NLnet Labs Unbound 1.12.0 up to and including 1.26.0 has a use-after-free vulnerability when compiled for DNS-over-HTTPs support with '--with-libnghttp2'. During failure code paths (i.e., RPZ drop query, jostle due to heavy traffic), a dropped DoH stream brings down the whole DoH session and does not account properly for other DoH streams in the same session. This leads to use-after-free in those code paths. If the prerequisites are satisfied (possible RPZ drop or heavy client traffic), a malicious actor can trigger the vulnerability with a single DoH connection and the appropriate traffic. Impact is limited as the reads are not user controlled and the use-after-free leads to early returns. However, a hardened allocator can catch the use-after-free and controllably terminate the process resulting to denial of service.

The remote Fedora 43 host has a package installed that is affected by multiple vulnerabilities as referenced in the FEDORA-2026-41f949afc4 advisory.

    # Update to 1.26.1 (rhbz#2535076)

    Security fix list from upstream:

    - Fix CVE-2026-81642, Heap buffer overflow and possible Remote Code Execution when digesting DNSKEY.
    Thanks to Yuqi Qiu and Xiang Li from Nankai University, AOSP Lab for the report.
    - Fix CVE-2026-81634, Possible heap buffer overflow during DNSSEC canonicalization. Thanks to Vlatko     Kosturjak with Marlink Cyber for the report.
    - Fix CVE-2026-82717, CNAME synthesis could lead to heap corruption. Thanks to Ben Morris from Anthropic     for the report.
    - Fix CVE-2026-77955, Possible ZONEMD verification bypass window. Thanks to Yuqi Qiu and Xiang Li from     Nankai University, AOSP Lab, In addition, thanks to Qifan Zhang from Palo Alto Networks for also reporting     this issue.
    - Fix CVE-2026-78227, Use-after-free in DoQ stream output buffer on reset re-transmission. Thanks to Yuqi     Qiu and Xiang Li from Nankai University, AOSP Lab for the report.
    - Fix CVE-2026-80225, Possible degradation of service from continuous queries on the same TCP/DoT     connection. Thanks to Qifan Zhang from Palo Alto Networks for the report.
    - Fix CVE-2026-82720, Use-after-free in DoH stream cleanup code path. Thanks to Yuqi Qiu and Xiang Li from     Nankai University, AOSP Lab for the report.
    - Fix CVE-2026-85501, Retrap: Novel Vulnerabilities to launch Algorithmic Complexity Attacks on DNSSEC.
    Thanks to Zuyao Xu and Xiang Li from Nankai University, AOSP Lab for report. In addition, thanks to Qifan     Zhang from Palo Alto Networks for a complimentary report.
    - Fix CVE-2026-77860, 'serve-expired' can bypass Unbound 'wait-limit'. Thanks to Xuanchao Xie, Lutong     Chen, and Kaiping Xue from the University of Science and Technology of China (USTC) for the report.

Tenable has extracted the preceding description block directly from the Fedora security advisory.

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

There are packages installed that are affected by a vulnerability referenced in the following CVE:

  - NLnet Labs Unbound 1.12.0 up to and including 1.26.0 has a use-after-free vulnerability when compiled for
    DNS-over-HTTPs support with '--with-libnghttp2'. During failure code paths (i.e., RPZ drop query, jostle
    due to heavy traffic), a dropped DoH stream brings down the whole DoH session and does not account
    properly for other DoH streams in the same session. This leads to use-after-free in those code paths. If
    the prerequisites are satisfied (possible RPZ drop or heavy client traffic), a malicious actor can trigger
    the vulnerability with a single DoH connection and the appropriate traffic. Impact is limited as the reads
    are not user controlled and the use-after-free leads to early returns. However, a hardened allocator can
    catch the use-after-free and controllably terminate the process resulting to denial of service.
    (CVE-2026-82720)

The AlmaLinux 9.2 host has packages installed that are affected by multiple vulnerabilities as referenced in the TuxCare ALMALINUX9.2:CLSA-2026:1790962865 advisory.

