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

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

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.

Products include Envoyproxy Envoy.

How do I fix CVE-2026-26309?

Upgrade to a fixed version.

Upgrade to the fixed version. Memory-safe languages, compiler hardening and fuzzing prevent most new cases. More about Memory corruption.

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 Trivial authentication bypass in Envoy Critical 10.0
• CVE-2019-18801 Memory corruption in n/a Critical 9.8
• CVE-2019-18802 Security feature bypass in n/a Critical 9.8
• CVE-2020-35470 Information disclosure in n/a High 8.8
• CVE-2024-23324 Envoy ext auth can be bypassed when Proxy protocol filter sets invalid UTF-8 metadata High 8.6

All Envoyproxy Envoy vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as BIT-envoy-2026-26309, GHSA-56cj-wgg3-x943.