ISC — CraftedSignal Threat Feed

CVE-2026-3039: BIND TKEY Authentication Memory Consumption Vulnerability

hello@craftedsignal.io — Wed, 20 May 2026 13:22:34 +0000

ISC BIND 9 is vulnerable to excessive memory consumption (CVE-2026-3039) when processing maliciously crafted packets targeting servers using TKEY-based authentication via GSS-API tokens. This configuration is often found in Active Directory-integrated DNS deployments or Kerberos-secured DNS environments. An attacker can exploit this vulnerability by sending specially crafted packets, causing the BIND server to consume excessive memory resources, potentially leading to denial of service. The affected versions include BIND 9 versions 9.0.0 through 9.16.50, 9.18.0 through 9.18.48, 9.20.0 through 9.20.22, 9.21.0 through 9.21.21, 9.9.3-S1 through 9.16.50-S1, 9.18.11-S1 through 9.18.48-S1, and 9.20.9-S1 through 9.20.22-S1. Defenders should monitor DNS server memory usage and implement rate limiting or packet filtering to mitigate the risk.

Attack Chain

The attacker identifies a BIND server configured to use TKEY-based authentication with GSS-API.
The attacker crafts a malicious DNS packet specifically designed to exploit the memory consumption vulnerability.
The attacker sends the crafted packet to the vulnerable BIND server.
The BIND server receives the packet and attempts to process the TKEY authentication.
Due to the malicious structure of the packet, the server allocates an excessive amount of memory during the authentication process.
The attacker repeats steps 3-5, sending multiple crafted packets to continually exhaust server memory.
The BIND server’s memory consumption increases significantly, impacting performance and stability.
The BIND server eventually crashes due to memory exhaustion, resulting in a denial-of-service condition.

Impact

Successful exploitation of CVE-2026-3039 leads to excessive memory consumption on the affected BIND server, potentially resulting in a denial-of-service condition. This can disrupt DNS resolution services for the affected domain or network, impacting users’ ability to access websites and online services. The vulnerability poses a significant risk to organizations relying on Active Directory-integrated DNS or Kerberos-secured DNS environments, potentially causing widespread service outages.

Recommendation

Upgrade BIND 9 to a patched version beyond 9.16.50, 9.18.48, 9.20.22, or 9.21.21 to remediate CVE-2026-3039.
Implement rate limiting on DNS traffic to mitigate the impact of malicious packets, protecting against memory exhaustion.
Monitor DNS server memory usage for unexpected spikes using system monitoring tools.
Deploy the Sigma rule Detect Excessive DNS Server Memory Allocation to identify potential exploitation attempts.
Review DNS server configurations to minimize the use of TKEY-based authentication where possible.

ISC BIND Use-After-Free Vulnerability Due to Race Condition (CVE-2026-5947)

hello@craftedsignal.io — Wed, 20 May 2026 13:20:01 +0000

A use-after-free vulnerability, tracked as CVE-2026-5947, exists in ISC BIND. Specifically, when BIND receives an incoming DNS message signed with SIG(0), it validates that signature. If the number of “recursive-clients” reaches the configured limit during this validation process due to a query flood, the DNS message may be discarded. However, a small window of time exists where the SIG(0) validation process might still attempt to read the now-discarded DNS message, leading to a use-after-free condition and undefined behavior. This issue affects BIND 9 versions 9.20.0 through 9.20.22, 9.21.0 through 9.21.21, and 9.20.9-S1 through 9.20.22-S1. BIND 9 versions 9.18.28 through 9.18.49 and 9.18.28-S1 through 9.18.49-S1 are not affected.

Attack Chain

An attacker sends a DNS query flood to a BIND server to exhaust the recursive-clients limit.
Simultaneously, the attacker sends a crafted DNS message signed with SIG(0).
The BIND server receives the crafted DNS message and begins SIG(0) signature validation.
While the signature validation is in progress, the recursive-clients limit is reached due to the query flood.
The BIND server discards the DNS message to enforce the recursive-clients limit.
The SIG(0) validation routine attempts to read the discarded DNS message.
A use-after-free vulnerability is triggered because the memory associated with the DNS message has been freed.
This can lead to undefined behavior, potentially causing the BIND server to crash or, in more severe cases, allow for remote code execution.