  - In NLnet Labs Unbound 1.13.2 up to and including 1.26.1, a vulnerability in ZONEMD configured zones     (zonemd-check: yes) which are located below (but not at) a trust anchor allow for an attack window where     (tampered with) zone contents are served (or stored to disk) prior to the ZONEMD integrity check. This is     caused by the needed DS/DNSKEY asynchronous resolution that needs to happen before the ZONEMD check     completes. If a zonefile is written to disk (zonefile: option) while the ZONEMD check failed, the tampered     data are reloaded on startup and available until ZONEMD verification concludes again. If verification     fails, the data is not served any more but still persists on disk for future reloads. (CVE-2026-77955)

  - NLnet Labs Unbound 1.12.0 up to and including 1.26.0 has a use-after-free vulnerability when compiled for     DNS-over-HTTPs support with '--with-libnghttp2'. During failure code paths (i.e., RPZ drop query, jostle     due to heavy traffic), a dropped DoH stream brings down the whole DoH session and does not account     properly for other DoH streams in the same session. This leads to use-after-free in those code paths. If     the prerequisites are satisfied (possible RPZ drop or heavy client traffic), a malicious actor can trigger     the vulnerability with a single DoH connection and the appropriate traffic. Impact is limited as the reads     are not user controlled and the use-after-free leads to early returns. However, a hardened allocator can     catch the use-after-free and controllably terminate the process resulting to denial of service.
    (CVE-2026-82720)

  - Novel vulnerabilities to launch algorithmic complexity attacks on DNSSEC have been researched under the     term 'ReTrap'. These result in degradation of service when malicious zones are used to serve the     algorithmic complexity vulnerabilities. NLnet Labs Unbound up to and including 1.26.0 is vulnerable to     some of them. TagTrap, where the triple(Zone, Algo, KeyTag) matching mechanism introduces a significant     attack vector when resolvers handle malicious responses containing numerous mismatched DNSKEY, RRSIG, and     DS record. DelegationTrap, where constructing the chain-of-trust requires iterative validation of DNSKEY     and DS records from the root zone downward. For deeply nested domains, this results in significant     computational overhead. NsecTrap, where responses with excessive invalid NSEC records compel the resolver     to validate each one. AdditionalTrap, where Unbound by default would try to DNSSEC validate the ADDITIONAL     section as well. This can be exploited to waste validation resources by malicious users. (CVE-2026-85501)

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

The remote CentOS Linux 9 host has packages installed that are affected by multiple vulnerabilities as referenced in the unbound-1.26.1-1.el9 build changelog.

  - In NLnet Labs Unbound up to and including 1.26.0, a vulnerability was found in that can progressively     corrupt heap memory and under certain systems and compilation options could lead to remote code execution.
    The vulnerability starts when CNAME synthesis during an upstream response needs to enforce(rewrite) a max     TTL value in the packet buffer. Coupled with a compression pointer that points to the overwritten value     and invalidates the domain name, it leads to an error path that does not properly move the buffer position     and allows for the heap buffer overflow. Since this is heavily reliant on heap memory layout, results are     memory corruption that eventually leads to a crash and under specific systems and compilation options     remote code execution. (CVE-2026-82717)

  - In NLnetLabs Unbound 1.20.0 up to and including 1.26.0, a vulnerability on the 'serve-expired' code path     can cause a double decrement on the 'wait-limit' counter per client IP essentially bypassing one of the     counter measures that was introduced for DNSBomb (CVE-2024-33655). A malicious actor can exploit this by     controlling an authoritative zone with short TTL, so cached entries expire quickly. Each 'slow' query, one     the attacker's authoritative never answers, is followed by one query for an expired cached name, which is     answered immediately via the 'serve-expired' path and decrements the counter twice. This second query was     named 'pump'. By alternating slow queries and pumps, the attacker keeps the per-client counter at or below     the configured 'wait-limit' indefinitely, and can hold an arbitrary number of pending queries from a     single source IP, up to the global mesh quota (num-queries-per-thread); eventually bypassing one of the     counter measures introduced for DNSBomb (CVE-2024-33655). This vulnerability is present on the 'serve-     expired' code path. (CVE-2026-77860)

