When asked to both use a `.netrc` file for credentials and to follow HTTP
redirects, libcurl could leak the password used for the first host to the
followed-to host under certain circumstances.
CVSS Score
5.3
EPSS Score
0.005
Published
2026-05-13
A vulnerability exists where a connection requiring TLS incorrectly reuses an
existing unencrypted connection from the same connection pool. If an initial
transfer is made in clear-text (via IMAP, SMTP, or POP3), a subsequent request
to that same host bypasses the TLS requirement and instead transmit data
unencrypted.
CVSS Score
5.9
EPSS Score
0.003
Published
2026-05-13
When an OAuth2 bearer token is used for an HTTP(S) transfer, and that transfer
performs a redirect to a second URL, curl could leak that token to the second
hostname under some circumstances.
If the hostname that the first request is redirected to has information in the
used .netrc file, with either of the `machine` or `default` keywords, curl
would pass on the bearer token set for the first host also to the second one.
CVSS Score
5.3
EPSS Score
0.003
Published
2026-03-11
curl would wrongly reuse an existing HTTP proxy connection doing CONNECT to a
server, even if the new request uses different credentials for the HTTP proxy.
The proper behavior is to create or use a separate connection.
CVSS Score
6.5
EPSS Score
0.003
Published
2026-03-11
libcurl can in some circumstances reuse the wrong connection when asked to do
an Negotiate-authenticated HTTP or HTTPS request.
libcurl features a pool of recent connections so that subsequent requests can
reuse an existing connection to avoid overhead.
When reusing a connection a range of criterion must first be met. Due to a
logical error in the code, a request that was issued by an application could
wrongfully reuse an existing connection to the same server that was
authenticated using different credentials. One underlying reason being that
Negotiate sometimes authenticates *connections* and not *requests*, contrary
to how HTTP is designed to work.
An application that allows Negotiate authentication to a server (that responds
wanting Negotiate) with `user1:password1` and then does another operation to
the same server also using Negotiate but with `user2:password2` (while the
previous connection is still alive) - the second request wrongly reused the
same connection and since it then sees that the Negotiate negotiation is
already made, it just sends the request over that connection thinking it uses
the user2 credentials when it is in fact still using the connection
authenticated for user1...
The set of authentication methods to use is set with `CURLOPT_HTTPAUTH`.
Applications can disable libcurl's reuse of connections and thus mitigate this
problem, by using one of the following libcurl options to alter how
connections are or are not reused: `CURLOPT_FRESH_CONNECT`,
`CURLOPT_MAXCONNECTS` and `CURLMOPT_MAX_HOST_CONNECTIONS` (if using the
curl_multi API).
CVSS Score
6.5
EPSS Score
0.003
Published
2026-03-11
When an OAuth2 bearer token is used for an HTTP(S) transfer, and that transfer
performs a cross-protocol redirect to a second URL that uses an IMAP, LDAP,
POP3 or SMTP scheme, curl might wrongly pass on the bearer token to the new
target host.
CVSS Score
5.3
EPSS Score
0.006
Published
2026-01-08
When doing multi-threaded LDAPS transfers (LDAP over TLS) with libcurl,
changing TLS options in one thread would inadvertently change them globally
and therefore possibly also affect other concurrently setup transfers.
Disabling certificate verification for a specific transfer could
unintentionally disable the feature for other threads as well.
CVSS Score
6.3
EPSS Score
0.001
Published
2026-01-08
When libcurl is asked to perform automatic gzip decompression of
content-encoded HTTP responses with the `CURLOPT_ACCEPT_ENCODING` option,
**using zlib 1.2.0.3 or older**, an attacker-controlled integer overflow would
make libcurl perform a buffer overflow.
CVSS Score
7.3
EPSS Score
0.012
Published
2025-02-05
When curl is told to use the Certificate Status Request TLS extension, often referred to as OCSP stapling, to verify that the server certificate is valid, it might fail to detect some OCSP problems and instead wrongly consider the response as fine. If the returned status reports another error than 'revoked' (like for example 'unauthorized') it is not treated as a bad certficate.
CVSS Score
6.5
EPSS Score
0.007
Published
2024-09-11
When an application tells libcurl it wants to allow HTTP/2 server push, and the amount of received headers for the push surpasses the maximum allowed limit (1000), libcurl aborts the server push. When aborting, libcurl inadvertently does not free all the previously allocated headers and instead leaks the memory. Further, this error condition fails silently and is therefore not easily detected by an application.
CVSS Score
8.6
EPSS Score
0.361
Published
2024-03-27