CVE-2026-26309Public exploitNetwork attack vector

Proxy crash via off-by-one JSON escaping

Published Mar 10, 2026 · Updated Mar 10, 2026

Off-by-one write in Envoy before 1.34.13, 1.35.0 through 1.35.8, 1.36.0 through 1.36.4, and 1.37.0 allows remote attackers to crash it. In JsonEscaper::escapeString(), sprintf writes a trailing null after a six-byte control-character escape, then result[position] overwrites one byte past a fully sized std::string when that escape ends the output. Request paths that escape attacker-controlled invalid header text can reach the flaw without authentication; later C-string use can crash the process or read beyond the string boundary.

CVSS severity5.3
Medium
EPSS probability0.36%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Off-by-one write in Envoy before 1.34.13, 1.35.0 through 1.35.8, 1.36.0 through 1.36.4, and 1.37.0 allows remote attackers to crash it. In JsonEscaper::escapeString(), sprintf writes a trailing null after a six-byte control-character escape, then result[position] overwrites one byte past a fully sized std::string when that escape ends the output. Request paths that escape attacker-controlled invalid header text can reach the flaw without authentication; later C-string use can crash the process or read beyond the string boundary.

The record

CVE
CVE-2026-26309
Published
Mar 10, 2026
Updated
Mar 10, 2026
Vendor
Envoy Proxy
Product
Envoy
Classifications
CWE-193, T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 10, 2026CVE publishedPublication date reported by the CVE source.
  2. Mar 10, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 1.34.13
  2. Affected versionversion=>= 1.35.0, < 1.35.9
  3. Affected versionversion=>= 1.36.0, < 1.36.5
  4. Affected versionversion=>= 1.37.0, < 1.37.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Envoy Proxy · Envoyversion=< 1.34.13; version=>= 1.35.0, < 1.35.9; version=>= 1.36.0, < 1.36.5; version=>= 1.37.0, < 1.37.1

Published CVSS scores

5.3GitHub, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-193Off-by-one Error
T1499Endpoint Denial of Service

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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