      • Egress netflow/Zeek: SEG appliance opening outbound sessions to anything not whitelisted (not a Cisco update endpoint, not the downstream MTA, not DNS/NTP) — TCP/443 to unknown hosts, 4444, 8080 or ICMP-tunnel patterns are strong post-exploit indicators.
      • Inventory monitoring: periodically compare the AsyncOS build string against the list of known-vulnerable builds and alert on regression (e.g. after a snapshot restore reintroduces a vulnerable image).

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    Description

    A vulnerability in the Advanced Malware Protection (AMP) feature of Cisco Secure Email Gateway could allow an unauthenticated, remote attacker to write arbitrary files on the underlying operating system. This vulnerability is due to improper handling of email attachments when the file reputation service is enabled. An attacker could exploit this vulnerability by sending an email that contains a crafted attachment through an affected device. A successful exploit could allow the attacker to write arbitrary files and execute commands with root privileges on the underlying operating system.

    Source: certeu_advisory, NVD (NIST), BSI CSAF, BSI WID Portal, cvelistv5, CISA KEV Catalog, cisco_csaf, CVE.org (MITRE)

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    VendorProductAffected versionsFixed in
    CiscoCisco Secure Email
    • 13.0.0-392
    • 13.0.5-007
    • 13.5.1-277
    • 13.5.4-038
    • 14.0.0-698
    • 14.2.0-620
    • 14.2.1-020
    • 14.3.0-032
    • 15.0.0-104
    • 15.0.1-030
    • 15.0.3-002
    • 15.0.5-016
    • 15.5.0-048
    • 15.5.1-055
    • 15.5.2-018
    • 15.5.3-022
    • 15.5.4-012
    • 16.0.0-050
    • 16.0.0-054
    • 16.0.1-017
    • 16.0.2-112
    • 16.0.3-044
    • 16.0.4-016
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    CiscoSecure Email Gateway
    • < 15.5.5-014
    • < 16.5.0-780
    • 15.5.5-014
    • 16.5.0-780

    Affected operating systems

    Affected operating systems
    FamilyVendorProductVersion
    Otherciscoasyncos—

    Weakness classes (CWE)

    • CWE-23Relative Path Traversal

      Base

      The product uses external input to construct a pathname that should be within a restricted directory, but it does not properly neutralize sequences such as ".." that can resolve to a location that is outside of that directory.

      Entry at MITRE CWE-23

    Public exploit references

    Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.

    Public exploit references
    SourceReferenceMaturityPublished
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited (KEV)
    PoC-in-GitHub (nomi-sec)CVE-2026-76519-POC (opens in a new tab)github.comPoC (publicly reported)reported, not verified

    References & sources

    Reanalysis & status changes

    Chronological NVD audit events for this CVE — reanalyses, CVSS updates, CPE diffs.

    1. Initial Analysisnvd@nist.gov

      • CPE Configuration: after AND OR *cpe:2.3:o:cisco:asyncos:*:*:*:*:*:*:*:* versions from (including) 15.0 up to (excluding) 15.5.5-014 *cpe:2.3:o:cisco:asyncos:*:*:*:*:*:*:*:* versions from (including) 16.0 up to (excluding) 16.5.0-780 OR cpe:2.3:a:cisco:secure_email_gateway_virtual_appliance_c100v:-:*:*:*:*:*:*:* cpe:2.3:a:cisco:secure_email_gateway_virtual_appliance_c300v:-:*:*:*:*:*:*:* cpe:2.3:a:cisco:secure_email_gateway_virtual_appliance_c600v:-:*:*:*:*:*:*:* cpe:2.3:h:cisco:secure_email_gateway_c195:-:*:*:*:*:*:*:* cpe:2.3:h:cisco:secure_email_gateway_c395:-:*:*:*:*:*:*:* cpe:2.3:h:cisco:secure_email_gateway_c695:-:*:*:*:*:*:*:*
      • Reference Type: after Cisco Systems, Inc.: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-esa-afw-zqYJkjxp Types: Vendor Advisory
      • Reference Type: after CISA-ADP: https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-76519 Types: US Government Resource

    2. CVE Modified134c704f-9b21-4f2e-91b3-4a467353bcc0

      • SSVC: before {"id":"CVE-2026-76519","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"yes"},{"technic…
      • SSVC: after {"id":"CVE-2026-76519","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"yes"},{"technic…

    3. CVE CISA KEV Update9119a7d8-5eab-497f-8521-727c672e3725

      • Date Added: after 2026-10-14
      • Due Date: after 2026-10-14
      • Required Action: after 2026-10-14
      • Vulnerability Name: after 2026-10-14

    4. CVE Modified134c704f-9b21-4f2e-91b3-4a467353bcc0

      • Reference: after https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-76519
      • SSVC: after {"id":"CVE-2026-76519","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"yes"},{"technic…

    5. New CVE Receivedpsirt@cisco.com

      • Description: after A vulnerability in the Advanced Malware Protection (AMP) feature of Cisco Secure Email Gateway could allow an unauthenticated, remote attacker to write arbitrary files on the underlying operating system. This vulnerability is due to improper handling of email attachments when the file reputation service is enabled. An attacker could exploit this vulnerability by sending an email that contains a crafted attachment through an affected device. A successful exploit could allow the attacker to write arbitrary files and execute commands with&nbsp;root privileges on the underlying operating system.
      • CVSS V3.1: after AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
      • CWE: after CWE-23
      • Affected: after New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/76xxx/CVE-2026-76519.json">CVE-2026-76519</a>

    Linked advisories