  - In NLnet Labs Unbound 1.13.2 up to and including 1.26.1, a vulnerability in ZONEMD configured zones     (zonemd-check: yes) which are located below (but not at) a trust anchor allow for an attack window where     (tampered with) zone contents are served (or stored to disk) prior to the ZONEMD integrity check. This is     caused by the needed DS/DNSKEY asynchronous resolution that needs to happen before the ZONEMD check     completes. If a zonefile is written to disk (zonefile: option) while the ZONEMD check failed, the tampered     data are reloaded on startup and available until ZONEMD verification concludes again. If verification     fails, the data is not served any more but still persists on disk for future reloads. (CVE-2026-77955)

  - NLnet Labs Unbound 1.22.0 up to and including 1.26.1, has a use-after-free vulnerability when compiled for     DNS-over-QUIC support with '--with-libngtcp2'. Each DoQ stream owns an output buffer that holds the DNS     response. ngtcp2's retransmission buffer keeps a shallow pointer into the output buffer for as long as a     STREAM frame may be resent. On a client RESET_STREAM, the output buffer is freed but ngtcp2 still holds     the matching retransmission entries. The next PTO timeout makes ngtcp2 re-encode the STREAM frame and copy     from the freed buffer. A malicious actor that can query Unbound over DoQ and that withholds ACKs, sends     RESET_STREAM, and waits for PTO, reaches this use-after-free with no privilege. This leads to     retransmissions against freed memory and eventually an abnormal server exit under a 20-query spray.
    (CVE-2026-78227)

  - In NLnetLabs Unbound up to and including 1.26.0, a degradation of service vulnerability is present in the     TCP/DoT reading procedure where there is no limit on consecutive reads. A malicious actor that can stream     and sustain a rate of distinct uncached names over the TCP/DoT connection, monopolizes a single worker's     entire event loop for as long as its writes stay ahead of the drain. (CVE-2026-80225)

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

It is, therefore, affected by multiple vulnerabilities as referenced in the ALAS2023-2026-3127 advisory.

    In NLnet Labs Unbound 1.13.2 up to and including 1.26.1, a vulnerability in ZONEMD configured zones     (zonemd-check: yes) which are located below (but not at) a trust anchor allow for an attack window where     (tampered with) zone contents are served (or stored to disk) prior to the ZONEMD integrity check. This is     caused by the needed DS/DNSKEY asynchronous resolution that needs to happen before the ZONEMD check     completes. If a zonefile is written to disk (zonefile: option) while the ZONEMD check failed, the tampered     data are reloaded on startup and available until ZONEMD verification concludes again. If verification     fails, the data is not served any more but still persists on disk for future reloads. (CVE-2026-77955)

    In NLnetLabs Unbound up to and including 1.26.0, a degradation of service vulnerability is present in the     TCP/DoT reading procedure where there is no limit on consecutive reads. A malicious actor that can stream     and sustain a rate of distinct uncached names over the TCP/DoT connection, monopolizes a single worker's     entire event loop for as long as its writes stay ahead of the drain. (CVE-2026-80225)

    In NLnet Labs Unbound up to and including 1.26.0, a 255 length query name with a large TCP response can     lead to a heap buffer overflow during the RRSet canonicalisation routine. This is caused by missing to add     the first owner name into the buffer length check. A malicious actor operating a malicious name server or     tampering with an incoming response to Unbound (canonicalisation happens before DNSSEC validation), can     trigger the vulnerability. (CVE-2026-81634)

