CVE-2025-13034: 
cURL vulnerability analysis and mitigation

CVE-2025-13034 is a certificate pinning bypass vulnerability in curl titled "No QUIC certificate pinning with GnuTLS." It affects curl versions 8.8.0 through 8.17.0 (inclusive) and was disclosed on January 7, 2026, with the fix released in curl 8.18.0 the same day. The vulnerability allows a network attacker to impersonate a server when public key pinning is configured but silently bypassed under specific conditions. It carries a CVSS v3.1 base score of 5.9 (Medium) (curl Advisory, Feedly).

The root cause is CWE-295 (Improper Certificate Validation): when curl uses the CURLOPT_PINNEDPUBLICKEY option (or --pinnedpubkey CLI flag), the public key pinning check is supposed to verify the server's certificate against a pinned key. However, this check is silently skipped when three conditions are simultaneously met: (1) the connection uses QUIC (HTTP/3) via the ngtcp2 backend, (2) ngtcp2 is built with GnuTLS as its TLS library, and (3) the user has explicitly disabled standard certificate verification. Other QUIC backends (e.g., quiche, OpenSSL-based ngtcp2) and HTTP/1 or HTTP/2 connections are not affected. The flaw was introduced in commit 3210101088dfa3d6a125 and fixed in commit 3d91ca8cdb3b434226e743946 (curl Advisory). This issue is noted as similar to CVE-2025-5025, which affected a different TLS library in the same code path.

Successful exploitation allows a network-positioned attacker to act as a man-in-the-middle, impersonating a server that the client believes it has verified via public key pinning. The primary impact is to integrity (CVSS I:H), as the attacker can intercept or modify data in transit without the client detecting the impostor. Confidentiality and availability are not directly impacted by this flaw. The scope is limited to applications using libcurl or the curl tool with QUIC/HTTP3 over ngtcp2+GnuTLS with pinning enabled and standard certificate verification disabled — a narrow but security-sensitive configuration (curl Advisory, Feedly).

1. Identify a target: Locate an application or system using libcurl or the curl tool configured with CURLOPT_PINNEDPUBLICKEY/--pinnedpubkey, connecting over HTTP/3 (QUIC) via ngtcp2 built with GnuTLS, and with standard certificate verification explicitly disabled (CURLOPT_SSL_VERIFYPEER=0 or --insecure).
2. Position for MitM: Gain a network-on-path position between the curl client and the target server (e.g., via ARP spoofing, DNS hijacking, or rogue Wi-Fi access point).
3. Present a rogue certificate: Serve a TLS certificate with a different public key than the pinned one. Under normal circumstances, curl would reject this connection; however, due to the bug, the pinning check is skipped in the vulnerable configuration.
4. Intercept or modify traffic: With the connection accepted, the attacker can read or tamper with the data exchanged between the curl client and the intended server, effectively acting as an undetected impostor (curl Advisory).

The primary remediation is to upgrade curl to version 8.18.0 or later, which contains the fix (curl Advisory). If an immediate upgrade is not possible, three workarounds are available: (A) rebuild curl/ngtcp2 with a different TLS library (e.g., OpenSSL or BoringSSL instead of GnuTLS), (B) avoid using HTTP/3 (QUIC) for connections that rely on public key pinning, or (C) do not disable standard certificate verification when using pinning. Downstream distributions including Slackware, Mageia, Ubuntu, Alpine Linux, FreeBSD, and Debian have released updated packages (IBM Advisory).

The vulnerability was reported by Stanislav Fort of Aisle Research on November 9, 2025, and the curl project awarded a bug bounty of 2,540 USD for the report (curl Advisory). curl lead developer Daniel Stenberg announced the curl 8.18.0 release and the advisory on his blog (Daniel Stenberg Blog). The vulnerability received moderate community attention given its narrow exploitation conditions, with discussion on oss-security and social media noting the similarity to CVE-2025-5025. A SolCyber blog post highlighted the issue as a reminder of the subtlety of cryptographic implementation bugs.