Envoy affected by off-by-one write in JsonEscaper::escapeString()

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

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

EPSS

• CVE-2026-26309: 0.39% estimated exploitation activity in the next 30 days; percentile 32.93 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary 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.

Details The bug is in the control-character escaping path in source/common/common/ jsonescapestring.h:67. - The function pre-sizes result to the final length: std::string result(input.size() + requiredsize, '\\'); - For control characters (0x00..0x1f), it emits a JSON escape sequence of length 6: \u00XX. - It uses sprintf(&result[position + 1], "u%04x", ...), which writes 5 chars + a trailing NUL (\0) starting at result[position + 1]. - Then it does position += 6; and writes result[position] = '\\'; to overwrite the NUL. - If the control character occurs at the end of the output (e.g., the input ends with \x01), then after position += 6, position == result.size(), so result[position] is one past the end (off-by-one), violating…

Published by the source

10 Mar 2026, 18:30 UTC

Last modified by the source

10 Mar 2026, 22:54 UTC

Source last checked

26 Oct 2026, 15:15 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-56cj-wgg3-x943

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# Patch Next advisory brief: CVE-2026-26309

Advisory: Envoy affected by off-by-one write in JsonEscaper::escapeString()
IDs: CVE-2026-26309, GHSA-56cj-wgg3-x943
Record: https://patchnext.com/advisories/cve-2026-26309 (revision 1, source checked 26 Oct 2026, 15:15 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor response not established. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/envoyproxy/envoy: affected >= 1.37.0, <= 1.37.0; >= 1.36.0, <= 1.36.4; >= 1.35.0, <= 1.35.8; <= 1.34.12

## Suggested inspection (read-only)
- Go github.com/envoyproxy/envoy: check the installed version with your package manager.

## Supported action
No complete vendor fix is recorded. Follow the primary source's mitigation guidance and check this record again before closing anything.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/envoyproxy/envoy.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-56cj-wgg3-x943 (GitHub-reviewed advisory GHSA-56cj-wgg3-x943 lists affected ranges: Go github.com/envoyproxy/envoy >=1.37.0 =1.36.0 =1.35.0 =0 <=1.34.12)

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