CVE-2021-22947
MediumCVSS 5.9Exploitation Probability (EPSS)
High risk87th percentile - higher than 87% of all known CVEs
Summary
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, instead continuing to use and trust responses received before the TLS handshake as if they were authenticated. This flaw allows a Man-In-The-Middle attacker to inject fake responses, pass through TLS traffic from the legitimate server, and trick curl into sending data back to the user thinking the injected data comes from the TLS-protected server.
Risk Assessment
An attacker could intercept or modify data transmitted via curl in IMAP/POP3 sessions, compromising confidentiality and integrity of communication.
Recommendation
Update curl to a version newer than 7.78.0, which includes a fix for this issue.
Other vulnerabilities in curl
See all- CVE-2024-9681Medium
When curl is asked to use HSTS, the expiry time for a subdomain might overwrite a parent domain's cache entry, making it end sooner or later than otherwise intended. This affects applications using HSTS and URLs with the insecure HTTP:// scheme.
- CVE-2026-94265Critical
When processing an HTTP redirect to a URL containing userinfo (a username:password component), curl might erroneously retain and re-transmit the original host's HTTP Negotiate authentication state. Consequently, a malicious server can trigger a redirect containing arbitrary userinfo to force curl to send a freshly minted Kerberos ticket to an untrusted third-party host, effectively bypassing the domain-scoping restrictions normally enforced by the absence of --location-trusted.
- CVE-2026-87866Critical
When built with wolfSSL, curl can successfully validate a pinned public key hash against a key the TLS peer does not possess. During the TLS handshake, wolfSSL validates the connection, but curl's subsequent re-parsing of the same certificate, tricked by a manipulated validity length field, extracts and matches a different embedded key, incorrectly authorizing the pinned public key.
- CVE-2026-87861Critical
When using ambient user credentials, an HTTP request directed to one origin may be dispatched over a connection previously authenticated via Negotiate for a different origin, leaving the request's intended credentials unused.
- CVE-2026-86414Critical
An HTTP proxy can trick libcurl into sending the origin's credentials (Basic password or a Negotiate/Kerberos ticket) to the proxy itself by answering the CONNECT tunnel request with 401 WWW-Authenticate instead of the expected 407 Proxy-Authenticate.
- CVE-2026-86413Critical
When using an ambient user in a first transfer doing Negotiate authentication and asked to follow redirects, curl could pass on its credential details to an unrelated second origin when asked to redirect there.
- CVE-2026-86411Critical
An ftp:// transfer configured with CURLUSESSL_NONE can reuse a pooled FTP control connection that a previous easy handle upgraded with STARTTLS and authenticated using a client certificate. The second handle performs neither a new TLS handshake nor a new FTP login, so the server continues to authorize it under the first handle's certificate identity.
- CVE-2026-8926Critical
When curl uses a .netrc file for credentials and a URL with a username (without password) is specified, curl may wrongly retrieve and use the password of another user from .netrc for that host if no match exists for the specified user.
- CVE-2026-8925Critical
A double-free vulnerability has been found in the curl library's SASL authentication handling. The GSASL context is cleaned up twice without clearing the pointer, causing free() to be called on the same memory region twice.
- CVE-2026-8924Critical
A flaw in curl's cookie parsing logic allows a malicious HTTP server to set "super cookies" that bypass the Public Suffix List check, enabling injection of cookies that curl scopes and sends to unrelated third-party domains.
Original NVD description (English source)
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.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS