CVE-2026-100208

Executive Summary

CVE-2026-100208 is an integer overflow or wraparound vulnerability (CWE-190) in Microsoft Office Outlook that permits an unauthorized attacker to achieve remote code execution over a network. Exploitation requires user interaction (UI:R) and is assessed at high attack complexity (AC:H), with no privileges required (PR:N) and full impact to confidentiality, integrity, and availability of the targeted component, yielding a CVSS 3.1 base score of 7.5 (High). Any organization running affected Outlook builds is in scope; the primary risk is code execution in the context of the Outlook client through a malicious message or document processed by the user.

Technical Analysis

The vulnerability is classified as CWE-190: Integer Overflow or Wraparound. In this class of defect, an arithmetic operation on a length, offset, counter, or size value produces a result outside the representable range of the integer type used. The wrapped value is subsequently used to allocate a buffer, compute a copy length, index memory, or bound a loop. The typical consequence is an undersized allocation paired with an oversized write, or an out-of-bounds read/write — memory-safety conditions that a skilled attacker can steer toward controlled code execution.

A meaningful technical detail in this case is the assignment of attack complexity "High" in the CVSS vector. An AC:H rating in memory-corruption scenarios usually signals that successful exploitation depends on conditions outside the attacker's direct control: heap layout grooming, allocator state, or timing. In other words, the overflow is reachable, but converting the corrupted state into reliable code execution is non-trivial and may require repeated attempts or environment-specific tuning. This lowers the likelihood of mass exploitation while keeping the severity high for targeted operations.

The attack surface is the Outlook client and its parsing of network-delivered content. Because the vector is AV:N and PR:N, the malicious input arrives remotely and the attacker needs no credentials. The UI:R component means the attack is not self-executing end-to-end: the user must take an action — typically opening or previewing the crafted content — for the vulnerable code path to be reached. Full impacts to C, I, and A with scope unchanged (S:U) indicate that compromise stays within the Outlook process and the security context of the logged-on user rather than crossing a privilege boundary by itself.

The NVD description does not identify the specific Outlook component, parser, file format, protocol handler, function, or API involved. It also does not enumerate affected builds, the trigger format, or the exact integer field that overflows. Those specifics should not be fabricated; they must be confirmed against the vendor advisory and the corresponding patch diff once available. What can be stated with confidence is the defect class, the delivery vector (network), the access requirement (none), the interaction requirement (user), and the resulting impact (code execution as the Outlook user).

Exploitation Scenarios

No public proof-of-concept, nuclei template, or exploit code is known for this CVE as of the data snapshot. The attack chain below is therefore inferred strictly from the vulnerability class and the CVSS characterization, without inventing function names, endpoints, file paths, or parameters.

1. Reconnaissance / target identification. The attacker identifies organizations using Microsoft Office Outlook. Because no version range is published in the available data, targeted reconnaissance focuses on confirming Outlook deployment and patch level through banner identification, mail-header analysis, tenant discovery, or social profiling of the victim organization. Outlook is a high-value, universally deployed collaboration client, so the population of candidate targets is broad.

2. Triggering the flaw class. The attacker crafts network-delivered content — a message body, attachment, or other Outlook-processed object — that carries an integer field the client parses and then uses in a size or offset computation. When the value is chosen so the arithmetic operation wraps (CWE-190), the resulting length or index is inconsistent with the true data size. The user must open or otherwise render the content (UI:R) for the vulnerable code path to execute. Because AC:H applies, the attacker may need to shape surrounding content — padding, field ordering, or repeated attempts — so the wrapped value lands on a memory layout that produces a useful corruption rather than a crash.

3. Payload / execution method at the class level. The wrapped integer leads to an out-of-bounds memory write or read. At the vulnerability-class level, the objective is to corrupt adjacent heap or stack structures to obtain control of an indirect call target or otherwise redirect execution into attacker-supplied data, ultimately achieving code execution in the context of the Outlook process. No specific shellcode, ROP chain, or gadget is asserted here, because the underlying implementation detail is not disclosed in the provided intelligence.

4. Post-exploitation impact. Successful exploitation yields code execution as the victim user. With scope unchanged, the attacker does not automatically obtain elevated privileges, but code running as the user can read mail stores, harvest credentials and tokens, enumerate the local host, reach internal network resources available to that user, and serve as an initial foothold for further lateral movement. Given the client-side nature of the bug, the principal lateral-movement vector is the user's own mailbox and contact graph — for example, using the compromised account to send further weaponized content internally (consistent with T1203, Exploitation for Client Execution).