    In NLnet Labs Unbound up to and including 1.26.0, a vulnerability was found in that can progressively     corrupt heap memory and under certain systems and compilation options could lead to remote code execution.
    The vulnerability starts when CNAME synthesis during an upstream response needs to enforce(rewrite) a max     TTL value in the packet buffer. Coupled with a compression pointer that points to the overwritten value     and invalidates the domain name, it leads to an error path that does not properly move the buffer position     and allows for the heap buffer overflow. Since this is heavily reliant on heap memory layout, results are     memory corruption that eventually leads to a crash and under specific systems and compilation options     remote code execution. (CVE-2026-82717)

    NLnet Labs Unbound 1.12.0 up to and including 1.26.0 has a use-after-free vulnerability when compiled for     DNS-over-HTTPs support with '--with-libnghttp2'. During failure code paths (i.e., RPZ drop query, jostle     due to heavy traffic), a dropped DoH stream brings down the whole DoH session and does not account     properly for other DoH streams in the same session. This leads to use-after-free in those code paths. If     the prerequisites are satisfied (possible RPZ drop or heavy client traffic), a malicious actor can trigger     the vulnerability with a single DoH connection and the appropriate traffic. Impact is limited as the reads     are not user controlled and the use-after-free leads to early returns. However, a hardened allocator can     catch the use-after-free and controllably terminate the process resulting to denial of service.
    (CVE-2026-82720)

    Novel vulnerabilities to launch algorithmic complexity attacks on DNSSEC have been researched under the     term 'ReTrap'. These result in degradation of service when malicious zones are used to serve the     algorithmic complexity vulnerabilities. NLnet Labs Unbound up to and including 1.26.0 is vulnerable to     some of them. TagTrap, where  the triple(Zone, Algo, KeyTag) matching mechanism introduces a significant     attack vector when resolvers handle malicious responses containing numerous mismatched DNSKEY, RRSIG, and     DS record. DelegationTrap, where constructing the chain-of-trust requires iterative validation of DNSKEY     and DS records from the root zone downward. For deeply nested domains, this results in significant     computational overhead. NsecTrap, where  responses with excessive invalid NSEC records compel the resolver     to validate each one. AdditionalTrap, where Unbound by default would try to DNSSEC validate the ADDITIONAL     section as well. This can be exploited to waste validation resources by malicious users. (CVE-2026-85501)

Tenable has extracted the preceding description block directly from the tested product security advisory.

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

The remote Fedora 44 host has a package installed that is affected by multiple vulnerabilities as referenced in the FEDORA-2026-996b326401 advisory.

    # Update to 1.26.1 (rhbz#2535076)

    Security fix list from upstream:

    - Fix CVE-2026-81642, Heap buffer overflow and possible Remote Code Execution when digesting DNSKEY.
    Thanks to Yuqi Qiu and Xiang Li from Nankai University, AOSP Lab for the report.
    - Fix CVE-2026-81634, Possible heap buffer overflow during DNSSEC canonicalization. Thanks to Vlatko     Kosturjak with Marlink Cyber for the report.
    - Fix CVE-2026-82717, CNAME synthesis could lead to heap corruption. Thanks to Ben Morris from Anthropic     for the report.
    - Fix CVE-2026-77955, Possible ZONEMD verification bypass window. Thanks to Yuqi Qiu and Xiang Li from     Nankai University, AOSP Lab, In addition, thanks to Qifan Zhang from Palo Alto Networks for also reporting     this issue.
    - Fix CVE-2026-78227, Use-after-free in DoQ stream output buffer on reset re-transmission. Thanks to Yuqi     Qiu and Xiang Li from Nankai University, AOSP Lab for the report.
    - Fix CVE-2026-80225, Possible degradation of service from continuous queries on the same TCP/DoT     connection. Thanks to Qifan Zhang from Palo Alto Networks for the report.
    - Fix CVE-2026-82720, Use-after-free in DoH stream cleanup code path. Thanks to Yuqi Qiu and Xiang Li from     Nankai University, AOSP Lab for the report.
    - Fix CVE-2026-85501, Retrap: Novel Vulnerabilities to launch Algorithmic Complexity Attacks on DNSSEC.
    Thanks to Zuyao Xu and Xiang Li from Nankai University, AOSP Lab for report. In addition, thanks to Qifan     Zhang from Palo Alto Networks for a complimentary report.
    - Fix CVE-2026-77860, 'serve-expired' can bypass Unbound 'wait-limit'. Thanks to Xuanchao Xie, Lutong     Chen, and Kaiping Xue from the University of Science and Technology of China (USTC) for the report.

