dbcve.org
ScreenshareApplication · On24

CVE-2021-34823

CRITICAL · 9.1 CVSS v3.1 Published 2021-08-13
Fix available
A fix is available. Upgrade to 2.0 or later.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
The ON24 ScreenShare (aka DesktopScreenShare.app) plugin before 2.0 for macOS allows remote file access via its built-in HTTP server. This allows unauthenticated remote users to retrieve files accessible to the logged-on macOS user. When a remote user sends a crafted HTTP request to the server, it triggers a code path that will download a configuration file from a specified remote machine over HTTP. There is an XXE flaw in processing of this configuration file that allows reading local (to macOS) files and uploading them to remote machines.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

The ON24 ScreenShare plugin for macOS contains a pre-authentication XXE vulnerability in its built-in HTTP server. Remote unauthenticated attackers can send crafted HTTP requests to trigger download of malicious XML configuration files from remote servers, exploiting XXE to read local files accessible to the logged-on user and exfiltrate them to attacker-controlled machines.

MitigationUpgrade to ON24 ScreenShare plugin version 2.0 or later. Until patched, disable the plugin, block external access to its HTTP server port, and restrict the application's network permissions.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
ScreenshareApplication
Affected:< 2.0

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check if ON24 ScreenShare is installed
    Look for ON24 ScreenShare in /Applications folder, or search using Spotlight (Cmd+Space) or 'ls /Applications' in Terminal
    Affected if ON24 ScreenShare appears in Applications or system searches
  2. Identify the installed version
    Right-click ON24 ScreenShare in Applications > Get Info, or right-click the app bundle and look in Contents/Info.plist for CFBundleVersion, or right-click > Show Package Contents > Contents/Info.plist
    Affected if Version displayed is below 2.0 (e.g., 1.x or any version number less than 2.0)
  3. Determine if the built-in HTTP server is active
    Check for listening ports by running 'lsof -i -P' or 'netstat -an | grep LISTEN' in Terminal; look for ports associated with ON24 (commonly 7842, 8080, or similar)
    Affected if A port associated with ON24 ScreenShare shows as LISTENING
  4. Assess network exposure of the HTTP server
    Run 'curl http://localhost:<port>' where <port> is the port found in the previous step, or use 'sudo lsof -i :<port>' to confirm binding to 0.0.0.0 vs 127.0.0.1
    Affected if The HTTP server responds to localhost requests AND binds to all interfaces (0.0.0.0) rather than localhost only

You are affected if ON24 ScreenShare is installed with a version lower than 2.0 AND its HTTP server is accessible, allowing remote attackers to reach it.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 2.0 or later
Fixed in 2.0
Interim mitigation

Upgrade to ON24 ScreenShare plugin version 2.0 or later. Until patched, disable the plugin, block external access to its HTTP server port, and restrict the application's network permissions.

Recommended fix High confidence

2.0

  1. Upgrade ON24 ScreenShare (DesktopScreenShare.app) to version 2.0 or later
  2. Verify the upgrade was successful by checking the installed version

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Screenshare Scoped from the published advisory
  • Consultation3.0 h
  • Implementation4.0 h
  • Testing2.0 h
  • Review / QA1.0 h
10.0 hours of engineering $1,800
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,880.

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-34823 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data