CVE-2026-86409: stale SigV4 headers sent unintended

Severity: high (CVSS 8.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.1% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-10-20 · updated 2026-10-29
Weakness: CWE-201

Description

Setting `CURLOPT_AWS_SIGV4` to NULL but does not disable SigV4 authentication, contrary to expectations and documentation. A subsequent transfer on the same handle still sends an `Authorization: AWS4-HMAC-SHA256` header to whatever URL isset next, leaking state information.

Affected products

curl: from 8.21.0 through 8.21.0; from 8.20.0 through 8.20.0; from 8.19.0 through 8.19.0; from 8.18.0 through 8.18.0; from 8.17.0 through 8.17.0; from 8.16.0 through 8.16.0

References

https://curl.se/docs/CVE-2026-86409.html
https://curl.se/docs/CVE-2026-86409.json
https://hackerone.com/reports/3973135
https://hackerone.com/reports/3973135

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