Impact Assessment

The CVSS 3.1 base score is 7.5 (High). Subscore analysis: AV:N confirms remote, network-borne delivery; AC:H reflects exploitation conditions that are difficult to control reliably; PR:N means no authentication or prior access is required; UI:R means a user must interact with the crafted content; S:U means the impact is confined to the vulnerable Outlook context; and C:H/I:H/A:H indicate total loss of confidentiality, integrity, and availability of that component.

Business impact hinges on the deployment ubiquity of Outlook. A single successful exploitation grants the attacker a code-execution foothold on an endpoint inside the corporate network — a high-value position for credential theft, mail exfiltration, business email compromise, and pivoting into shared files and internal services. Because one crafted message can be sent to many recipients, the blast radius scales with the recipient list, even though the AC:H rating reduces the probability that any individual attempt succeeds. The UI:R requirement also means security-awareness posture and mail-handling hygiene directly modulate organizational risk.

There is currently no evidence of exploitation in the wild: CISA KEV status is NO, EPSS has not yet been scored by FIRST.org, and no public PoC exists. Consequently, real-world risk today is dominated by the residual risk of an unpatched client rather than by active adversary activity. That balance can shift quickly if an exploit becomes public or if the vulnerability is folded into commodity exploitation frameworks.

Detection Guidance

Sigma Rule

title: Potential Outlook Integer Overflow Exploitation (CVE-2026-100208)
id: 8f1c2b6e-4a7d-4c91-9f3a-0d5e6f7a8b9c
status: experimental
description: Detects suspicious child process creation from Outlook, consistent with code execution following a client-side memory corruption vulnerability.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-100208
author: Threat Intelligence
date: 2026-01-01
logsource:
  category: process_creation
  product: windows
detection:
  selection_parent:
    ParentImage|endswith:
      - '\OUTLOOK.EXE'
  selection_susp_child:
    Image|endswith:
      - '\cmd.exe'
      - '\powershell.exe'
      - '\pwsh.exe'
      - '\wscript.exe'
      - '\cscript.exe'
      - '\mshta.exe'
      - '\rundll32.exe'
      - '\regsvr32.exe'
  condition: selection_parent and selection_susp_child
falsepositives:
  - Legitimate add-ins or administrative scripts spawned by Outlook
  - Documented enterprise automation invoking Outlook via COM
level: high
tags:
  - attack.t1203
  - attack.execution
  - cve.2026.100208

YARA Rule

rule CVE_2026_100208_Outlook_Integer_Overflow_Generic
{
    meta:
        description = "Generic heuristic detection for memory-corruption exploitation attempts targeting Outlook integer overflow (CVE-2026-100208). No public PoC exists; rule targets generic exploit-marker behavior."
        author = "Threat Intelligence"
        date = "2026-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-100208"
        cve = "CVE-2026-100208"
        severity = "high"

    strings:
        $mz = { 4D 5A }
        $pe = { 50 45 00 00 }

        // Generic shellcode/marker heuristics - high false positive potential
        $sc1 = { 90 90 90 90 90 90 90 90 }
        $sc2 = { FC 48 83 E4 F0 }           // common x64 prologue
        $sc3 = { 6A 40 68 }                 // push pattern

        $mark1 = "CVE-2026-100208" ascii nocase
        $mark2 = "OutlookOverflow" ascii nocase

    condition:
        uint16(0) == 0x5A4D and
        (
            any of ($sc*) or
            all of ($mark*)
        )
}

Suricata IDS Rule

alert smtp any any -> any any (msg:"CVE-2026-100208 Possible Outlook Integer Overflow Exploitation Attempt"; flow:established,to_client; file_data; content:"OutlookOverflow"; nocase; sid:2026000101; rev:1; metadata:cve CVE-2026-100208, attack_target Client_Endpoint, deployment Perimeter, signature_severity Major; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-100208; classtype:attempted-user;)

Elastic Detection

sequence by host.id, user.name with maxspan=30s
  [ process where event.type == "start" and process.parent.name : "outlook.exe" ] by process.entity_id
  [ process where event.type == "start" and process.name : ("cmd.exe", "powershell.exe", "pwsh.exe", "wscript.exe", "cscript.exe", "mshta.exe", "rundll32.exe", "regsvr32.exe") ] by process.parent.entity_id

