CVE-2026-55400

Executive Summary

CVE-2026-55400 is an integer underflow vulnerability (CWE-190) affecting Ivanti Secure Access servers prior to version 14.57. An attacker holding an authenticated session can send specially crafted traffic to a server running a non-default configuration and trigger a persistent denial of service (DoS), taking the remote-access gateway offline for an extended period. As of this report there is no confirmed vendor patch, no CISA KEV listing, no public exploit code, and a low EPSS score (0.00228, 13.9th percentile), indicating low near-term exploitation likelihood but a high potential impact if exploited against an unpatched edge device.

Technical Analysis

The vulnerability is classified as an integer underflow (CWE-190: Integer Overflow or Wraparound). Integer underflow occurs when an arithmetic operation produces a value below the minimum representable value of the integer data type. In unsigned integer arithmetic, the result wraps around to a very large positive value; in signed arithmetic, the result becomes undefined or negative. Network protocol parsers commonly use integer length or offset fields to compute buffer boundaries, loop counts, or allocation sizes. When a length calculation underflows, the parser may allocate an undersized buffer and then write a large amount of data into it, or it may compute an incorrect offset that causes out-of-bounds reads and writes. In the context of a VPN gateway implemented largely in C/C++, this class of flaw frequently leads to memory corruption and process crashes.

According to the NVD description, three conditions are required for successful exploitation:

1. Authenticated session — the attacker must hold valid credentials for the Secure Access portal (a VPN user, an administrative account, or a guest account depending on configuration).
2. Non-default configuration — some feature, module, or protocol handler that is not enabled out of the box must be active on the target server. The NVD description does not specify which configuration option is involved; insufficient technical detail is available to identify the exact feature.
3. Specially crafted traffic — the attacker sends a crafted sequence or payload to the exposed interface that exercises the vulnerable arithmetic path.

The reported impact is a persistent denial of service. This distinguishes the bug from a transient crash that the service recovers from automatically. A persistent DoS implies the gateway process enters a crash loop, the appliance becomes unresponsive, or the underlying OS requires manual intervention (reboot, console access, or vendor support) to restore service. No privilege escalation, authentication bypass, or remote code execution is claimed in the NVD description; the impact appears confined to availability, but on an edge gateway that availability impact is organization-wide.

Because the precise vulnerable function, the affected parser, and the triggering input are not disclosed, exploitation mechanics can only be described at the vulnerability-class level. The general mechanism is: the attacker submits protocol fields containing length/offset/size values that cause an unsigned subtraction to wrap past zero, producing a corrupted length used in subsequent memory operations, which terminates the service.

Exploitation Scenarios

No public PoC exists, no Nuclei template is available, and no exploit code has been observed in the wild as of this report. The attack chain below is inferred strictly from the NVD description and the vulnerability class (CWE-190). Where implementation specifics are unknown, this is stated explicitly rather than filled in with assumptions.

Step 1 — Reconnaissance and target identification. The attacker identifies Internet-facing Ivanti Secure Access gateways using asset search engines (Shodan, Censys, FOFA) with banners or titles containing "Ivanti Secure Access" or legacy "Pulse Secure" branding, or by scanning TLS certificate subject names on ports 443 and 8443. Version fingerprinting may be performed via login-page markers, favicon hashes, or documented behavior differences between releases. The attacker determines whether the target is running a version prior to 14.57.

Step 2 — Gaining an authenticated session. The vulnerability explicitly requires an authenticated session. The attacker obtains valid credentials through phishing targeting remote workers, credential stuffing against the VPN portal, or by compromising a legitimate user account through other means. Alternatively, the threat actor may already be an insider with a valid VPN account. No authentication bypass is claimed for this CVE, so credential acquisition is a mandatory precursor.

