# CVE-2025-66220: envoy — Fixed in 1.36.3 🐹 🐹 Go Not in CISA KEV MEDIUM severity # CVE-2025-66220 — envoy MEDIUM CVE-2025-66220 is a medium-severity (CVSS 5) CWE-170 vulnerability in github.com/envoyproxy/envoy. A fix is available for github.com/envoyproxy/envoy — see the affected versions and patch details below. Envoy’s TLS certificate matcher for \`match\_typed\_subject\_alt\_names\` may incorrectly treat certificates containing an embedded null byte Also known asBIT-envoy-2025-66220[GHSA-rwjg-c3h2-f57p](/vulnerability/GHSA-rwjg-c3h2-f57p) Published Dec 3, 2025 Updated Oct 28, 2026 Affected 4 pkgs Patched 4 / 4 Exploits None indexed Exploitation data as of Nov 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS) ## Exploitation Status ### Proof-of-concept exploit code exists - CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation. Exploitation and automatability from CISA’s SSVC triage for CVE-2025-66220. ## EPSS Exploitation Probability [via FIRST.org ↗](https://www.first.org/epss/) 0.2%probability of exploitation in next 30 days Lower Risk+0.01% Lower risk than most CVEs8th percentile — riskier than 8% of all scored CVEsHighest risk 0.00%0.23%0.45%0.68%0.0%0.2%May 26Sep 26Nov 26 Probability of exploitation in the next 30 days, from [FIRST.org EPSS](https://www.first.org/epss/). ## How urgent is this, really ## Where this sits among everything scored ## Real-World Exposure 4 pkgs affected 🐹`github.com/envoyproxy/envoy`🐹`github.com/envoyproxy/envoy`🐹`github.com/envoyproxy/envoy`🐹`github.com/envoyproxy/envoy` Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems. ## Description #### Summary 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. #### Details This occurs when the SAN is encoded as a `BMPSTRING` or `UNIVERSALSTRING`, and its UTF-8 conversion result is truncated at the first null byte during string assignment. As a result, `"victim\0evil"` may match an exact: `"victim"` rule and be accepted by Envoy. #### PoC Create a CA and a server certificate signed by that CA. Create two client certificates signed by the same CA: client\_evil with OTHERNAME BMPSTRING = "evil" client\_null with OTHERNAME BMPSTRING = "victim\\0evil" Configure Envoy with require\_client\_certificate: true and a match\_typed\_subject\_alt\_names entry for the OTHERNAME OID with matcher.exact: "victim". Connect without a client cert → connection rejected. Connect with client\_evil → connection rejected. Connect with client\_null → connection accepted (but shouldn't!). #### Impact An attacker who can obtain a trusted client certificate with a null byte embedded in an OTHERNAME SAN can exploit this vulnerability. The practical impact is unauthorized impersonation of the matched identity, enabling access to services or APIs protected by that exact OTHERNAME check. #### Credit [markevich.nikita1@gmail.com](mailto:markevich.nikita1@gmail.com) ## Affected Packages 4 total 4 fixed Ecosystem Package Vulnerable range Fix 🐹Go `github.com/envoyproxy/envoy` ≥ 1.36.0&&< 1.36.3 1.36.3`go get github.com/envoyproxy/envoy@v1.36.3` 🐹Go `github.com/envoyproxy/envoy` ≥ 1.35.0&&< 1.35.7 1.35.7`go get github.com/envoyproxy/envoy@v1.35.7` 🐹Go `github.com/envoyproxy/envoy` ≥ 1.34.0&&< 1.34.11 1.34.11`go get github.com/envoyproxy/envoy@v1.34.11` 🐹Go `github.com/envoyproxy/envoy` all versions 1.33.13`go get github.com/envoyproxy/envoy@v1.33.13` ## Affected Products 1 product · 4 configurations Application `envoy`envoyproxy ≥ 1.36.0 && < 1.36.3 range ## Detection & mitigation playbook Open-source dependency 1. ### Detect Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/envoyproxy/envoy, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version. 2. ### Fix Update github.com/envoyproxy/envoy to 1.36.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2025-66220 is resolved across your whole dependency graph. 3. ### Workarounds If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands. ## Fixing This On Your OS If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project. Red HatModerate This vulnerability is rated Moderate for Red Hat products. Envoy's mTLS certificate matcher, when configured with \`match\_typed\_subject\_alt\_names\`, may incorrectly validate certificates containing an embedded null byte in the OTHERNAME SAN field. This could allow an attacker to bypass mTLS authentication if they can… Workaround published by Red Hat > Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability. Source: [Red Hat security advisory for CVE-2025-66220](https://access.redhat.com/security/cve/cve-2025-66220) (CC BY 4.0) ## Frequently Asked Questions \### Summary 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. ### Details This occurs when the SAN is encoded as a \`BMPSTRING\` or \`UNIVERSALSTRING\`, and its UTF-8 conversion result is truncated at the first null byte during string assignment. As a result, \`"victim\\0evil"\` may match an exact: \`"victim"\` rule and be accepted by Envoy. ### PoC Create a CA and a server certificate signed by that CA. Create two client certificates signed by the same CA: cl O3 Security · Impact-Aware SCA ### Is CVE-2025-66220 in your dependencies? Find it across Go, including transitive dependencies. [Scan my dependencies](/book-demo) [How O3 SCA works](/impact-aware-sca)