CVE-2023-20109
CVE-2023-20109 is a security flaw in Cisco IOS and IOS XE, the operating system on Cisco’s routers and switches. Attackers can run their own code on Cisco IOS and IOS XE. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 10 Oct 2023.[1]
Cisco IOS and IOS XE is business software or equipment. Most families won’t have it, but attackers break into it to get inside companies, and from there to what those companies hold about their customers, patients and staff. Cisco has published a fix or advice.
Description
CISA calls it a group encrypted transport VPN out-of-bounds write in Cisco IOS and IOS XE. In its words: “Cisco IOS and IOS XE contain an out-of-bounds write vulnerability in the Group Encrypted Transport VPN (GET VPN) feature that could allow an authenticated, remote attacker who has administrative control of either a group member or a key server to execute malicious code or cause a device to crash.”[1]
The CVE record adds: “A vulnerability in the Cisco Group Encrypted Transport VPN (GET VPN) feature of Cisco IOS Software and Cisco IOS XE Software could allow an authenticated, remote attacker who has administrative control of either a group member or a key server to execute arbitrary code on an affected device or cause the device to crash. This vulnerability is due to insufficient validation of attributes in the Group Domain of Interpretation (GDOI) and G-IKEv2 protocols of the GET VPN feature. An attacker could exploit this vulnerability by either compromising an installed key server or modifying the configuration of a group member to point to a key server that is controlled by the attacker. A successful exploit could allow the attacker to execute arbitrary code and gain full control of the affected system or cause the affected system to reload, resulting in a denial of service (DoS)…”[2]
The underlying weakness is out-of-bounds write (CWE-787), in MITRE’s list of common software weaknesses.
It scores 6.6 out of 10 on the Common Vulnerability Scoring System, which rates it medium, scored by NVD using CVSS 3.1.[2]
Affected versions
NVD’s record doesn’t list version ranges for it; Cisco’s advisory has them[3]. CISA names the product as Cisco IOS and IOS XE.
Most families won’t have Cisco IOS and IOS XE; it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 10 Oct 2023, 13 days after it was made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 31 Oct 2023 to fix it.[1]
FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at 3%. That’s in the top 16% of the flaws it scores.[4]
CISA assesses it as not automatable: each attack takes some work.
CISA doesn’t say who was attacked, or how.
Fix
Cisco has published a fix or advice.[3] CISA’s required action: “Apply mitigations per vendor instructions or discontinue use of the product if mitigations are unavailable.”[1]
- Check whether you use it.Cisco IOS and IOS XE is business software or equipment, so most families won’t have it. If you run a business, or work from home on company equipment, ask whoever looks after your IT.
- Send this page to your IT person.Ask them to confirm it’s fixed, or not in use. CISA gave US government agencies until 31 Oct 2023 to fix it.
- If it can’t be fixed, take it offline.CISA’s advice when there’s no fix is to stop using the product, or take it off the internet, until there is one.
You can’t fix someone else’s server, but you can watch for what follows a break-in like this: a breach notice, a password reset you didn’t ask for, or a scam message that knows things it shouldn’t. HARD watches for those for you and your family.
Timeline
| Date | Event |
|---|---|
| The flaw is made public in the National Vulnerability Database, with a severity score of 6.6.[2] | |
| CISA adds it to its catalog of flaws known to be used in attacks.[1] | |
| CISA’s deadline for US government agencies to fix it.[1] | |
| FIRST puts the odds of it being used in attacks in the next 30 days at 3%.[4] |
See also
Other Cisco IOS and IOS XE flaws
- CVE-2025-20352: software SNMP denial of service and remote code execution ·
- CVE-2017-3881: remote code execution ·
- CVE-2018-0171: software smart install remote code execution ·
Other Cisco flaws
- CVE-2026-76418, Cisco Secure Firewall ASA Software and Cisco Secure FTD Software: unchecked return value to NULL pointer dereference ·
- CVE-2026-76519, Cisco Secure Email Gateway: relative path traversal ·
- CVE-2026-76504, Cisco Catalyst SD-WAN Manager: hex encoding ·
- CVE-2026-76460, Cisco Identity Services Engine: incorrect use of privileged APIs ·
- CVE-2026-76461, Cisco Secure Email Gateway: SQL injection ·
Other out-of-bounds write flaws
- CVE-2026-86950, Apple devices: out-of-bounds write ·
- CVE-2026-53266, Linux Kernel: out-of-bounds write ·
- CVE-2026-87491, Chrome and other Chromium browsers: out of bounds write ·
- CVE-2026-53362, Linux Kernel: unspecified ·
- CVE-2022-0995, Linux Kernel: out-of-bounds write ·
Added the same day
- CVE-2023-44487, IETF HTTP/2: rapid reset attack ·
- CVE-2023-41763, Microsoft Skype for Business: privilege escalation ·
- CVE-2023-36563, Microsoft WordPad: information disclosure ·
- CVE-2023-21608, Adobe Acrobat and Reader: use-after-free ·
Every Cisco flaw on CISA’s list · Everything added in October 2023
References
- ^ Cybersecurity and Infrastructure Security Agency. “Cisco IOS and IOS XE Group Encrypted Transport VPN Out-of-Bounds Write Vulnerability”. Known Exploited Vulnerabilities Catalog, added 10 Oct 2023.
- ^ National Institute of Standards and Technology. “CVE-2023-20109 Detail”. National Vulnerability Database, published 27 Sep 2023.
- ^ Cisco. Advisories and fixes: sec.cloudapps.cisco.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.025, percentile 84.7, 31 Oct 2026.
- CVE Program. “CVE-2023-20109”. The official CVE record.
This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.