Step 3 — Triggering the integer underflow. The attacker sends specially crafted traffic to the server. At the vulnerability-class level, this involves posting protocol data with malformed or boundary-length fields — for example, length values that cause a subtract operation to underflow during parsing. The attack only succeeds if the server is running the non-default configuration that exposes the vulnerable code path. This step may require several repeated attempts to hit the exact arithmetical boundary condition. Because the specific URI, parameter, and payload structure are unknown, a precise byte-level payload cannot be described here.

Step 4 — Post-exploitation outcome. The crafted traffic triggers the underflow condition, leading to memory corruption or an uncontrolled arithmetic result that crashes the gateway service. The DoS is persistent: the service does not automatically recover, requiring manual intervention such as a reboot or vendor support engagement. During the outage, all remote-access VPN users, and any services that depend on the gateway (authentication federation, internal application access, administrative access to the network), are unavailable.

Realistic attacker profiles: (a) a disgruntled or malicious insider with legitimate VPN credentials aiming to disrupt operations; (b) an external actor who compromises low-privilege credentials and uses this flaw to knock the edge device offline, possibly as a precursor to other activity or as pure disruption; (c) hacktivists seeking high-visibility outages.

Impact Assessment

CVSS context: No CVSS score or vector has been published (CVSS: N/A). Analysts should therefore treat severity as conditional and monitor for an updated score. In the absence of a published vector, the practical severity should be evaluated based on the product's role: Ivanti Secure Access is a perimeter remote-access gateway, and availability loss directly translates to loss of business continuity for all remote workers and any services network-dependent on the VPN.

Business impact: Persistent DoS of a VPN gateway halts remote work, blocks access to internal applications, disrupts authentication to federated services, and can break active sessions for hundreds or thousands of users. Recovery may require an on-site reboot or vendor intervention, extending MTTR from minutes to hours or days depending on appliance accessibility. For organizations that run the non-default configuration required for exploitation, the risk is materially higher.

Blast radius: The impact is confined to the affected gateway instance and its dependent user population; there is no claimed data confidentiality or integrity impact. Lateral movement is not described as a consequence of this vulnerability. However, because the device sits at the network edge, a persistent outage can also degrade security monitoring, MDM enrollment, and remote troubleshooting capabilities. The CISA KEV catalog does not list this CVE, and EPSS places it in the 13.9th percentile of exploited CVEs, suggesting that mass exploitation is not currently occurring.

Systemic risk note: Ivanti Secure Access (formerly Pulse Connect Secure) has been a repeated target of state-sponsored exploitation in prior years across other CVEs. Even though this specific CVE has no known active exploitation, organizations running unpatched Ivanti edge devices should treat any new advisory with elevated urgency.

Detection Guidance

The following detection content is heuristic. Because the NVD description does not disclose the vulnerable URI, parameter, or payload format, these rules focus on behavioral indicators: malformed or oversized HTTP requests from authenticated sessions, repeated 5xx responses, and abnormal service termination. Tune thresholds to your environment's baseline traffic before enabling in alert-only mode.

Sigma Rule

title: Potential CVE-2026-55400 Integer Underflow DoS Attempt - Ivanti Secure Access
id: 6b9e2c2a-5540-4a00-9c10-2c6e4a1b3d00
status: experimental
description: Detects a high volume of unusually large HTTP requests directed at an Ivanti Secure Access gateway from a single source, followed by 5xx responses, which may indicate repeated triggering of CVE-2026-55400 integer underflow conditions. Heuristic rule; no known URI signature.
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2026-55400
author: Valters IT Threat Intelligence
date: 2026/06/01
logsource:
    category: webserver
    product: generic
detection:
    selection:
        sc_status:
            - '500'
            - '502'
            - '503'
    filter_baseline:
        sc_bytes|lt: 1048576
    condition: selection and not filter_baseline
timeframe: 5m
level: high
fields:
    - src_ip
    - sc_status
    - cs_username
    - dest_host
falsepositives:
    - Legitimate large file uploads through the VPN portal
    - Malformed client requests from old browsers
    - Administrative activity during maintenance windows

