Bug 2536076 (CVE-2026-93007)

Summary: CVE-2026-93007 dnsmasq: dnsmasq: TCP/53 child-slot exhaustion starves DNSSEC validation
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: akhatavk, amctagga, aoconnor, aos-team-art-private, asdas, bniver, dpaolell, eglynn, flucifre, gmeno, groman, jdelft, jjoyce, jpretori, jschluet, jupierce, lgarciaa, lhh, mbenjamin, mbiarnes, mburns, mgarciac, mhackett, ppalepu, ppostler, prdhamdh, rhel-process-autobot, security-response-team, sghai, sidsharm, sostapov, suppawar, vereddy, vlaad, watson-tool-maintainers
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in dnsmasq. The TCP DNS service uses a small fixed pool of child processes and does not set a receive timeout on client connections. A remote attacker can open idle TCP connections to port 53 and occupy the entire pool, preventing new TCP DNS connections and DNSSEC validation that requires TCP fallback. UDP DNS resolution continues, and occupied slots are released after a fixed lifetime.
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Description OSIDB Bzimport 2026-09-17 14:47:02 UTC 
A resource-exhaustion issue was found in dnsmasq's TCP DNS handler. Incoming TCP connections on port 53 cause fork() of a child process tracked in fixed-size tcp_pids/tcp_pipes arrays bounded by MAX_PROCS (default 30, configurable via --max-tcp-connections). Children block on read_write() of the client socket with no SO_RCVTIMEO. The only timeout is CHILD_LIFETIME (150 seconds via SIGALRM).

An unauthenticated remote attacker can open MAX_PROCS idle TCP connections and hold all slots for up to 150 seconds. During exhaustion, set_dns_listeners() stops polling TCP listen sockets, and DNSSEC swap_to_tcp() (which shares the same slot pool) returns STAT_ABANDONED. UDP DNS continues to function.

This is a slowloris-style exhaustion of the fork-per-connection TCP architecture. SO_RCVTIMEO is already applied to upstream server sockets (forward.c) but not to accepted client sockets.

Affected versions: all current versions (fundamental TCP architecture).
Fix: Not yet available (reporter coordinating with upstream).
Reporter: Shubham Raj (Causal Security)
PSIRT Ticket: PSIRTSUPT-23743