CVE-2026-77166: Page Emoji Update UI Injection Vulnerability

CVE-2026-77166 Overview

CVE-2026-77166 is a low-severity input validation vulnerability affecting the page emoji update endpoint of an unspecified web application. The emoji field fails to properly validate user-supplied input. Attackers with high privileges can inject long text strings and line break characters into this field. The malformed content breaks the sidebar layout and can visually hide other interface elements. The issue is categorized under [CWE-840] (Business Logic Errors) and requires user interaction to trigger the layout disruption for other users.

Critical Impact
The flaw enables limited integrity impact by corrupting the sidebar rendering, which can obscure navigation elements and interfere with the intended user experience.

Affected Products

Specific vendor and product not disclosed in the published advisory
Web application exposing a page emoji update endpoint
Reported through HackerOne report #3599470

Discovery Timeline

2026-09-21 - CVE-2026-77166 published to the National Vulnerability Database (NVD)
2026-09-22 - Last updated in the NVD database

Technical Details for CVE-2026-77166

Vulnerability Analysis

The vulnerability resides in the server-side handler responsible for updating the emoji associated with a page. The endpoint accepts arbitrary content in the emoji field without enforcing length, character set, or format constraints. An authenticated user with elevated privileges can submit long strings or embedded newline characters. When the persisted value is later rendered in the sidebar, the layout breaks and adjacent navigation items may be pushed out of view.

The issue is a business logic and input validation weakness rather than a memory safety or code execution flaw. It does not permit confidentiality loss or availability disruption based on the published metrics. However, hiding sidebar items can obstruct navigation and mask legitimate content from other users of the workspace.

Root Cause

The root cause is missing server-side validation on the emoji field. The application assumes the field will contain a short Unicode emoji character. It does not enforce a maximum length, reject line breaks, or validate that the input matches an expected emoji code point range. The rendered sidebar component trusts the stored value and applies no client-side truncation or sanitization.

Attack Vector

An attacker must authenticate with high privileges to reach the emoji update endpoint. The attacker sends a crafted request to the page emoji update endpoint with an oversized string or embedded newlines in the emoji parameter. When another user loads the affected page or workspace, the rendered sidebar layout breaks. Exploitation requires user interaction because the layout effect only manifests when a victim renders the affected view.

No verified exploit code is publicly available. Technical details are documented in the HackerOne Report #3599470.

Detection Methods for CVE-2026-77166

Indicators of Compromise

Requests to the page emoji update endpoint containing payloads that exceed the length of a normal Unicode emoji code point
Emoji field values containing newline characters (\n, \r, or %0A, %0D in URL-encoded form)
User reports of sidebar navigation items disappearing or being visually cut off

Detection Strategies

Inspect application logs for PATCH or POST requests to the page emoji update endpoint with abnormally large request bodies
Query the persistence layer for stored emoji values that exceed a reasonable byte length or contain control characters
Correlate privileged account activity with subsequent user reports of UI rendering anomalies

Monitoring Recommendations

Enable request-size and payload-content logging on the emoji update endpoint
Alert on any emoji field write that contains line breaks or exceeds the maximum expected Unicode grapheme length
Review audit trails for high-privilege accounts modifying page metadata outside of normal working patterns

How to Mitigate CVE-2026-77166

Immediate Actions Required

Apply input validation on the server side to restrict the emoji field to a single valid Unicode emoji grapheme
Reject or strip line break characters and other whitespace control characters before persisting the value
Audit existing stored emoji values and normalize any records containing oversized or malformed content

Patch Information

No vendor patch identifier is listed in the published CVE record. Consult the HackerOne Report #3599470 for remediation details published by the reporting party. Organizations operating the affected application should track vendor advisories for a formal fix.

Workarounds

Restrict access to the page emoji update endpoint to trusted administrative accounts until validation is in place
Apply a web application firewall (WAF) rule that rejects requests to the emoji update endpoint containing newline characters or payloads over a defined byte threshold
Enforce client-side rendering truncation on the sidebar component to limit visual impact from malformed stored values

bash

# Example WAF rule concept: reject emoji update requests with newlines or oversized payloads
# ModSecurity-style rule
SecRule REQUEST_URI "@endsWith /page/emoji" \
  "id:1077166,phase:2,deny,status:400,\
   chain,msg:'CVE-2026-77166 emoji field validation'"
  SecRule ARGS:emoji "@rx (\r|\n|.{33,})" "t:none"