TLS weakening via shared Schannel cipher state
Published Jun 11, 2021 · Updated May 28, 2026
Incorrect session isolation in curl libcurl 7.61.0 through 7.76.1 allows remote attackers to weaken TLS cipher selection on Windows. The Schannel set_ssl_ciphers path stored parsed CURLOPT_SSL_CIPHER_LIST algorithms in one static array, so the last concurrent transfer overwrote the cipher set used by other transfers. Exploitation requires a Schannel build, concurrent TLS transfers, and application use of CURLOPT_SSL_CIPHER_LIST; the bounded consequence is negotiation with unintended ciphers.
Summary
What happened
Incorrect session isolation in curl libcurl 7.61.0 through 7.76.1 allows remote attackers to weaken TLS cipher selection on Windows. The Schannel set_ssl_ciphers path stored parsed CURLOPT_SSL_CIPHER_LIST algorithms in one static array, so the last concurrent transfer overwrote the cipher set used by other transfers. Exploitation requires a Schannel build, concurrent TLS transfers, and application use of CURLOPT_SSL_CIPHER_LIST; the bounded consequence is negotiation with unintended ciphers.
The record
- CVE
- CVE-2021-22897
- Published
- Jun 11, 2021
- Updated
- May 28, 2026
- Vendor
- curl
- Product
- libcurl
- Classifications
- CWE-668, CWE-488
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Jun 11, 2021CVE publishedPublication date reported by the CVE source.
- May 28, 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.
- Affected versionversion=7.61.0 through 7.76.1
What conditions does exploitation require?
What is affected?
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.
Weakness, pattern, technique
Public exploit references
- HackerOne report 1172857proof of concept · demonstrated
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Technologies
Your stack
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Your stack
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