CVE-2025-66220VulnerabilityPublished 3 Dec 2025

CVE-2025-66220: envoyproxy envoy · Improper Null Termination

Affects envoyproxy envoySource CVE Database V5

Not known to be exploited · CVSS 5.0 Medium.

Data as of 26 Oct 2026, 06:26 UTC

CVSS 3.1 base score

5.0Medium

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

EPSS: 0.1% chance of exploitation in 30 days – higher than 5% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.

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

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

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

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: High: A successful attack depends on conditions beyond the attacker's control. That is, a successful attack cannot be accomplished at will, but requires the attacker to invest in some measurable amount of effort in preparation or execution against the vulnerable component before a successful attack can be expected. For example, a successful attack may require an attacker to: gather knowledge about the environment in which the vulnerable target/component exists; prepare the target environment to improve exploit reliability; or inject themselves into the logical network path between the target and the resource requested by the victim in order to read and/or modify network communications (e.g., a man in the middle attack).
Privileges Required: High: The attacker is authorized with (i.e., requires) privileges that provide significant (e.g., administrative) control over the vulnerable component that could affect component-wide settings and files.
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: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact on the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 3 Dec 2025
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: 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 2026-10-26.

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.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC 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
GitHubenvoyproxy/envoy`pkg:github/envoyproxy/envoy`Other threats	Affected <=1.33.12	Fixed in–

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

Organizations should immediately inventory their Envoy deployments to identify affected versions (<=1.33.12, 1.34.0 to 1.34.10, 1.35.0 to 1.35.6, and 1.36.0 to 1.36.2). In the interim, consider implementing stricter certificate validation policies, such as rejecting certificates with embedded null bytes or other malformed SAN entries at the ingress or certificate issuance level. Employ network segmentation and zero-trust principles to limit the exposure of Envoy instances to untrusted networks. Conduct thorough audits of mTLS configurations to ensure they adhere to best practices and do not rely solely on vulnerable matching logic. Additionally, enhance monitoring and logging around certificate validation failures and mTLS handshakes to detect anomalous activity that could indicate exploitation attempts. Engage with certificate authorities to enforce strict certificate issuance policies that prevent malformed certificates.

Description

CVE-2025-66220 is a medium severity vulnerability in Envoy proxy versions up to 1.36.2, where improper null termination in the mTLS certificate matcher allows certificates with embedded null bytes in OTHERNAME SAN values to be incorrectly accepted as valid. This flaw could lead to unauthorized access by bypassing certificate validation, impacting confidentiality and integrity without requiring user interaction but needing high privileges and network access. No known exploits are currently reported. European organizations using affected Envoy versions in their service mesh or edge proxy infrastructure should prioritize patching to prevent potential misuse. Countries with significant cloud, telecom, and financial sectors relying on Envoy are at higher risk. Mitigation involves upgrading to fixed versions once available and implementing strict certificate validation policies. The vulnerability’s CVSS score is 5.0, reflecting moderate risk due to the complexity of exploitation and limited scope of impact.

Impact

For European organizations, this vulnerability poses a risk primarily to the confidentiality and integrity of internal and external communications secured by Envoy’s mTLS. Attackers exploiting this flaw could impersonate legitimate services or intercept sensitive data by bypassing certificate validation, potentially leading to data breaches or unauthorized access to critical systems. Sectors such as finance, telecommunications, cloud service providers, and critical infrastructure operators that rely heavily on Envoy for secure service mesh or edge proxy deployments are particularly vulnerable. The impact is heightened in environments where strict identity verification is essential for compliance with GDPR and other data protection regulations. Although the vulnerability does not affect availability, the compromise of confidentiality and integrity could result in regulatory penalties, reputational damage, and operational disruptions. The lack of known exploits provides a window for proactive mitigation, but the medium severity score suggests that organizations should not delay remediation.

Summary

CVE-2025-66220 is a vulnerability identified in the Envoy proxy, a widely used high-performance edge, middle, and service proxy component. The issue stems from improper handling of null termination (CWE-170) in the mTLS certificate matcher, specifically in the match_typed_subject_alt_names feature. Envoy versions 1.33.12 and earlier, as well as versions up to 1.36.2, incorrectly process certificates that contain an embedded null byte (\0) within an OTHERNAME Subject Alternative Name (SAN) field. This improper null termination causes Envoy to treat such malformed certificates as valid matches during mutual TLS (mTLS) authentication. The consequence is a potential bypass of certificate validation, allowing an attacker with the ability to present a crafted certificate containing an embedded null byte to gain unauthorized access or impersonate a legitimate service. The vulnerability requires network access and privileges to present certificates but does not require user interaction. The CVSS v3.1 base score is 5.0, indicating a medium severity level, with high impact on confidentiality, limited impact on integrity, and no impact on availability. No public exploits or active exploitation in the wild have been reported as of the publication date. The vulnerability affects multiple Envoy versions commonly deployed in cloud-native environments, service meshes, and edge proxy setups, making it relevant for organizations relying on Envoy for secure service-to-service communication.

CWE-170Improper Null Termination (opens in a new tab)
Base weaknessLikelihood of exploit: Medium
The product does not terminate or incorrectly terminates a string or array with a null character or equivalent terminator.
Requirements
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.
Implementation
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.
Implementation
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.
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)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

github.com/envoyproxy/envoy/security/advisories/GHSA-rwjg-c3h2-f57p (opens in a new tab)

Community reviews

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 10 Dec 2025, 20:03 UTC

WeaknessesCWE-170

Technical details

References

Community reviews

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