YARA Rule

rule CVE_2026_55400_Ivanti_SecureAccess_DoS_Heuristic
{
    meta:
        author = "Valters IT Threat Intelligence"
        description = "Heuristic YARA rule for traffic captures and web logs consistent with CVE-2026-55400 integer underflow DoS attempts against Ivanti Secure Access. Matches on repeated oversized HTTP requests and 5xx crash responses."
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-55400"
        cve = "CVE-2026-55400"
        date = "2026-06-01"
        threat_level = "medium"
    strings:
        $s_ivanti = "Ivanti" ascii wide nocase
        $s_secure = "Secure Access" ascii wide nocase
        $s_clen = "Content-Length:" ascii wide nocase
        $s_500 = "HTTP/1.1 500" ascii wide nocase
        $s_502 = "HTTP/1.1 502" ascii wide nocase
        $s_503 = "HTTP/1.1 503" ascii wide nocase
    condition:
        filesize < 20MB and
        ($s_ivanti or $s_secure) and
        (#s_clen >= 5) and
        (#s_500 > 0 or #s_502 > 0 or #s_503 > 0)
}

Suricata IDS Rule

alert http any any -> $HOME_NET any (msg:"ET EXPLOIT Possible CVE-2026-55400 Ivanti Secure Access Integer Underflow DoS Attempt"; flow:established,to_server; content:"POST"; http.method; http.content_len; content_len:>10000000; threshold:type both, track by_src, count 5, seconds 60; classtype:attempted-dos; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-55400; sid:2026554; rev:1;)

Rule notes: This rule alerts when a single source sends five or more HTTP POST requests with a Content-Length greater than 10 MB to the gateway within 60 seconds. It should be paired with an outbound rule watching for repeated HTTP 500/502/503 responses from the gateway to confirm a crash condition. Adjust the content-length threshold and the count/seconds values to your organization's legitimate traffic profile.

Elastic Detection

sequence by source.ip with maxspan=10m
  [ http where http.request.bytes > 10000000 and destination.port in (443, 8443) ]
  [ http where http.response.status_code >= 500 ]

// Companion rule: detect gateway service instability (host-level)
// Adjust process names to match the Secure Access daemon on your deployment.
sequence by host.name with maxspan=15m
  [ process where event.action in ("process_ended", "process_stopped") and
    process.executable contains "pulse" ]
  [ network where destination.port in (443, 8443) ]

Splunk SPL Query

index=web OR index=proxy sourcetype=access_combined
    dest_host="*secureaccess*" OR dest_host="*pulse*"
    status IN (500, 502, 503) AND bytes > 10000000
| bucket _time span=5m
| stats count as bad_requests by src_ip, dest_host, _time
| where bad_requests >= 5
| sort - bad_requests
| table _time, src_ip, dest_host, bad_requests

// Availability monitor: detect gateway process/health-check failures
index=main sourcetype=network_health OR sourcetype=system
    host="*secureaccess*"
    ("DOWN" OR "unreachable" OR "connection refused" OR "timeout")
| stats count by host, _time
| where count > 3

Wazuh OSSEC Rule

<group name="ivanti,">
    <!-- Heuristic: gateway crash / persistent DoS indicators for CVE-2026-55400 -->
    <rule id="554001" level="12">
        <decoded_as>syslog</decoded_as>
        <match>Secure Access|Ivanti</match>
        <regex>segfault|core dumped|SIGSEGV|SIGABRT|out of memory|Fatal error|abnormal termination</regex>
        <description>CVE-2026-55400: Possible persistent DoS / service crash on Ivanti Secure Access gateway</description>
        <group>dos,</group>
        <cve>CVE-2026-55400</cve>
    </rule>

    <rule id="554002" level="10">
        <if_sid>554001</if_sid>
        <match>restarting|reloading|watchdog</match>
        <description>CVE-2026-55400: Ivanti Secure Access gateway entering crash/reload loop</description>
        <group>dos,</group>
    </rule>
</group>

Wazuh deployment note: Ensure the Ivanti Secure Access appliance logs web-server errors and system messages to syslog and that Wazuh is configured to ingest that source. The <cve> tag allows correlation with vulnerability scanners using the Wazuh vulnerability-detection module.