Splunk SPL Query

index=windows source="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational" EventCode=1
| eval parent=lower(ParentImage), child=lower(Image)
| where like(parent, "%\\outlook.exe")
| where match(child, "(cmd|powershell|pwsh|wscript|cscript|mshta|rundll32|regsvr32)\.exe$")
| stats count min(_time) as firstTime max(_time) as lastTime values(child) as children by host, User, parent
| convert ctime(firstTime) ctime(lastTime)
| sort - count

Wazuh OSSEC Rule

<group name="windows,sysmon,outlook,">
  <rule id="100208" level="12">
    <if_sid>61603</if_sid>
    <field name="win.eventdata.parentImage">\\OUTLOOK\.EXE$</field>
    <field name="win.eventdata.image">\\(cmd|powershell|pwsh|wscript|cscript|mshta|rundll32|regsvr32)\.exe$</field>
    <description>Possible CVE-2026-100208 exploitation: $(win.eventdata.image) spawned by Outlook</description>
    <mitre>
      <id>T1203</id>
    </mitre>
    <group>cve_2026_100208,attack_execution,</group>
  </rule>
</group>

Note on detection quality: because no PoC, no exploit code, and no technical specifics (affected file format, trigger field, or endpoint) are published for CVE-2026-100208, the rules above detect the generic post-exploitation behavior of client-side code execution from Outlook rather than the vulnerable parsing path itself. They will generate false positives from legitimate Outlook add-ins and automation. Tune exclusions against authorized add-in inventories before alerting.

Mitigation & Patch Guidance

Patch confidence is 2/4 — CLAIMED. A single source (NVD) indicates a patch exists, but this has not been independently confirmed, and no vendor advisory, KB article, or fixed version number is enumerated in the available data. Treat remediation as provisional until Microsoft's own advisory is located.

1. Immediate mitigations (while patch status is unverified):

   • Restrict the attack surface where operationally feasible. Enforce Mark-of-the-Web and Outlook attachment handling policies; block or quarantine risky attachment types and disable automatic preview of untrusted content where policy permits.
   • Harden the client: enforce Protected View for content from the internet, and consider aggressive attachment and ActiveX/scripting restrictions through Group Policy or the Trust Center.
   • Apply email gateway and endpoint filtering for anomalous or malformed message structures, and enable exploit-protection features (attack surface reduction, DEP, CFG, heap integrity) on endpoints running Outlook.
   • Since exploitation requires user interaction (UI:R), reinforce user reporting of suspicious mail and reduce the population of users who open unsolicited content.
   • Increase monitoring for Outlook spawning child processes (see Detection Guidance), and isolate suspicious hosts quickly.

2. Patch / upgrade steps:

   • Obtain the official Microsoft security update for the affected Outlook build through Microsoft Update, WSUS, or Configuration Manager. Because the fixed version is not enumerated in the available data, verify the patch level against Microsoft's published advisory rather than assuming a version.
   • Prioritize endpoints where Outlook version or channel is unclear, then confirm update deployment and reboot where required. Inventory to ensure no endpoint remains on an unpatched build.
   • Validate the patch on a representative ring before broad rollout, and re-verify that detection rules still fire on known-benign activity after patching.

3. Long-term hardening:

   • Maintain a reliable asset inventory of Office/Outlook versions and update channels, and enforce automatic patching to reduce exposure windows for client-side memory-safety flaws.
   • Treat the client as untrusted: segment endpoints, limit lateral reachability, and apply least privilege so that code execution as a user does not translate easily into domain-wide compromise.
   • Track EPSS and CISA KEV for this CVE; if it is added to KEV or a PoC becomes public, escalate to emergency patching and tighten detections accordingly.
   • Build add-in allow-listing and code-integrity controls to suppress the noisy false positives the detection rules above will otherwise generate.

References

• NVD — CVE-2026-100208: https://nvd.nist.gov/vuln/detail/CVE-2026-100208
• MITRE CWE-190 — Integer Overflow or Wraparound: https://cwe.mitre.org/data/definitions/190.html
• FIRST EPSS (not yet scored for this CVE): https://www.first.org/epss/
• CISA Known Exploited Vulnerabilities Catalog (CVE not listed; status NO): https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• Microsoft Security Update Guide (authoritative source for the vendor fix and affected builds; no advisory enumerated in the supplied data): https://msrc.microsoft.com/update-guide/

No known threat actor campaigns or in-the-wild exploitation evidence is associated with CVE-2026-100208 at the time of this report. CISA KEV status is NO, EPSS is unscored, and no public PoC, exploit code, or nuclei template exists.