Last updated 2 July 2026

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.

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uses a proper HTTP/3 server for the initial transfers, and when it makes a second transfer to the same site it has been replaced by the attacker's impostor machine - without a valid certificate.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 (CURLOPTSSLSESSIONIDCACHE is not disabled) and early data enabled (the CURLSSLOPTEARLYDATA bit is set in CURLOPTSSLOPTIONS), libcurl might send off the second request's bytes on that new connection before enforcing the certificate verification failure. Potentially leaking sensitive information.

Calling curleasypause() within the event-based CURLMOPTSOCKETFUNCTION 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.

Last updated 2 July 2026

incomplete mTLS config matching in conn reuse

env-set cross-proxy Digest auth state leak

Last updated 2 July 2026

Last updated 2 July 2026

A flaw in curl’s cookie parsing logic allows a malicious HTTP server to set 'super cookies' that bypass the Public Suffix List check. This enables an attacker-controlled origin to inject cookies that curl subsequently scopes and transmits to unrelated third-party domains.

Last updated 2 July 2026

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.

Last updated 10 July 2026

cross-origin Digest auth state leak

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.

Last updated 10 July 2026

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 use-after-free vulnerability exists in libcurl when an application configures an HTTP/2 stream-dependency tree via CURLOPTSTREAMDEPENDS or CURLOPTSTREAMDEPENDSE, subsequently invokes curleasyreset(), and finally terminates the handle with curleasycleanup(). During this final cleanup phase, libcurl attempts to access and modify an internal structure that was already freed during the reset operation.

cross-proxy Digest auth state leak

stale custom cookie host causes cookie leak

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 fails to detect OCSP problems and instead wrongly consider the response as fine.

curl might erroneously pass on credentials for a first proxy to a second proxy.

This can happen when the following conditions are true:

1. curl is setup to use specific different proxies for different URL schemes 2. the first proxy needs credentials 3. the second proxy uses no credentials 4. while using the first proxy (using say http://), curl is asked to follow a redirect to a URL using another scheme (say https://), accessed using a second, different, proxy

netrc credential leak with reused proxy connection

Last updated 10 July 2026

Last updated 10 July 2026

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.

When doing a second SMB request to the same host again, curl would wrongly use a data pointer pointing into already freed memory.

Accessibility. This issue was addressed through improved state management.

Accessibility. This issue was addressed through improved state management.

bad reuse of HTTP Negotiate connection