yaml-cpp EUVD-2026-84854

The consuming application must parse untrusted/attacker-controlled YAML using yaml-cpp 0.9.0. The specific trigger is a malformed YAML document with an unverified simple key (e.g. a plain scalar spanning a line break, as in issue #1475's input '!\n: |\nb\n>\n|\n !\n>') that leaves a stale simple key referencing an indent that gets popped, driving Scanner::PopAllIndents to underflow the indentation stack. Limiting factors: no public weaponized exploit for data theft (only a crash reproducer); impact is mostly a denial-of-parse/crash with at most Low confidentiality leakage; CVSS marks the vector local (AV:L), so an attacker needs a path to supply YAML to the vulnerable parser, and only yaml-cpp 0.9.0 is implicated.

Signals point to a modest, lower-priority issue despite the 'sensitive information' framing. CVSS base is 5.1 (Medium) with vector AV:L/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:L, i.e. local access, low complexity, no authentication, and only Low confidentiality plus Low availability impact. EPSS is 0.28% (21st percentile), indicating low predicted exploitation; this CVE is NOT in CISA KEV, so there is no confirmed active exploitation. Two conflicts must be flagged explicitly: (1) the description says 'remote attacker' but the CVSS vector is AV:L (local) - the real attack surface depends on how the consuming application feeds YAML to the parser, so verify the deployment context rather than assuming remote reachability; (2) the description and CWE-822 (untrusted pointer dereference / information disclosure) conflict with the actual fix in PR #1476, which guards against an indentation-stack underflow in Scanner::PopAllIndents - behavior that is primarily a parser crash/DoS (consistent with the A:L in the vector) rather than a reliable data-leak primitive. Net: treat as a routine library-hardening patch for anyone parsing untrusted YAML with yaml-cpp 0.9.0, not an emergency.

An application that embeds yaml-cpp 0.9.0 parses attacker-supplied YAML; a crafted document containing a stale, unverified simple key that spans a line break drives Scanner::PopIndent()/PushIndentTo() to pop from an empty indentation stack, causing an invalid dereference and a parser crash (with potential limited memory exposure). A concrete reproducer input exists publicly in GitHub issue #1475 and as a regression test in PR #1476. Exploitation is low-complexity but, per the CVSS vector, requires local access (AV:L) to reach the parser with the malicious input.