envoy: Improper Null Termination
CVE-2025-66220 is a high-severity improper null termination vulnerability in envoy from Envoyproxy, with a CVSS 3.1 base score of 7.1 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 envoy versions: >= 1.36.0, <= 1.36.2; >= 1.35.0, <= 1.35.6; >= 1.34.0, <= 1.34.10, 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 0.18% 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 envoy you run against the affected versions below, and install the vendor's latest release.
- Read the advisory on github.com for workarounds and exact fixed builds.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Envoyproxy | envoy | >= 1.36.0, <= 1.36.2; >= 1.35.0, <= 1.35.6; >= 1.34.0, <= 1.34.10; <= 1.33.12 | 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
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- HighAll data handled by the component can be read.
- Integrity
- LowLimited modification of data is possible.
- Availability
- NoneAvailability is not affected.
Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N. Scored by NVD.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 3.1 | 5.0 | Medium | CNA (github.com) |
| CVSS 3.1 | 7.1 | High | NVD |
Official description
Envoy is a high-performance edge/middle/service proxy. In 1.33.12, 1.34.10, 1.35.6, 1.36.2, and earlier, Envoy’s mTLS certificate matcher for match_typed_subject_alt_names may incorrectly treat certificates containing an embedded null byte (\0) inside an OTHERNAME SAN value as valid matches.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Improper Null Termination
The product does not terminate or incorrectly terminates a string or array with a null character or equivalent terminator.
How developers prevent this class of flaw
- Use a language that is not susceptible to these issues. However, be careful of null byte interaction errors (CWE-626) with lower-level constructs that may be written in a language that is susceptible.
- Ensure that all string functions used are understood fully as to how they append null characters. Also, be wary of off-by-one errors when appending nulls to the end of strings.
- If performance constraints permit, special code can be added that validates null-termination of string buffers, this is a rather naive and error-prone solution.
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: poc, automatable: no, technical impact: partial).
- Record last changed materially (score, affected products, status or references).
References
Advisories
Other vulnerabilities in envoy
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 7.4 High severity
Recent Improper Null Termination vulnerabilities
- 7.5 High severity
- 7.8 High severity
- 7.5 High severity
- 7.5 High severity
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