CVE-2026-77166

Executive Summary

CVE-2026-77166 is an input validation vulnerability (CWE-840: Business Logic Errors) affecting the "emoji field" in a page emoji update endpoint. A high-privileged, authenticated attacker can inject long text and line breaks into the emoji field, which is not properly validated, causing the sidebar layout to break and potentially hiding other sidebar items. Per the CVSS vector, exploitation requires high privileges (PR:H) and user interaction (UI:R), resulting in low integrity impact only (I:L) with no confidentiality or availability impact, yielding a base score of 2.4. No public proof-of-concept, exploit code, or Nuclei template exists, and no patch has been released.

Technical Analysis

This vulnerability resides in the page emoji update endpoint, where an "emoji field" accepts user-supplied input without proper validation. Per the NVD description, the flaw allows an attacker to inject "long text and line breaks" into that field. The described effect is a client-side presentation-layer failure: the sidebar layout becomes broken and can hide other items.

The weakness maps to CWE-840 (Business Logic Errors), indicating that the failure is not a classic memory-safety or injection flaw (e.g., SQLi or XSS as such), but rather an overall failure to enforce expected input constraints specific to the "emoji" concept. An emoji field should logically be constrained to a small set of short glyph sequences; the code does not enforce that constraint and does not reject line breaks or oversized input.

Attack surface: the update endpoint is network-reachable (AV:N), but requires an authenticated, privileged actor (PR:H) and some form of user interaction (UI:R) — for example, a victim viewing or being routed to the affected page/sidebar after the malicious emoji value is stored. The scope is unchanged (S:U) and there is no direct confidentiality or availability impact (C:N/A:N). Integrity of data displayed in the application UI is affected (I:L), since the sidebar can be rendered incorrectly or conceal other items.

Because the NVD description is sparse, the following cannot be asserted and are not claimed here: the exact HTTP method, URL path, parameter name, request/response schema, storage location (e.g., database field length limits), or rendering framework. No specific function names, class names, or API paths are provided by the source data, so none are invented.

Exploitation Scenarios

The following chain is inferred at the vulnerability-class level (input validation / business logic failure), consistent with the description. No public PoC exists.

1. Reconnaissance / target identification: The attacker identifies an instance of the affected product reachable over the network. Since no vendor, product, or version range is provided, targeting relies on the operator knowing which asset exposes the affected "page emoji update" functionality. No Shodan fingerprint specific to the product is available, so the dorks above are generic and may produce false positives.

2. Triggering the flaw class: The attacker is an authenticated user with high privileges (PR:H) — i.e., someone permitted to update page configuration, including the emoji field. The attacker submits a crafted value for the emoji field containing excessive text and embedded line-break characters, bypassing the (absent) validation. The exact transport (form-encoded, JSON, multipart) is not specified in the source data and is therefore not asserted.

3. Payload/execution method: At the class level, this is a content-injection into a stored configuration value. The "payload" is simply an overlong, multi-line string rather than executable code. No script execution, command execution, or query injection is described. It is plausible — but not confirmed by the source — that the value is persisted, in which case impact would recur for every viewer of the affected page (stored) rather than being transient (reflected). This stored-vs-reflected distinction is not stated in the NVD description and should be verified on the affected deployment.

4. Post-exploitation impact: The rendered sidebar breaks and can hide other items. The practical effect is UI spoofing/degradation and possible concealment of navigation or status elements from other users viewing the page. There is no described data exfiltration, privilege escalation, or denial of service beyond the degraded rendering. Integrity impact is rated Low (I:L) and availability impact None (A:N) in the CVSS vector.

Impact Assessment

Business impact is limited. The CVSS base score is 2.4, reflecting AV:N/AC:L/PR:H/UI:R/S:U/C:N/I:L/A:N. Key implications:

• Confidentiality: None (C:N). No data exposure is described.
• Integrity: Low (I:L). The visible layout/sidebar can be altered, potentially hiding other items — a UI deception/presentation integrity issue.
• Availability: None (A:N) per the vector; the described "hiding" of items is a rendering effect, not loss of availability of the underlying service.
• Privileges Required: High (PR:H). Only powerful, authenticated actors can reach the endpoint.
• User Interaction: Required (UI:R). A user must view/trigger the affected rendering.

Blast radius: confined to instances of the unidentified product that expose the page emoji update endpoint and render the affected sidebar. Because vendor, product, and affected versions are unknown, asset inventory cannot be scoped from this CVE record alone. No CISA KEV listing and no exploit code exist, so no active exploitation campaign is indicated. EPSS is not yet scored by FIRST.org; no EPSS value is asserted here.

Overall risk is Low, but consequential in environments where sidebar navigation conveys authorization cues or critical status, since hiding items could be used for low-grade social engineering or to obscure warnings.

Detection Guidance

Because the flaw is a logic/input-validation issue at the application layer with no public exploit, detection should focus on anomalous emoji-field submissions (oversized values, embedded line breaks) and on the resulting rendering behavior. Rules below are written at the application/anomaly level and should be tuned to the specific product once the field name and endpoint are identified from the deployment.

