IOS: Improper Handling of Syntactically Invalid Structure
CVE-2026-20125 is a high-severity improper handling of syntactically invalid structure vulnerability in IOS from Cisco, with a CVSS 3.1 base score of 7.7 out of 10. It can be exploited remotely over the network by an attacker with a low-privileged account, without any action from a user. Affected IOS versions: 12.2(33)CY; 12.2(33)CY1; 12.2(33)CY2, and others listed below. The record does not name a fixed version, so check the vendor advisory. There is no confirmed exploitation so far. EPSS estimates a 0.3% chance of exploitation within 30 days, which is low compared with other CVEs.
High severity with no evidence of exploitation so far.
What to do
- Check which version of IOS you run against the affected versions below, and install the vendor's latest release.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Cisco | IOS | 12.2(33)CY; 12.2(33)CY1; 12.2(33)CY2; 12.2(55)SE; 12.2(55)SE3; 12.2(55)SE2; 12.2(58)SE; 12.2(55)SE1; and 32 more ranges | Not stated |
| Cisco | Cisco IOS XE Software | 3.5.0E; 3.5.1E; 3.5.2E; 3.5.3E; 3.11.1S; 3.11.2S; 3.11.0S; 3.11.3S; and 32 more ranges | Not stated |
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- LowA basic user account is enough.
- User interaction
- NoneNo action by a victim is required.
- Scope
- ChangedThe attack can reach beyond the vulnerable component into other systems.
- Confidentiality
- NoneNo data is disclosed.
- Integrity
- NoneNothing can be modified.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:H. Scored by CNA (cisco.com).
Official description
A vulnerability in the HTTP Server feature of Cisco IOS Software and Cisco IOS XE Software Release 3E could allow an authenticated, remote attacker to cause an affected device to reload unexpectedly, resulting in a denial of service (DoS) condition. This vulnerability is due to improper validation of user-supplied input. An attacker could exploit this vulnerability by sending malformed HTTP requests to an affected device. A successful exploit could allow the attacker to cause a watchdog timer to expire and the device to reload, resulting in a DoS condition. To exploit this vulnerability, the attacker must have a valid user account.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Improper Handling of Syntactically Invalid Structure
The product does not handle or incorrectly handles input that is not syntactically well-formed with respect to the associated specification.
More Improper Handling of Syntactically Invalid Structure vulnerabilities (CWE-228)
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: no, technical impact: partial).
- Record last changed materially (score, affected products, status or references).
References
Other references
Other vulnerabilities in IOS
- 8.6 High severity
- 8.6 High severity
- 9.0 Critical severity
- 7.7 High severity
- 7.7 High severity
- 8.1 High severity
Recent Improper Handling of Syntactically Invalid Structure vulnerabilities
- 9.8 Critical severity
- 9.8 Critical severity
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
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