Squidbleed — Squid FTP-gateway heap over-read

Researchers at Calif.io disclosed CVE-2026-47729, nicknamed Squidbleed: a heap buffer over-read in the Squid proxy's FTP-over-HTTP gateway (src/FtpGateway.cc) introduced by a 1997 code commit (Calif.io, 2026-06-18; The Hacker News, 2026-06-22). The root cause is a whitespace-skipping loop that calls strchr(w_space, *copyFrom) without first checking for the string terminator: strchr returns a non-NULL pointer when the search character is the embedded \0, so the parser walks past the end of the FTP directory-listing buffer into adjacent heap memory containing other users' cached HTTP requests. An attacker who controls an FTP server and can induce the proxy to fetch from it (FTP support and TCP/21 are in Squid's default Safe_ports ACL) can leak Authorization headers, session cookies, API keys and other cleartext request content from concurrent users sharing the same proxy worker (SecurityWeek, 2026-06-22). HTTPS relayed via CONNECT tunnels is not exposed; only cleartext HTTP and TLS-terminating proxy setups are. SUSE rates it moderate (CVSS 6.5) and there is no confirmed in-the-wild exploitation. The fixed-version picture is disputed upstream: the patch was merged in spring 2026, but the Squid maintainer first attributed the fix to 7.6 (released 8 June 2026) then corrected that to 7.7, while Debian's assessment is that the referenced commit is already present in 7.6, and SecurityWeek reports the fix shipped in 7.6 (The Hacker News, 2026-06-22; SecurityWeek, 2026-06-22). The safe reading for defenders is to treat the fixed version as uncertain and verify against your own build rather than assuming a single release line is clean — see § 7. Calif.io credits an AI model (Anthropic's "Claude Mythos") with surfacing the strchr edge case during AI-assisted fuzzing — another data point in the AI-assisted-vulnerability-discovery pattern the W25 weekly tracked.

Why it matters to us: Squid is widely deployed as a forward / caching / web-filtering proxy across EU public-sector networks, university perimeters and ISP infrastructure — exactly the multi-user environments where the cross-user leak has impact. Interim mitigation that does not depend on resolving the fixed-version dispute: disable FTP proxying (acl ftp proto FTP + http_access deny ftp, or drop FTP from Safe_ports) where it is not needed, and restrict who can reach the proxy from untrusted/multi-tenant segments. Confirm the fix is present in your actual build (RHEL/Debian/Ubuntu ship 4.x–6.x — check for a backport) rather than trusting a version number. Detection: monitor Squid access.log for ftp://-scheme requests from unusual clients and for worker heap-corruption / crash signals (T1190 Exploit Public-Facing Application; effective outcome resembles T1040 Network Sniffing).