bad reuse of HTTP Negotiate connection

libcurl wrongly allows cookies to be set for Top Level Domains (TLDs) if thehost name is provided with a trailing dot.curl can be told to receive and send cookies. curl's "cookie engine" can bebuilt with or without Public Suffix Listawareness. If PSL support not provided a more rudimentary check exists to atleast prevent cookies from being set on TLDs. This check was broken if thehost name in the URL uses a trailing dot.This can allow arbitrary sites to set cookies that then would get sent to adifferent and unrelated site or domain.

A flaw was found in libcurl. A part of a password may be prepended to the host name before the host name is resolved, leading to a leak of the partial password over the network and to DNS servers. This highest threat from this vulnerability is to data confidentiality.

Last updated 10 July 2026

Last updated 10 July 2026

By default, curl automatically responds to WebSocket PING frames. Because curl lacks an upper bound on memory allocation for unacknowledged frames, a malicious server can exhaust all available memory by flooding curl with rapid, sequential PING messages.

incomplete mTLS config matching in conn reuse

An issue in curl’s QUIC UDP receive function allows a malicious HTTP/3 server to trigger a remote denial of service against a curl or libcurl client. Because the helper function discards zero-length UDP datagrams before counting them toward the per-call packet budget, a connected QUIC peer can continuously stream empty datagrams to indefinitely stall the client.

A vulnerability in libcurl caused the HTTP Referer: header to persist even when explicitly cleared. While the documentation states that passing NULL to CURLOPTREFERER suppresses the header, the option failed to clear the internal state. As a result, the previous referrer string was erroneously reused and sent in subsequent requests, potentially leaking sensitive information to unintended servers.

A flaw in libcurl's handling of HTTP/2 Server Push streams, when the parent handle is set to share connections with other handles, can lead to use-after-free in the cleanup process.

A flaw in libcurl makes it wrongly reuse an HTTP connection setup for a given hostname using Negotiate authentication, when the initial request is done using empty credentials. This can make user B's request get sent over user A's previously authenticated connection.

domain-scoped PSL domain cookie

Last updated 2 July 2026

cross-origin Digest auth state leak

A Set-Cookie: header using tab (horizontal tab, ASCII code 9) instead of space (ascii code 32) immediately before the Secure attribute causes curl to store the cookie without its Secure flag. The cookie might then wrongfully be sent over plaintext HTTP on subsequent requests to the same host.

A malicious server can serve excessive amounts of Set-Cookie: headers in a HTTP response to curl and curl < 7.84.0 stores all of them. A sufficiently large amount of (big) cookies make subsequent HTTP requests to this, or other servers to which the cookies match, create requests that become larger than the threshold that curl uses internally to avoid sending crazy large requests (1048576 bytes) and instead returns an error.This denial state might remain for as long as the same cookies are kept, match and haven't expired. Due to cookie matching rules, a server on foo.example.com can set cookies that also would match for bar.example.com, making it it possible for a "sister server" to effectively cause a denial of service for a sibling site on the same second level domain using this method.

A vulnerability was found in curl. This issue occurs because when curl saves cookies, alt-svc, and HSTS data to local files, it makes the operation atomic by finalizing the process with a rename from a temporary name to the final target file name. This flaw leads to unpreserved file permissions, either by mistake or by a malicious actor.

A vulnerability was found in curl. This issue occurs because it mishandles message verification failures when curl does FTP transfers secured by krb5. This flaw makes it possible for a Man-in-the-middle attack to go unnoticed and allows data injection into the client.

Using its HSTS support, curl can be instructed to use HTTPS directly insteadof using an insecure clear-text HTTP step even when HTTP is provided in theURL. This mechanism could be bypassed if the host name in the given URL used atrailing dot while not using one when it built the HSTS cache. Or the otherway around - by having the trailing dot in the HSTS cache and not using thetrailing dot in the URL.

