CVE-2026-42009 Overview
A flaw exists in GnuTLS affecting the Datagram Transport Layer Security (DTLS) packet reordering logic. The comparator function responsible for ordering DTLS packets by sequence number does not correctly handle packets with duplicate sequence numbers. A remote attacker can exploit this defect to trigger unstable packet ordering or undefined behavior, leading to a denial of service against any application linking GnuTLS for DTLS communication. The weakness is classified under [CWE-475] (Undefined Behavior for Input to API).
Critical Impact
A network-based attacker without authentication can cause a denial of service in DTLS endpoints by sending crafted packets with duplicate sequence numbers.
Affected Products
- GnuTLS (versions implementing the affected DTLS packet reordering comparator)
- Red Hat distributions shipping the affected GnuTLS package
- Applications and services using GnuTLS for DTLS communication
Discovery Timeline
- 2026-05-18 - CVE-2026-42009 published to the National Vulnerability Database (NVD)
- 2026-05-18 - Last updated in the NVD database
Technical Details for CVE-2026-42009
Vulnerability Analysis
The vulnerability resides in the DTLS packet reordering subsystem of GnuTLS. DTLS preserves the security guarantees of TLS over unreliable datagram transports such as UDP. Because datagrams may arrive out of order, GnuTLS sorts buffered records by their DTLS sequence number using a comparator function.
The comparator does not define a stable ordering when two records carry identical sequence numbers. Comparator functions that return inconsistent results for equal inputs violate the contract required by standard sort routines, which depend on a strict weak ordering. The result is undefined behavior in the buffering and reordering paths.
An unauthenticated remote attacker who can deliver DTLS packets to a vulnerable peer can trigger this state by sending records with duplicated sequence numbers. The downstream effect is process instability or termination, producing a denial of service. The flaw affects availability only, with no impact on confidentiality or integrity.
Root Cause
The root cause is improper handling of equality in the DTLS sequence-number comparator. When the comparator receives two records with the same sequence number, it does not return a consistent ordering, leaving the sort result undefined. This maps to [CWE-475], where input to an API produces undefined behavior because the contract for the comparator is not satisfied.
Attack Vector
Exploitation requires only network reachability to a DTLS endpoint. No authentication, user interaction, or prior session state is needed beyond what a normal DTLS handshake or association allows. The attacker transmits DTLS records crafted with duplicate sequence numbers to drive the reordering buffer into the undefined code path.
No public proof-of-concept or in-the-wild exploitation has been reported. See the Red Hat CVE-2026-42009 Advisory and Red Hat Bug Report #2467279 for vendor technical details.
Detection Methods for CVE-2026-42009
Indicators of Compromise
- Unexpected crashes, aborts, or restarts of services that terminate DTLS associations using GnuTLS
- DTLS peers logging anomalous reordering, buffer state, or sequence-number errors
- Inbound UDP traffic to DTLS ports containing repeated records with identical sequence numbers
Detection Strategies
- Inventory hosts running GnuTLS and identify services exposing DTLS endpoints such as VPN concentrators, SIP, WebRTC, and CoAP gateways
- Monitor process supervision logs for repeated termination of DTLS-handling services correlated with external UDP traffic
- Inspect DTLS traffic with network analyzers for sequences of records sharing identical DTLS sequence numbers
Monitoring Recommendations
- Alert on abnormal crash rates of services linked against libgnutls
- Track packet capture statistics for duplicate DTLS sequence numbers from a single source
- Correlate UDP source addresses associated with crashes for blocklisting at the network edge
How to Mitigate CVE-2026-42009
Immediate Actions Required
- Apply vendor-supplied GnuTLS updates as soon as patched packages become available from your Linux distribution
- Restrict network exposure of DTLS services to trusted networks and authenticated peers where feasible
- Enable automatic restart of critical DTLS services through process supervision to limit the duration of service outages
Patch Information
At the time of publication, refer to the Red Hat CVE-2026-42009 Advisory and Red Hat Bug Report #2467279 for current package status and fix availability. Apply distribution updates for the gnutls package once released, then restart all services that link against the library so the patched code is loaded.
Workarounds
- Limit DTLS endpoint exposure by firewalling UDP listeners to known client subnets
- Where DTLS is not required, disable DTLS support in the application and use TLS over a reliable transport instead
- Place rate limiting and anomaly detection on UDP traffic destined for DTLS services to reduce abuse potential
# Configuration example: identify GnuTLS version and restart dependent services after patching
rpm -q gnutls
sudo dnf update gnutls
sudo systemctl restart <dtls-service-name>
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