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Debian LinuxOperating system · Debian

CVE-2021-46837

MEDIUM · 6.5 CVSS v3.1 Published 2022-08-30
Fix available
A fix is available. Upgrade to 16.16.2 / 17.9.3 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
res_pjsip_t38 in Sangoma Asterisk 16.x before 16.16.2, 17.x before 17.9.3, and 18.x before 18.2.2, and Certified Asterisk before 16.8-cert7, allows an attacker to trigger a crash by sending an m=image line and zero port in a response to a T.38 re-invite initiated by Asterisk. This is a re-occurrence of the CVE-2019-15297 symptoms but not for exactly the same reason. The crash occurs because there is an append operation relative to the active topology, but this should instead be a replace operation.

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 denial of service vulnerability in Asterisk's PJSIP T.38 module allows an attacker to crash the server by sending a malicious SDP response containing an m=image line with zero port during a T.38 re-invite. The crash occurs because the code incorrectly uses an append operation on the active SDP topology when it should use a replace operation.

MitigationUpgrade Asterisk to version 16.16.2, 17.9.3, 18.2.2, 16.8-cert7 or later to patch the SDP topology handling logic.

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
Debian LinuxOperating system
Affected:= 9.0= 10.0= 11.0
Certified AsteriskApplication
Affected:= 16.8.0
AsteriskApplication
Affected:>= 16.0.0, < 16.16.2>= 17.0.0, < 17.9.3>= 18.0.0, < 18.2.2

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 'asterisk --version' to get the installed version number
    Affected if Version matches Digium Asterisk 16.0.0-16.16.1, 17.0.0-17.9.2, or 18.0.0-18.2.1, OR matches Certified 16.8.0, OR is from Debian 9.0/10.0/11.0 packages within these ranges
  2. Confirm PJSIP channel driver is in use
    Check if chan_pjsip is loaded: run 'module show like chan_pjsip' in Asterisk CLI, or check res_pjsip.so existence
    Affected if PJSIP is the active channel driver handling SIP traffic
  3. Verify T.38 fax support is enabled
    Check for T.38 configuration in pjsip.conf or sip.conf, look for 't38pt_udptl' or 'fax' related settings; run 'pjsip show transport' to confirm endpoint setup
    Affected if T.38 fax passthrough or gateway mode is configured on any endpoint
  4. Check for crash logs related to T.38 or SDP
    Review Asterisk crash logs (typically in /var/log/asterisk or /var/log/messages) for segfaults occurring during T.38 re-invite handling
    Affected if Recent crashes occurred with call flow involving m=image or T.38 re-invite

Environment is affected if running a vulnerable Asterisk version with PJSIP and T.38 fax enabled, as the crash only triggers when processing a malicious SDP with zero port in m=image line during T.38 re-invite.

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 16.16.2 / 17.9.3 / 18.2.2 or later
Fixed in 16.16.217.9.318.2.2
Interim mitigation

Upgrade Asterisk to version 16.16.2, 17.9.3, 18.2.2, 16.8-cert7 or later to patch the SDP topology handling logic.

Recommended fix High confidence

Upgrade to Asterisk 16.16.2, 17.9.3, or 18.2.2 (or Certified Asterisk 16.8-cert7) depending on your current branch

  1. Check current Asterisk version with 'asterisk -V' or 'core show version'
  2. For Asterisk 16.x: upgrade to version 16.16.2
  3. For Asterisk 17.x: upgrade to version 17.9.3
  4. For Asterisk 18.x: upgrade to version 18.2.2
  5. For Certified Asterisk 16.x: upgrade to version 16.8-cert7
  6. After upgrade, verify Asterisk starts successfully and test T.38 fax functionality
Caveat Point releases are typically backward-compatible; test thoroughly in non-production environment before deploying to production

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

Fix this in Debian Linux Scoped from the published advisory
  • Consultation2.0 h
  • Implementation1.0 h
  • Testing4.0 h
  • Review / QA2.0 h
9.0 hours of engineering $1,540
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Scan for this in your stack

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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-2021-46837 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