CVE-2026-1584: 
GnuTLS vulnerability analysis and mitigation

CVE-2026-1584 is a NULL pointer dereference vulnerability in GnuTLS that allows a remote, unauthenticated attacker to crash a TLS server by sending a specially crafted ClientHello message with an invalid Pre-Shared Key (PSK) binder value during the TLS handshake, resulting in a Denial of Service (DoS) condition. The vulnerability affects GNU GnuTLS and Red Hat Hardened Images. It was first reported on February 9, 2026, and publicly disclosed via NVD on April 9, 2026. The fixed version is GnuTLS 3.8.12, addressed in Red Hat errata RHSA-2026:7477. It carries a CVSS v3.1 base score of 7.5 (High) (Red Hat CVE, Red Hat Bugzilla, Github Advisory).

The vulnerability is classified as CWE-476 (NULL Pointer Dereference) and resides in the PSK binder verification logic within pre_shared_key.c in GnuTLS. When a server issues a NewSessionTicket and a client subsequently sends a ClientHello referencing the ticket identity, _gnutls_get_cred() returns NULL for the PSK credentials. However, the conditional guard if (pskcred == NULL && (session->internals.flags & GNUTLS_NO_TICKETS)) does not return early in this scenario, so the NULL pskcred pointer is passed to server_recv_params(). Inside that function, if the received PSK binder size matches the PRF MAC length but the binder value is incorrect, the code dereferences pskcred->binder_algo, causing a NULL pointer dereference and server crash. The attack requires no authentication, no user interaction, and is network-accessible with low complexity (Red Hat Bugzilla, Github Advisory).

Successful exploitation causes the GnuTLS-based TLS server process to crash, resulting in a complete loss of availability for any service relying on GnuTLS for TLS termination. There is no impact on confidentiality or data integrity — the vulnerability is purely a DoS condition. Any application or service using GnuTLS for TLS 1.3 with session ticket support is potentially affected, including web servers, VPN endpoints, and other network services (Red Hat CVE, Red Hat Bugzilla).

No public exploit code has been released; the original researcher noted a minimal Python-based PoC exists but was withheld from public disclosure at the time of reporting. There is no evidence of in-the-wild exploitation or threat actor attribution as of the latest available data. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. The EPSS score is approximately 0.081% (roughly the 29th percentile), indicating a low near-term exploitation probability (Red Hat Bugzilla, Github Advisory).

1. Reconnaissance: Identify internet-facing servers using GnuTLS for TLS 1.3 (e.g., via banner grabbing with tools like nmap or sslyze) that have session ticket support enabled.
2. Establish initial TLS connection: Connect to the target server and complete a TLS 1.3 handshake to receive a NewSessionTicket from the server.
3. Craft malicious ClientHello: Construct a new ClientHello message that includes a pre_shared_key extension referencing the ticket identity obtained in step 2, but with an invalid PSK binder value whose size matches the expected PRF MAC length (e.g., 32 bytes for SHA-256).
4. Send crafted message: Transmit the malformed ClientHello to the server during a new TLS handshake attempt.
5. Trigger crash: The server's PSK binder verification code dereferences the NULL pskcred pointer when checking pskcred->binder_algo, causing a NULL pointer dereference and crashing the server process, achieving a DoS condition (Red Hat Bugzilla).

• Network: Repeated TLS ClientHello messages from the same source IP targeting a GnuTLS-based service, particularly those initiating new handshakes with pre_shared_key extensions referencing previously issued session ticket identities.
• Logs: Sudden server process crashes or restarts correlated with incoming TLS connection attempts; application logs showing GnuTLS assertion failures or segmentation faults around PSK binder verification.
• Process: Unexpected termination of the GnuTLS-based server process (e.g., core dumps, SIGSEGV signals) without corresponding application-level errors; repeated service restarts in a short time window.

Red Hat has released a patched version of GnuTLS (3.8.12) via errata RHSA-2026:7477 for Red Hat Hardened Images, including packages such as gnutls-3.8.12-1.1.hum1 for aarch64 and x86_64. Fedora updates for Fedora 42 and 43 have also been issued. Administrators should upgrade GnuTLS to version 3.8.12 or later as the primary remediation. As a temporary workaround, disabling TLS session tickets (setting GNUTLS_NO_TICKETS) on the server side prevents the vulnerable code path from being reached, since the NULL pointer dereference only occurs when the server has issued a NewSessionTicket (Red Hat Errata, Red Hat Bugzilla).

Security news outlet SecurityOnline.info covered the GnuTLS 3.8.12 release, highlighting both the TLS 1.3 crash (CVE-2026-1584) and a CPU exhaustion issue as the primary fixes. The vulnerability was included in CISA's weekly vulnerability bulletin (SB26-103) for the week of April 6, 2026. Downstream distributions including Fedora, Slackware, Yocto Project, and Unraid issued updates referencing this CVE, and vendors such as Meinberg and Splunk also published advisories noting the dependency on GnuTLS (SecurityOnline, CISA Bulletin).