Haxx: Security Vulnerabilities
When asking curl to use a `.netrc` file to find credentials and at the same
time specifying a URL with a username(without a password), like
`https://user@example.com/`, curl could wrongly get and use the password for
*another* user set in the `.netrc` file for that host if such a one exists and
there is no match for the specified user.
CVSS Score
9.1
EPSS Score
0.006
Published
2026-07-03
When reusing a libcurl handle for sequential transfers driven by
environment-variable proxy configuration, libcurl fails to clear the proxy
authentication state between requests. Specifically, if the initial transfer
authenticates against `proxyA` using Digest auth, a subsequent transfer routed
through `proxyB` erroneously leaks the `Proxy-Authorization:` header intended
solely for `proxyA`.
CVSS Score
9.1
EPSS Score
0.008
Published
2026-07-03
libcurl would reuse a previously created connection even when some mTLS config
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, some TLS
settings related to client certificates were left out from the configuration
match checks, making them match too easily. In particular options related to
the private key.
CVSS Score
7.5
EPSS Score
0.004
Published
2026-07-03
libcurl had a flaw that when instructed to clear proxy authentication
credentials which made it not do so, leaving the old credentials around to get
used for subsequent transfers that should not know nor use them.
CVSS Score
9.8
EPSS Score
0.011
Published
2026-07-03
Calling `curl_easy_pause()` within the event-based `CURLMOPT_SOCKETFUNCTION`
callback triggers a use-after-free vulnerability, where libcurl attempts to
store a flag using a dangling struct pointer immediately after that pointer's
memory has been freed.
CVSS Score
7.3
EPSS Score
0.005
Published
2026-07-03
In this scenario, libcurl first uses a proper HTTP/3 server for the initial
transfers, and when it makes a second transfer to the same site it has been
replaced by the attacker's impostor machine - without a valid certificate.
When libcurl returns to the hostname the second time with a cached SSL session
(`CURLOPT_SSL_SESSIONID_CACHE` is not disabled) and early data enabled (the
`CURLSSLOPT_EARLYDATA` bit is set in `CURLOPT_SSL_OPTIONS`), libcurl might
send off the second request's bytes on that new connection *before* enforcing
the certificate verification failure. Potentially leaking sensitive
information.
CVSS Score
7.5
EPSS Score
0.004
Published
2026-07-03
A vulnerability in libcurl caused the HTTP `Referer:` header to persist even
when explicitly cleared. While the documentation states that passing NULL to
`CURLOPT_REFERER` 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.
CVSS Score
7.5
EPSS Score
0.007
Published
2026-07-03
When a libcurl-based application performs transfers via `SCP://` or `SFTP://`
and utilizes the `CURLOPT_SSH_KEYFUNCTION` callback, it may silently accept an
untrusted server. This vulnerability occurs when a server presents a host key
type that does not match the specific key type already recorded for that host
in the `known_hosts` file. Instead of rejecting the mismatch, the callback
mechanism fails to properly enforce the restriction, allowing the connection
to succeed without warning and risking a potential man-in-the-middle attack.
CVSS Score
7.4
EPSS Score
0.005
Published
2026-07-03
When a user invokes curl using a schemeless URL combined with
`--proto-default` sftp (or scp), a disconnect occurs between the tool layer
and libcurl. The tool layer incorrectly infers the URL scheme, which
erroneously bypasses the initialization of critical SSH security options like
CURLOPT_SSH_HOST_PUBLIC_KEY_SHA256 and CURLOPT_SSH_KNOWNHOSTS. Conversely, the
libcurl runtime successfully honors CURLOPT_DEFAULT_PROTOCOL and establishes
the connection via SFTP/SCP as specified. Because the tool layer skipped the
security configuration, these SSH host verification options are silently
omitted, causing curl to connect to an unverified SSH remote host without
throwing an error.
CVSS Score
7.5
EPSS Score
0.006
Published
2026-07-03
A vulnerability exists where a new transfer that uses STARTTLS to upgrade the
connection might reuse an existing live connection even though the TLS
configuration mismatches so it should not.
CVSS Score
8.1
EPSS Score
0.005
Published
2026-07-03
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