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AsteriskApplication · Digium

CVE-2021-26713

MEDIUM · 6.5 CVSS v3.1 Published 2021-02-19
Fix available
A fix is available. Upgrade to 16.16.1 / 17.9.2 or later.
See remediation →
71/100
Remediation priority · Elevated
Remotely reachable 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
A stack-based buffer overflow in res_rtp_asterisk.c in Sangoma Asterisk before 16.16.1, 17.x before 17.9.2, and 18.x before 18.2.1 and Certified Asterisk before 16.8-cert6 allows an authenticated WebRTC client to cause an Asterisk crash by sending multiple hold/unhold requests in quick succession. This is caused by a signedness comparison mismatch.

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

A stack-based buffer overflow in res_rtp_asterisk.c in Sangoma Asterisk allows an authenticated WebRTC client to crash the server by sending multiple hold/unhold requests in rapid succession. The vulnerability stems from a signedness comparison mismatch that leads to incorrect buffer size calculations during RTP session handling.

MitigationUpgrade Asterisk to version 16.16.1, 17.9.2, 18.2.1 or later (or Certified Asterisk 16.8-cert6 or later). If immediate patching is not feasible, restrict WebRTC client access to trusted users only or implement rate limiting on hold/unhold requests.

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
AsteriskApplication
Affected:>= 16.0.0, < 16.16.1>= 17.0.0, < 17.9.2>= 18.0.0, < 18.2.1
Certified AsteriskApplication
Affected:= 16.8

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
Low
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

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

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. Identify Asterisk version
    Run 'asterisk -V' or check /etc/asterisk/asterisk.conf for the version string, or use 'core show version' in the Asterisk CLI
    Affected if The installed version is >= 16.0.0 and < 16.16.1, OR >= 17.0.0 and < 17.9.2, OR >= 18.0.0 and < 18.2.1, OR is Digium Certified Asterisk 16.8 (not 16.8-cert6 or later)
  2. Verify res_rtp_asterisk module is loaded
    Run 'module show like res_rtp_asterisk' in the Asterisk CLI to confirm the RTP module is loaded
    Affected if The module is loaded and the version is in the vulnerable range from step 1
  3. Confirm WebRTC is enabled
    Check /etc/asterisk/pjsip.conf for endpoint configuration with 'webrtc=yes' or 'rtp_symmetric=yes' and 'ice_support=yes' settings that enable WebRTC functionality
    Affected if WebRTC endpoints are configured and the version is vulnerable
  4. Check for exposed WebRTC access
    Review /etc/asterisk/http.conf and firewall rules to determine if WebRTC ports (typically 8088/8089 or configured WS/WSS ports) are externally accessible
    Affected if WebRTC interface is exposed and the version is vulnerable
  5. Inspect RTP session configuration
    Run 'rtp set debug on' in the Asterisk CLI and monitor for rapid hold/unhold requests, or review sip/endpoint configuration for RTP settings in /etc/asterisk/
    Affected if RTP sessions are active and the system is vulnerable

You are affected if you are running a vulnerable Asterisk version (16.0.0-16.16.0, 17.0.0-17.9.1, 18.0.0-18.2.0, or Certified 16.8) with WebRTC and the res_rtp_asterisk module enabled, and the WebRTC interface is accessible to authenticated clients.

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.

From vendor data
Upgrade available Upgrade to 16.16.1 / 17.9.2 / 18.2.1 or later
Fixed in 16.16.117.9.218.2.1
Interim mitigation

Upgrade Asterisk to version 16.16.1, 17.9.2, 18.2.1 or later (or Certified Asterisk 16.8-cert6 or later). If immediate patching is not feasible, restrict WebRTC client access to trusted users only or implement rate limiting on hold/unhold requests.

Fix this in Asterisk Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing4.0 h
  • Review / QA2.0 h
12.0 hours of engineering $2,080
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,328.

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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-2021-26713 in production — separate from our analysis above.

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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