CVE-2026-56123: socat Heap Buffer Overflow
CVE-2026-56123 is a critical heap buffer overflow vulnerability in socat, published in the National Vulnerability Database (NVD) on June 25, 2026. It carries a CVSS v3.1 base score of 9.8 (Critical). According to its CVSS vector, it can be exploited over the network, needs no privileges and no user interaction.
What is CVE-2026-56123?
A heap buffer overflow in socat allows a malicious SOCKS5 server to overwrite memory.
The NVD description reads:
socat versions 1.8.0.0 through 1.8.1.1 contain a heap-based buffer overflow vulnerability that allows a malicious SOCKS5 proxy server to overwrite adjacent heap memory by exploiting a sign-extension flaw in the DOMAINNAME reply parser. During connection setup, the domain name length byte is read through a signed char field causing a negative bytes_to_read value that is implicitly converted to size_t, resulting in an unbounded heap write into the 262-byte reply buffer with attacker-controlled size and content.
NVD maps this issue to CWE-122 (Heap-based Buffer Overflow).
Affected versions and packages
NVD's CPE configuration lists these vulnerable version ranges:
- socat from 1.8.0.0 before 1.8.1.2
Based on those ranges, the first versions that are no longer listed as vulnerable include: 1.8.1.2.
Echo's advisory data tracks 1 package for CVE-2026-56123: socat.
Echo-built fixed versions recorded for this CVE:
- socat: 1.8.1.3-2+e1
Other package versions listed in Echo's advisory data (compare them with the versions you run):
- socat: 1.8.0.3-1
Severity and CVSS score
A score of 9.8 puts CVE-2026-56123 in the Critical range (9.0 to 10.0), the highest CVSS band. The vector shows the attack path and the impact on confidentiality, integrity and availability. For help weighing exploit likelihood against raw severity, see our guide to EPSS vs CVSS prioritization.
Impact: Heap Buffer Overflow
The CVSS vector rates the impact on confidentiality as high, integrity as high and availability as high. Exploitation requires no privileges and no user interaction, and the attacker reaches the target over the network.
What this means in practice depends on how socat is used in your environment: how widely it is deployed, whether the affected code path is reachable, and what the process can access. Treat the exposed systems that run a listed version as in scope until you have confirmed otherwise.
Is CVE-2026-56123 being exploited in the wild?
Known exploitation: CISA KEV
No. As of November 4, 2026, CVE-2026-56123 is not listed in CISA's Known Exploited Vulnerabilities (KEV) catalog (catalog version 2026.11.03). KEV only lists vulnerabilities with confirmed exploitation, so absence from the catalog is not proof that nobody is exploiting the flaw.
EPSS score and percentile
The Exploit Prediction Scoring System (EPSS) from FIRST estimates the probability that a CVE will see exploitation activity in the next 30 days. On November 3, 2026, CVE-2026-56123 had an EPSS score of 0.003, which corresponds to a probability of 0.31%. It sits at the 24th percentile, which means it is scored as more likely to be exploited than roughly 24% of scored CVEs, placing it below the median of scored CVEs. EPSS is recalculated daily and is a likelihood estimate, not a confirmation, so read it together with KEV status, exposure and CVSS severity.
Attack vector, complexity, privileges and user interaction
Based on the CVSS v3.1 vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H (source: NVD), this is how an attacker has to approach CVE-2026-56123:
- Attack vector: Network (AV:N): the flaw can be reached remotely over the network, so the attacker does not need to be on the same host or LAN.
- Attack complexity: Low (AC:L): no special conditions are needed, so the attack can be repeated reliably.
- Privileges required: None (PR:N): no account or credentials are needed.
- User interaction: None (UI:N): no action by a user is required.
- Scope: Unchanged (S:U): the impact stays within the security scope of the vulnerable component.
Is there a public proof of concept or exploit?
CISA's SSVC assessment recorded in NVD (dated June 26, 2026) lists the exploitation status as "none", which means there was no evidence of active exploitation and no public proof of concept at that time. None of the references in the NVD record is tagged as an exploit. This reflects NVD and CISA records as of November 4, 2026, and public exploit code can appear at any time.
Practical exposure conditions
- Only systems that run an affected version are exposed. Compare the socat versions in your hosts and container images against the vulnerable ranges listed above.
- The vector is network-based, so socat deployments that process input from untrusted networks carry the most risk, especially internet-facing ones. Internal-only deployments are still exposed to anyone who can reach them.
- No credentials are needed, so exposure does not depend on how well accounts are protected.
- No user has to click or open anything, so the flaw can be triggered without any user action.
- CISA's SSVC assessment marks the issue as not automatable, which means attacks are harder to run at scale.
How to fix CVE-2026-56123
- Inventory. Find every host, container image and base image that includes socat, including the packages listed above (socat).
- Upgrade. Upgrade to 1.8.1.2 or later where NVD lists a fixed release, or to the Echo-built fixed package listed above.
- Rebuild and restart. Rebuild container images and restart the services that load socat so the patched code is actually used.
- Verify. Re-scan your images and hosts to confirm that no package still matches a vulnerable version of CVE-2026-56123.
Interim mitigations if you cannot patch immediately
These are general hardening steps, not a vendor-specific workaround. Updating is the only complete fix.
- Restrict network access to services that use socat to trusted sources with firewall rules or allow-lists until you can patch.
- Monitor logs and runtime behavior on affected systems for unusual activity, and prioritize them in your next patch window.
How Echo helps with CVE-2026-56123
Echo is a registered CVE Numbering Authority (CNA) and ships patched, vetted open source packages and container images, so teams can remove vulnerable components instead of tracking an endless backlog. For this CVE, Echo's advisory data tracks 1 package (socat) and includes Echo-built fixed versions. Read how automated patch SLAs reduce enterprise risk in how automated patch SLAs reduce enterprise risk and why how to deal with CVE backlogs matters.
Frequently asked questions about CVE-2026-56123
What is CVE-2026-56123?
CVE-2026-56123 is a critical heap buffer overflow vulnerability in socat. A heap buffer overflow in socat allows a malicious SOCKS5 server to overwrite memory. It has a CVSS base score of 9.8.
Which versions are affected by CVE-2026-56123?
NVD lists these vulnerable ranges: socat from 1.8.0.0 before 1.8.1.2.
Is CVE-2026-56123 actively exploited?
It is not listed in CISA's KEV catalog as of November 4, 2026 (catalog version 2026.11.03), but that does not rule out exploitation. FIRST's EPSS model puts the probability of exploitation activity in the next 30 days at 0.31%, at the 24th percentile (November 3, 2026). On public exploit code: CISA's SSVC assessment in NVD (dated June 26, 2026) lists no public proof of concept.
How do I fix CVE-2026-56123?
Upgrade to 1.8.1.2 or later where NVD lists a fixed release, or to the Echo-built fixed package listed above. Then rebuild images, restart affected services and re-scan to confirm the fix.
How severe is CVE-2026-56123?
It is rated Critical with a CVSS base score of 9.8. The vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H means it can be exploited over the network, with no privileges and no user interaction.
What type of weakness is CVE-2026-56123?
NVD classifies it as CWE-122 (Heap-based Buffer Overflow).