Impact

Successful exploitation of CVE-2026-5947 can cause a denial-of-service (DoS) condition on the affected BIND server, disrupting DNS resolution services. In a worst-case scenario, it could lead to remote code execution, potentially allowing an attacker to gain control of the server. Given the critical role of DNS servers in network infrastructure, this vulnerability poses a significant risk. While no specific victim counts are available, the widespread use of BIND makes many organizations vulnerable.

Recommendation

Upgrade to a patched version of BIND 9 to address CVE-2026-5947. Versions 9.18.28 through 9.18.49 and 9.18.28-S1 through 9.18.49-S1 are not affected.
Monitor DNS server logs for errors related to SIG(0) validation, which may indicate exploitation attempts. Deploy the Sigma rule Detect SIG(0) validation failure to detect these events.
Rate limit incoming DNS queries to prevent query floods and reduce the likelihood of triggering the recursive-clients limit and the race condition.

CVE-2026-5946: BIND 9 `named` Assertion Failure Vulnerability

hello@craftedsignal.io — Wed, 20 May 2026 13:19:46 +0000

CVE-2026-5946 identifies multiple vulnerabilities within the named component of BIND 9, arising from improper handling of DNS messages employing a CLASS other than Internet (IN), such as CHAOS or HESIOD, or DNS messages with meta-classes (ANY or NONE) in the question section. An attacker can trigger these flaws by sending specially crafted DNS requests to a vulnerable BIND 9 server. The affected code paths include recursion, dynamic updates (UPDATE), zone change notifications (NOTIFY), and processing of IN-specific record types within non-IN data. Successful exploitation can lead to assertion failures in named, potentially causing a denial-of-service condition. The vulnerability impacts BIND 9 versions 9.11.0 through 9.16.50, 9.18.0 through 9.18.48, 9.20.0 through 9.20.22, 9.21.0 through 9.21.21, 9.11.3-S1 through 9.16.50-S1, 9.18.11-S1 through 9.18.48-S1, and 9.20.9-S1 through 9.20.22-S1.

Attack Chain

The attacker identifies a vulnerable BIND 9 server.
The attacker crafts a malicious DNS request. This request leverages a DNS CLASS other than IN, such as CHAOS, or includes meta-classes such as ANY or NONE in the question section.
The attacker sends the crafted DNS request to the target BIND 9 server.
The named process receives and parses the malicious DNS request.
Due to the unexpected CLASS or meta-class, the named process enters a vulnerable code path during recursion, dynamic updates, zone change notifications, or processing of IN-specific record types in non-IN data.
Within the vulnerable code path, the named process attempts an invalid operation based on the malicious request.
This invalid operation triggers an assertion failure within the named process.
The assertion failure may cause the named process to terminate or become unstable, resulting in a denial-of-service.

Impact

Successful exploitation of CVE-2026-5946 leads to assertion failures within the named process, causing potential instability or termination of the service. This results in a denial-of-service condition, disrupting DNS resolution services for affected networks and users. The severity of the impact depends on the role of the affected BIND 9 server; critical infrastructure DNS servers experiencing this issue can cause widespread outages.

Recommendation

Upgrade BIND 9 to a patched version (>= 9.16.51, >= 9.18.49, >= 9.20.23, >= 9.21.22) to remediate CVE-2026-5946.
Deploy the Sigma rule “Detect DNS queries with non-IN class” to identify potentially malicious DNS requests targeting this vulnerability.
Monitor DNS server logs for assertion failures in the named process, which may indicate exploitation attempts related to CVE-2026-5946.
Consider implementing rate limiting and request filtering to mitigate the impact of malicious DNS requests.