# CVE-2026-26309: Envoy has an off-by-one write in JsonEscaper::escapeString()

> An off-by-one write in Envoy's JSON escaper can corrupt string termination, leading to crashes or out-of-bounds reads.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 10 Mar 2026
- **Updated:** 10 Mar 2026
- **Also known as:** BIT-envoy-2026-26309, GHSA-56cj-wgg3-x943

## What is CVE-2026-26309?

CVE-2026-26309 is a medium-severity memory corruption vulnerability in Envoyproxy Envoy, published on 10 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Envoy is a high-performance edge/middle/service proxy. Prior to 1.37.1, 1.36.5, 1.35.8, and 1.34.13, an off-by-one write in Envoy::JsonEscaper::escapeString() can corrupt std::string null-termination, causing undefined behavior and potentially leading to crashes or out-of-bounds reads when the resulting string is later treated as a C-string. This vulnerability is fixed in 1.37.1, 1.36.5, 1.35.8, and 1.34.13.

Weakness: CWE-193 (Off-by-one Error)

## Which versions are affected by CVE-2026-26309?

9 open source packages ship an affected version, including github.com/envoyproxy/envoy and envoy. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `github.com/envoyproxy/envoy` | Go | All versions | None |
| `github.com/envoyproxy/envoy` | Go | All versions | None |
| `github.com/envoyproxy/envoy` | Go | All versions to 1.34.12 | None |
| `github.com/envoyproxy/envoy` | Go | All versions to 1.34.12 | None |
| `github.com/envoyproxy/envoy` | Go | From 1.35.0 to 1.35.8 | None |
| `github.com/envoyproxy/envoy` | Go | From 1.35.0 to 1.35.8 | None |
| `github.com/envoyproxy/envoy` | Go | From 1.36.0 to 1.36.4 | None |
| `github.com/envoyproxy/envoy` | Go | From 1.36.0 to 1.36.4 | None |
| `envoy` | Bitnami | All versions | 1.34.13, 1.35.9, 1.36.5, 1.37.1 |

Products: Envoyproxy Envoy.

## How do I fix CVE-2026-26309?

Upgrade to a fixed version.

## Is CVE-2026-26309 being exploited?

There is no known exploitation of CVE-2026-26309 in the wild and no public exploit code.

## Other vulnerabilities in Envoyproxy Envoy

- [CVE-2022-29226](https://insights.sbomify.com/CVE-2022-29226/): Trivial authentication bypass in Envoy (critical 10.0)
- [CVE-2019-18801](https://insights.sbomify.com/CVE-2019-18801/): Memory corruption in n/a (critical 9.8)
- [CVE-2019-18802](https://insights.sbomify.com/CVE-2019-18802/): Security feature bypass in n/a (critical 9.8)
- [CVE-2020-35470](https://insights.sbomify.com/CVE-2020-35470/): Information disclosure in n/a (high 8.8)
- [CVE-2024-23324](https://insights.sbomify.com/CVE-2024-23324/): Envoy ext auth can be bypassed when Proxy protocol filter sets invalid UTF-8 metadata (high 8.6)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-26309 and fix it if it is affected.

What it is: CVE-2026-26309 is a medium-severity memory corruption vulnerability in Envoyproxy Envoy, published on 10 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-26309 in the wild and no public exploit code.

Affected packages and fixed versions:
- github.com/envoyproxy/envoy (Go): affected all versions; no fix listed
- github.com/envoyproxy/envoy (Go): affected all versions; no fix listed
- github.com/envoyproxy/envoy (Go): affected all versions up to 1.34.12; no fix listed
- github.com/envoyproxy/envoy (Go): affected all versions up to 1.34.12; no fix listed
- github.com/envoyproxy/envoy (Go): affected from 1.35.0 up to 1.35.8; no fix listed
- github.com/envoyproxy/envoy (Go): affected from 1.35.0 up to 1.35.8; no fix listed
- github.com/envoyproxy/envoy (Go): affected from 1.36.0 up to 1.36.4; no fix listed
- github.com/envoyproxy/envoy (Go): affected from 1.36.0 up to 1.36.4; no fix listed
- envoy (Bitnami): affected all versions; fixed in 1.34.13, 1.35.9, 1.36.5, 1.37.1

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-26309/ (Markdown: https://insights.sbomify.com/CVE-2026-26309/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-26309
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-26309
- OSV: https://osv.dev/vulnerability/CVE-2026-26309

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-26309/
Summaries are generated and may contain mistakes; the official description above is the record.