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libcurl TLS Session Resumption Issues Disclosed

libcurl TLS Session Resumption Issues Disclosed

First seen 15 Sep 2026, 19:22 UTC • •
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ThreatCluster AI
ThreatCluster •September 15, 2026 at 19:55 UTC
  • •libcurl's TLS session resumption can expose user identities.
  • •Mutual TLS connections may lead to identity mix-ups due to ENGINE misconfigurations.
  • •Both issues were classified as informative, with no CVEs assigned.

Two security issues related to libcurl's handling of TLS sessions were disclosed. The first issue allows a TLS session to be reused without a client certificate, potentially exposing user identities. The second issue involves mutual TLS (mTLS) connections where different OpenSSL ENGINEs could lead to identity mix-ups when reusing connections. Both vulnerabilities were reported to HackerOne and classified as informative rather than critical vulnerabilities. The curl security team emphasized that the issues stemmed from documentation gaps rather than exploitable flaws. No CVEs were assigned, and the reports were closed without a bounty. Users of libcurl are advised to review the updated documentation to avoid potential misuse of the CURLOPT_SSL_CTX_FUNCTION option.

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Timeline

2026-09-14
TLS session resumption issue reported
A researcher disclosed that libcurl could reuse TLS sessions without client certificates, risking identity exposure.
Redpacketsecurity
2026-09-14
mTLS connection reuse issue reported
Another researcher found that curl's mTLS support omitted ENGINE selection, risking identity mix-ups.
Redpacketsecurity
2026-09-15
Reports closed as informative
Both issues were classified as informative, with no security vulnerabilities identified and no CVEs assigned.
Redpacketsecurity

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