curl libcurl: Information exposure via stale AWS SigV4 authorization header
Summary
libcurl does not remove the SigV4 authentication setting when CURLOPT_AWS_SIGV4 is set to NULL, so the next transfer on the same easy handle still sends an Authorization: AWS4-HMAC-SHA256 header. An attacker who can influence the destination URL of such a request can receive the access-key identifier and a request-bound signature; the secret key is not sent. The issue requires an application that reuses a handle, keeps CURLOPT_USERPWD credentials, relies on the NULL reset and then requests an attacker-influenced URL. The curl command line tool is not affected.
Affected
- curl 7.75.0 up to and including 8.21.0 (libcurl)
- Not affected: curl before 7.75.0, 8.22.0 and later, 8.20.2 and later in the 8.20.X branch, 8.16.2 and later in the 8.16.X branch, 8.14.3 and later in the 8.14.X branch
Fixed in
- curl 8.22.0
- Patch commit c3c70440e8d9938c0bb732595751e8aebc614f24
- Backport fixes per branch: 8.14.3, 8.16.2, 8.20.2
- Distribution fix status varies; verify with your vendor
Workaround
- Make sure the application never sets CURLOPT_AWS_SIGV4 to NULL to disable SigV4 (official vendor recommendation)
- Apply the vendor patch to the local libcurl build if an upgrade is not possible
Exploitation
- CISA SSVC assessment lists exploitation as 'poc' and automatable as 'no'
- No evidence of in-the-wild exploitation found
- No public proof of concept found in the research
Our recommendation
- Check whether libcurl is embedded in your own software, containers or vendor appliances, not only the OS curl package
- Upgrade libcurl to 8.22.0 or a fixed backport, or apply the patch, during your normal update cycle
- Review code that calls AWS-style APIs with SigV4 and reuses handles for requests to user-supplied URLs
- Verify the exact affected branch boundaries against the live vendor page before planning changes
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- curl security advisory CVE-2026-86409 curl.se
- curl CVE JSON metadata CVE-2026-86409 curl.se
- HackerOne original report 3973135 hackerone.com
- curl fix pull request 22681 github.com
- OSV entry CURL-CVE-2026-86409 osv.dev
- Debian Security Tracker CVE-2026-86409 security-tracker.debian.org
- Amazon Linux Security Advisory CVE-2026-86409 explore.alas.aws.amazon.com
Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 1 Nov 2026, 18:37.
Original description (NVD)
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.
Sources
The tracker combines official, freely usable data sources – updated continuously:
- NVD (NIST) – CVEs, rating (CVSS), affected products
- CISA KEV – vulnerabilities confirmed by CISA as actively exploited
- CISA Vulnrichment – CISA assessment (SSVC) in the official CVE record
- FIRST EPSS – probability of exploitation
- Microsoft MSRC – Patch Tuesday: vulnerabilities, severity, updates
- PoC-in-GitHub – public proof-of-concept repositories
- Metasploit (Rapid7) – modules of the Metasploit Framework
- Exploit-DB (OffSec) – archive of public exploits (links only)
- Patch days: official vendor pages (linked per vendor) (Vendor patch days)
This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.
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