An information disclosure vulnerability exists in curl 7.65.0 to 7.82.0 are vulnerable that by using an IPv6 address that was in the connection pool but with a different zone id it could reuse a connection instead.

A insufficiently protected credentials vulnerability in fixed in curl 7.83.0 might leak authentication or cookie header data on HTTP redirects to the same host but another port number.

libcurl would reuse a previously created connection even when a TLS or SSH related option had been changed that should have prohibited reuse. libcurl keeps previously used connections in a connection pool for subsequent transfers to reuse if one of them matches the setup. However, several TLS and SSH settings were left out from the configuration match checks, making them match too easily.

An improper authentication vulnerability exists in curl 7.33.0 to and including 7.82.0 which might allow reuse OAUTH2-authenticated connections without properly making sure that the connection was authenticated with the same credentials as set for this transfer. This affects SASL-enabled protocols: SMPTP(S) IMAP(S) POP3(S) and LDAP(S) (openldap only).

A vulnerability was found in curl. This issue occurs because the number of acceptable "links" in the "decompression chain" was unbounded, allowing a malicious server to insert a virtually unlimited number of compression steps. This flaw leads to a denial of service, either by mistake or by a malicious actor.

A vulnerability was found in curl. The issue occurs because curl's HSTS check can be bypassed to trick it to keep using HTTP. Using its HSTS support, it can instruct curl to use HTTPS directly instead of using an insecure clear-text HTTP step even when HTTP is provided in the URL. This mechanism can be bypassed if the hostname in the given URL uses IDN characters that get replaced with ASCII counterparts as part of the IDN conversion. Like using the character UTF-8 U+3002 (IDEOGRAPHIC FULL STOP) instead of the common ASCII full stop (U+002E) ..

A vulnerability was found in curl. The issue occurs if curl is told to use an HTTP proxy for a transfer with a non-HTTP(S) URL. It sets up the connection to the remote server by issuing a CONNECT request to the proxy and then tunnels the rest of the protocol through. An HTTP proxy might refuse this request (HTTP proxies often only allow outgoing connections to specific port numbers, like 443 for HTTPS) and instead return a non-200 response code to the client. Due to flaws in the error/cleanup handling, this could trigger a double-free issue in curl if using one of the following schemes in the URL for the transfer: dict, gopher, gophers, ldap, ldaps, rtmp, rtmps, telnet.

A vulnerability exists in curl <7.87.0 HSTS check that could be bypassed to trick it to keep using HTTP. Using its HSTS support, curl can be instructed to use HTTPS instead of using an insecure clear-text HTTP step even when HTTP is provided in the URL. However, the HSTS mechanism could be bypassed if the host name in the given URL first uses IDN characters that get replaced to ASCII counterparts as part of the IDN conversion. Like using the character UTF-8 U+3002 (IDEOGRAPHIC FULL STOP) instead of the common ASCII full stop (U+002E) .. Then in a subsequent request, it does not detect the HSTS state and makes a clear text transfer. Because it would store the info IDN encoded but look for it IDN decoded.

curl can be told to parse a .netrc file for credentials. If that file endsin a line with 4095 consecutive non-white space letters and no newline, curlwould first read past the end of the stack-based buffer, and if the readworks, write a zero byte beyond its boundary.This will in most cases cause a segfault or similar, but circumstances might also cause different outcomes.If a malicious user can provide a custom netrc file to an application or otherwise affect its contents, this flaw could be used as denial-of-service.

A vulnerability was found in curl. The issue occurs when doing HTTP(S) transfers, where curl might erroneously use the read callback (CURLOPTREADFUNCTION) to ask for data to send, even when the CURLOPTPOSTFIELDS option has been set if it previously used the same handle to issue a PUT request which used that callback. This flaw may surprise the application and cause it to misbehave and either send off the wrong data or use memory after free or similar in the subsequent POST request.

A use of incorrectly resolved name vulnerability fixed in 7.83.1 might remove the wrong file when --no-clobber is used together with --remove-on-error.