Sigma Rule

title: Potential CVE-2026-77166 - Oversized or Multiline Emoji Field Update
id: 6f2a9c1e-8b4d-4e77-9a10-cve202677166
status: experimental
description: Detects updates to a page emoji field containing excessive length or embedded line breaks, consistent with CVE-2026-77166 business-logic input validation weakness.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-77166
author: valtersit.com
date: 2026/01/01
logsource:
  category: application
  product: web_application
detection:
  selection_endpoint:
    http.url|contains:
      - 'emoji'
  selection_field:
    # emoji field key varies by product; adjust to observed schema
    emoji|re: '(?s).{50,}'
  selection_linebreaks:
    emoji|contains:
      - '\n'
      - '\r'
      - '%0a'
      - '%0d'
      - '
'
      - '
'
  condition: selection_endpoint and (selection_field or selection_linebreaks)
falsepositives:
  - Legitimate rich emoji sequences (rarely exceed 50 bytes; tune threshold)
  - Encoded newlines in unrelated fields
level: low

YARA Rule

rule CVE_2026_77166_Emoji_Field_Linebreak_Injection
{
    meta:
        description = "Detects requests or payloads targeting the page emoji update endpoint with embedded line breaks or oversized emoji values (CVE-2026-77166)"
        author = "valtersit.com"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-77166"
        date = "2026-01-01"
        severity = "low"

    strings:
        $ep1 = "emoji" ascii nocase
        $ep2 = "emoji" wide nocase
        $lb1 = "%0a" ascii
        $lb2 = "%0d" ascii
        $lb3 = "
" ascii
        $lb4 = "
" ascii
        $crlf = { 0D 0A }

    condition:
        any of ($ep1, $ep2) and
        (2 of ($lb1, $lb2, $lb3, $lb4) or $crlf)
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-77166 Possible Emoji Field Linebreak Injection"; flow:established,to_server; http.method; content:"POST"; http.uri; content:"emoji"; nocase; http.request_body; content:"%0a"; nocase; http.request_body; content:"%0d"; nocase; classtype:attempted-recon; sid:202677166; rev:1; metadata:reference nvd,cve,CVE-2026-77166;)

Elastic Detection

sequence by user.name, source.ip with maxspan=30s
  [ network where network.protocol == "http" and
      http.request.method == "POST" and
      wildcard(http.request.body, "*emoji*") and
      (wildcard(http.request.body, "*%0a*") or wildcard(http.request.body, "*%0d*")) ]

Splunk SPL Query

index=web sourcetype=access_combined OR sourcetype=proxy
| rex field=uri "emoji"
| search uri="*emoji*" AND (uri="*%0a*" OR uri="*%0d*" OR request_body="*%0a*" OR request_body="*%0d*")
| stats count min(_time) as first_seen max(_time) as last_seen by src_ip, user, uri, status
| sort - count

Wazuh OSSEC Rule

<group name="web,appsec,cve-2026-77166,">
  <rule id="202677"><!-- local rule id -->
    <decoded_as>json</decoded_as>
    <field name="http.request.body">emoji</field>
    <field name="http.request.body">%0a|%0d</field>
    <description>CVE-2026-77166: Emoji field linebreak injection attempt detected</description>
    <mitre>
      <id>T1190</id>
    </mitre>
    <group>app_input_validation,pci_dss_6.5,</group>
    <level>4</level>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). No vendor advisory, no OSV package data, and no affected-version range are available. Do not assume a fixed version exists.

1. Immediate mitigations:

• Apply server-side input validation on the emoji field: enforce a strict maximum length and reject control characters, including CR/LF, before the value is stored or rendered. Client-side validation alone is insufficient.
• Encode/escape all stored configuration values (including the emoji field) at render time so embedded line breaks and long strings cannot alter layout structure.
• Constrain layout rendering using CSS that is resilient to long unbroken strings (e.g., word-break/overflow controls) to reduce the "hide other items" effect.
• Restrict who can update page emoji configuration; this is already PR:H in the vector, but confirm that only intended privileged roles reach the endpoint.
• Because the product is unidentified in this record, verify whether the affected endpoint exists in your inventory, and log/alert on anomalous emoji-field submissions using the detection rules above.

2. Patch/upgrade steps:

• Monitor the NVD record and the (currently unknown) vendor advisory for a fixed version. No patch reference exists at this time; there is nothing to upgrade to based on the provided evidence.
• When a vendor fix is published, inventory affected instances, test the fix in a staging environment, and deploy promptly.

3. Long-term hardening:

• Adopt allowlist-based validation for any field with an inherently bounded format (like an emoji field) rather than denylist filtering.
• Add automated regression tests that submit oversized and multiline inputs to configuration endpoints and assert rejection.
• Include output encoding and responsive-layout robustness in secure coding standards and code review checklists (CWE-840 class).
• Track EPSS and KEV for this CVE; currently not KEV-listed and not EPSS-scored, so re-evaluate if that changes.

References

• NVD — CVE-2026-77166: https://nvd.nist.gov/vuln/detail/CVE-2026-77166
• CWE-840: Business Logic Errors — https://cwe.mitre.org/data/definitions/840.html
• MITRE ATT&CK T1190 — Exploit Public-Facing Application: https://attack.mitre.org/techniques/T1190/
• FIRST EPSS (currently unscored for this CVE): https://www.first.org/epss/
• CISA Known Exploited Vulnerabilities Catalog (CVE-2026-77166 NOT listed as of this report): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No known threat actor campaigns, exploit-in-the-wild evidence, public PoC, exploit code, or Nuclei template are associated with CVE-2026-77166 based on the available data.