VulnerabilityPublished 6 Aug 2026Updated 8 Nov 2026

CVE-2026-5856: Contiki-NG · Out-of-bounds Read

Affects Contiki-NGSource CVE Database V5

Compare

Not known to be exploited · CVSS 7.1 High.

Data as of 8 Nov 2026, 14:35 UTC

CVSS 3.1 base score

7.1High

How severe the flaw is on the 0–10 CVSS scale.

Thin evidence

One scorer or none

CVSS scores by source
CNA (VulnCheck)	CVSS 3.1	7.1
CNA (VulnCheck)	CVSS 4.0	7.1

Exploitation: No evidence of active exploitation and no public proof of concept
Automatable: Attackers cannot reliably automate steps 1–4 of the kill chain
Technical impact: Limited control over the software or limited information exposure

CISA SSVC assessment, 2026-08-07.

CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:L

Attack Vector: Adjacent
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: High
Integrity: None
Availability: Low

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Adjacent: The vulnerable component is bound to the network stack, but the attack is limited at the protocol level to a logically adjacent topology. This can mean an attack must be launched from the same shared physical (e.g., Bluetooth or IEEE 802.11) or logical (e.g., local IP subnet) network, or from within a secure or otherwise limited administrative domain (e.g., MPLS, secure VPN to an administrative network zone).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: High: There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: Low: Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the impacted component are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 6 Aug 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 8 Nov 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Affected software

Product	Affected	Fixed in
Contiki-NGContiki-NG	Affected =0	Fixed in–

AI-suggested

Until a fix is available, network administrators should consider restricting or monitoring multicast UDP traffic on port 5353 and DNS UDP responses from untrusted sources to limit exposure. Additionally, disabling RESOLV_CONF_SUPPORTS_MDNS if not required may reduce risk. Follow vendor advisories for updates and apply patches promptly once released.

Description

CVE-2026-5856 is an out-of-bounds read vulnerability in Contiki-NG's DNS/mDNS resolver component. The flaw occurs in the skip_name() function, which processes DNS wire-format name labels without proper packet-boundary checks. An attacker can exploit this by sending specially crafted DNS or multicast DNS packets that cause the resolver to read beyond the UDP packet buffer, potentially disclosing memory contents or causing a crash. This vulnerability affects builds with RESOLV_CONF_SUPPORTS_MDNS enabled and standard DNS builds under certain conditions.

Impact

This vulnerability allows an unauthenticated attacker to cause out-of-bounds reads of memory adjacent to the UDP packet buffer in Contiki-NG's DNS resolver. This can lead to disclosure of sensitive memory contents or denial of service via resolver crashes. On mDNS-enabled builds, exploitation can be triggered by any local network peer without requiring an outstanding query. On standard DNS builds, exploitation requires the ability to inject spoofed UDP responses during an active DNS query. The CVSS 3.1 base score is 7.1, indicating high severity with high confidentiality impact, no integrity impact, and low availability impact.

Summary

The vulnerability in Contiki-NG arises from the skip_name() function in os/services/resolv/resolv.c, which walks DNS name labels without verifying packet boundaries. The caller function newdata() invokes skip_name() in a loop based on the nquestions field from an attacker-controlled DNS header before validating the transaction ID. If nquestions is set higher than the actual number of questions, skip_name() reads past the UDP packet buffer. The returned pointer is then cast to a dns_answer structure, leading to out-of-bounds memory reads. On builds with RESOLV_CONF_SUPPORTS_MDNS enabled, any local peer can trigger this via multicast UDP packets on port 5353 without authentication or outstanding queries. On standard DNS builds, an attacker capable of injecting UDP responses from port 53 during an active query can trigger the same issue. The impact includes disclosure of memory contents or crashing the resolver due to invalid memory access.

CWE-125Out-of-bounds Read (opens in a new tab)
Base weaknessCWE Top 25 (2025): #8
The product reads data past the end, or before the beginning, of the intended buffer.
Implementation · Input Validation
Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
Architecture and Design · Language Selection
Use a language that provides appropriate memory abstractions.
OWASP Cheat Sheets
XML Security (opens in a new tab)
Mass Assignment (opens in a new tab)
OWASP ASVS 4.0.3
V13.3.1 (opens in a new tab)
V5.1.4 (opens in a new tab)
V5.1.3 (opens in a new tab)
Attack patterns
Overread Buffers (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

Community reviews

Loading reviews…

Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 8 Nov 2026, 14:35 UTC

WeaknessesCWE-125

Technical details

References

Community reviews

Related threats