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Findings

CVE-2021-22947

When curl >= 7.20.0 and <= 7.78.0 connects to an IMAP or POP3 server to retrieve data using STARTTLS to upgrade to TLS security, the server can respond and send back m…

Published Sep 29, 2021 · updated Oct 14, 2026 · haxx curl

When curl >= 7.20.0 and <= 7.78.0 connects to an IMAP or POP3 server to retrieve data using STARTTLS to upgrade to TLS security, the server can respond and send back multiple responses at once that curl caches. curl would then upgrade to TLS but not flush the in-queue of cached responses but instead continue using and trustingthe responses it got *before* the TLS handshake as if they were authenticated.Using this flaw, it allows a Man-In-The-Middle attacker to first inject the fake responses, then pass-through the TLS traffic from the legitimate server and trick curl into sending data back to the user thinking the attacker's injected data comes from the TLS-protected server.

Timeline

Detection, updates, and resolution for this record.

  1. Sep 29, 2021 · NIST NVD

    Detected

    When curl >= 7.20.0 and <= 7.78.0 connects to an IMAP or POP3 server to retrieve data using STARTTLS to upgrade to TLS security, the server can respond and send back multiple responses at once that curl caches. curl would then upgrade to TLS but not flush the in-queue of cached responses but instead continue using and trustingthe responses it got *before* the TLS handshake as if they were authenti

  2. Oct 14, 2026 · NIST NVD

    Updated

    When curl >= 7.20.0 and <= 7.78.0 connects to an IMAP or POP3 server to retrieve data using STARTTLS to upgrade to TLS security, the server can respond and send back multiple responses at once that curl caches. curl would then upgrade to TLS but not flush the in-queue of cached responses but instead continue using and trustingthe responses it got *before* the TLS handshake as if they were authenti

Affected

  • cpe debian/debian linux10.0
  • cpe debian/debian linux11.0
  • cpe debian/debian linux9.0
  • cpe fedoraproject/fedora33
  • cpe fedoraproject/fedora35
  • cpe haxx/curlversion unspecified
  • cpe netapp/cloud backupversion unspecified
  • cpe netapp/clustered data ontapversion unspecified
  • cpe netapp/h300e firmwareversion unspecified
  • cpe netapp/h300s firmwareversion unspecified
  • cpe netapp/h500s firmwareversion unspecified
  • cpe netapp/h700s firmwareversion unspecified

Sources

  • CVE.orgCVE-2021-22947

    Official CVE identifier

    cve.org
  • NIST NVDCVE-2021-22947

    When curl >= 7.20.0 and <= 7.78.0 connects to an IMAP or POP3 server to retrieve data using STARTTLS to upgrade to TLS security, the server can respond and send back multiple responses at once that curl caches. curl would then upgrade to TLS but not flush the in-queue of cached r

    nvd.nist.gov

References