Mitigation & Patch Guidance

Patch confidence: 0/4 — UNPATCHED. No patch evidence exists from any tracked source. The NVD description identifies version 14.57 as the first non-affected release, but a formal vendor patch/advisory has not been confirmed. Treat the vulnerability as unpatched until Ivanti publishes an advisory and a verified release.

Immediate mitigations (no patch available):

1. Audit and disable non-default configurations. The vulnerability is only reachable when a non-default configuration option is enabled. Review the gateway configuration against the vendor's hardening guide and disable any features, modules, or protocol handlers that are not strictly required for business operation. Until the specific feature is identified, minimize the attack surface by removing all non-essential services.
2. Restrict authenticated access. Enforce phishing-resistant multi-factor authentication (MFA) on all VPN accounts. Revoke dormant, contractor, and shared accounts. Apply least-privilege policies to reduce the pool of accounts that can satisfy the authenticated-session requirement.
3. Limit source exposure. Where possible, restrict access to the gateway's management interface to trusted administrative source IPs. Consider geo-blocking or allow-listing at the perimeter for the VPN portal if business constraints permit.
4. Deploy compensating controls. Place a web application firewall (WAF) or reverse proxy in front of the gateway with request-size limits, rate limiting, and malformed-request filtering tuned to block the oversized or malformed traffic pattern described above.
5. Prepare an outage response plan. Because the DoS is persistent, document the manual recovery procedure (console access, reboot sequence, vendor escalation path) and ensure on-site or out-of-band administrative access is available if the gateway goes down.

Patch/upgrade steps:

• Subscribe to Ivanti's security advisory notifications and monitor for the official advisory referencing CVE-2026-55400.
• Plan an upgrade to Secure Access version 14.57 or later in a staging environment first, validating third-party integrations (LDAP/AD, SAML SSO, endpoint posture checks) before production rollout.
• If the organization cannot upgrade immediately, apply any interim hotfix or configuration workaround Ivanti publishes, and document compensating controls in the risk register.

Long-term hardening:

• Maintain a standing patching cadence for all edge devices, with Ivanti Secure Access classified as a critical system with a patching SLA of no more than 7 days after advisory release.
• Perform periodic configuration reviews against the vendor hardening checklist; disable unused features by default.
• Deploy a secondary or redundant gateway to ensure high availability against future single-device failures.
• Ensure comprehensive logging (web access logs, syslog, process-level events) is forwarded to the SIEM so the detection rules in this report are actionable.
• Track this CVE in CISA KEV and EPSS feeds; revisit prioritization if the exploitation status changes.

References

1. NVD Entry — CVE-2026-55400: https://nvd.nist.gov/vuln/detail/CVE-2026-55400
2. CWE-190 Integer Overflow or Wraparound — MITRE: https://cwe.mitre.org/data/definitions/190.html
3. Ivanti Security Advisories Portal: https://www.ivanti.com/support/security-advisories
4. CISA Known Exploited Vulnerabilities Catalog (CVE-2026-55400 not currently listed): https://www.cisa.gov/known-exploited-vulnerabilities-catalog
5. FIRST.org EPSS (CVE-2026-55400 score: 0.00228, 13.9th percentile): https://www.first.org/epss/

Threat-actor campaign status: No known exploit-in-the-wild activity, public exploit code, or ransomware/general intrusion campaign is associated with CVE-2026-55400 as of this report. However, Ivanti Secure Access / Pulse Connect Secure appliances have historically been prime targets for state-sponsored and financially motivated actors (prior zero-day exploitation of Ivanti edge products in 2023–2025). SOC teams should treat any unpatched Ivanti edge device with heightened suspicion and prioritize log review for the detection patterns above, even in the absence of active exploitation of this CVE.