Tenable has extracted the preceding description block directly from the Fedora security advisory.

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

The remote Debian 13 host has packages installed that are affected by multiple vulnerabilities as referenced in the dsa-6507 advisory.

    -------------------------------------------------------------------------     Debian Security Advisory DSA-6507-1                   security@debian.org     https://www.debian.org/security/                       Moritz Muehlenhoff     September 19, 2026                    https://www.debian.org/security/faq
    - -------------------------------------------------------------------------

    Package        : unbound     CVE ID         : CVE-2026-14586 CVE-2026-32665 CVE-2026-40622 CVE-2026-40691                      CVE-2026-41637 CVE-2026-42955 CVE-2026-44621 CVE-2026-44687                      CVE-2026-44690 CVE-2026-46582 CVE-2026-50045 CVE-2026-50046                      CVE-2026-50243 CVE-2026-50248 CVE-2026-50251 CVE-2026-50252                      CVE-2026-52863 CVE-2026-54478 CVE-2026-55708 CVE-2026-55717                      CVE-2026-55973 CVE-2026-55990 CVE-2026-55991 CVE-2026-56416                      CVE-2026-56444 CVE-2026-77860 CVE-2026-77955 CVE-2026-78227                      CVE-2026-80225 CVE-2026-81634 CVE-2026-81642 CVE-2026-82717                      CVE-2026-82720 CVE-2026-85501

    Multiple security vulnerabilities were discovered in Unbound, a validating,     recursive, caching DNS resolver, which could result in denial of service,     cache poisoning, DNS cookie bypass or potentially the execution of     arbitrary code.

    For the stable distribution (trixie), these problems have been fixed in     version 1.26.1-0+deb13u1.

    We recommend that you upgrade your unbound packages.

    For the detailed security status of unbound please refer to     its security tracker page at:
    https://security-tracker.debian.org/tracker/unbound

    Further information about Debian Security Advisories, how to apply     these updates to your system and frequently asked questions can be     found at: https://www.debian.org/security/

    Mailing list: debian-security-announce@lists.debian.org

Tenable has extracted the preceding description block directly from the Debian security advisory.

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

The remote Fedora 45 host has a package installed that is affected by multiple vulnerabilities as referenced in the FEDORA-2026-3baacede89 advisory.

    # Update to 1.26.1 (rhbz#2535076)

    Security fix list from upstream:

    - Fix CVE-2026-81642, Heap buffer overflow and possible Remote Code Execution when digesting DNSKEY.
    Thanks to Yuqi Qiu and Xiang Li from Nankai University, AOSP Lab for the report.
    - Fix CVE-2026-81634, Possible heap buffer overflow during DNSSEC canonicalization. Thanks to Vlatko     Kosturjak with Marlink Cyber for the report.
    - Fix CVE-2026-82717, CNAME synthesis could lead to heap corruption. Thanks to Ben Morris from Anthropic     for the report.
    - Fix CVE-2026-77955, Possible ZONEMD verification bypass window. Thanks to Yuqi Qiu and Xiang Li from     Nankai University, AOSP Lab, In addition, thanks to Qifan Zhang from Palo Alto Networks for also reporting     this issue.
    - Fix CVE-2026-78227, Use-after-free in DoQ stream output buffer on reset re-transmission. Thanks to Yuqi     Qiu and Xiang Li from Nankai University, AOSP Lab for the report.
    - Fix CVE-2026-80225, Possible degradation of service from continuous queries on the same TCP/DoT     connection. Thanks to Qifan Zhang from Palo Alto Networks for the report.
    - Fix CVE-2026-82720, Use-after-free in DoH stream cleanup code path. Thanks to Yuqi Qiu and Xiang Li from     Nankai University, AOSP Lab for the report.
    - Fix CVE-2026-85501, Retrap: Novel Vulnerabilities to launch Algorithmic Complexity Attacks on DNSSEC.
    Thanks to Zuyao Xu and Xiang Li from Nankai University, AOSP Lab for report. In addition, thanks to Qifan     Zhang from Palo Alto Networks for a complimentary report.
    - Fix CVE-2026-77860, 'serve-expired' can bypass Unbound 'wait-limit'. Thanks to Xuanchao Xie, Lutong     Chen, and Kaiping Xue from the University of Science and Technology of China (USTC) for the report.

Tenable has extracted the preceding description block directly from the Fedora security advisory.

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

The version of FreeBSD installed on the remote host is prior to tested version. It is, therefore, affected by multiple vulnerabilities as referenced in the 3f4d285d-b1f6-11f1-8f5e-3c7c3fba4204 advisory.

    NLnet Labs reports:
    This release consolidates security fixes for issues reported over               a period of time. There are fixes for:

Tenable has extracted the preceding description block directly from the FreeBSD security advisory.

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

The Linux/Unix host has one or more packages installed that are impacted by a vulnerability without a vendor supplied patch available.

  - NLnet Labs Unbound 1.12.0 up to and including 1.26.0 has a use-after-free vulnerability when compiled for     DNS-over-HTTPs support with '--with-libnghttp2'. During failure code paths (i.e., RPZ drop query, jostle     due to heavy traffic), a dropped DoH stream brings down the whole DoH session and does not account     properly for other DoH streams in the same session. This leads to use-after-free in those code paths. If     the prerequisites are satisfied (possible RPZ drop or heavy client traffic), a malicious actor can trigger     the vulnerability with a single DoH connection and the appropriate traffic. Impact is limited as the reads     are not user controlled and the use-after-free leads to early returns. However, a hardened allocator can     catch the use-after-free and controllably terminate the process resulting to denial of service.
    (CVE-2026-82720)

Note that Nessus relies on the presence of the package as reported by the vendor.

ID	Name	Product	Family	Type	Severity
362832	Fedora 43 : unbound (2026-41f949afc4)	Nessus	Fedora Local Security Checks	Local	critical
458310	Echo: unbound: security update to 1.26.1	Tenable Cloud Security	Echo Local Security Checks	Local	medium
458310	Echo: unbound: security update to 1.26.1	Tenable Self-Hosted Container Security	Echo Local Security Checks	Local	medium
362706	AlmaLinux 9.2 [TuxCare] Security Update: python3-unbound / unbound / unbound-devel / unbound-libs Multiple Vulnerabilities (ALMALINUX9.2:CLSA-2026:1790962865)	Nessus	Alma Linux Local Security Checks	Local	high
351124	CentOS 9 : unbound-1.26.1-1.el9	Nessus	CentOS Local Security Checks	Local	critical
351052	Amazon Linux 2023 : python3-unbound, unbound, unbound-anchor (ALAS2023-2026-3127)	Nessus	Amazon Linux Local Security Checks	Local	critical
349620	Fedora 44 : unbound (2026-996b326401)	Nessus	Fedora Local Security Checks	Local	critical
348572	Debian dsa-6507 : libunbound-dev - security update	Nessus	Debian Local Security Checks	Local	critical
348363	Fedora 45 : unbound (2026-3baacede89)	Nessus	Fedora Local Security Checks	Local	critical
346906	FreeBSD : Unbound -- multiple vulnerabilities (3f4d285d-b1f6-11f1-8f5e-3c7c3fba4204)	Nessus	FreeBSD Local Security Checks	Local	critical
346325	Linux Distros Unpatched Vulnerability : CVE-2026-82720	Nessus	Misc.	Local	medium

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