a2a-dotnet EUVD-2026-110072

Exploitation requires reaching the A2A.AspNetCore JSON-RPC endpoint handled by A2AJsonRpcProcessor.ProcessRequestAsync and sending a body that fails JSON parsing; the vulnerable code path is the System.Text.Json exception handler that embeds ex.Message (JSON paths, line/byte positions, exception type and library names) in the -32700 ParseError response returned to the client. Affected: a2a-dotnet up to and including 1.0.0-preview2. Limiting factors: the disclosed data is internal parser/diagnostic metadata only - no credentials, user data, integrity or availability impact - and the issue is fully resolved by upgrading to 1.0.0-preview3, which the vendor states is sufficient; whether any authentication gates the endpoint before the parse step is not established by the available sources.

This is a genuine but low-severity information-disclosure issue (CWE-209), and the signals agree on that. The VulDB-supplied CVSS 4.0 vector (AV:N/AC:L/AT:N/PR:L/UI:N) with VC:L and no integrity or availability impact yields 5.3 (Medium), and the vuln.today decision label is 'this_month' rather than act_now/emergency - consistent with a flaw that leaks internal parser metadata, not credentials or system control. No KEV listing and no EPSS score are present in the input, and no public exploit is referenced; exploitation is trivially easy in mechanical terms (send malformed JSON) but the payoff is limited to System.Text.Json error internals. One signal to flag: the input vector sets PR:L (authenticated), yet the confirmed trigger is a JSON parse failure (see patch diff), which typically occurs before any application-level authentication - I independently assess PR:N and note the discrepancy. This is a hygiene/defense-in-depth fix to prioritize on a normal patch cycle, not an emergency.

A remote attacker sends a malformed JSON body (e.g. '{invalid json' or truncated JSON-RPC) to the a2a-dotnet ASP.NET Core JSON-RPC endpoint. The server catches the System.Text.Json exception and returns its raw ex.Message inside the JSON-RPC ParseError (-32700) response, disclosing parser details such as JSON paths, line and byte positions, exception type and library names that aid reconnaissance. No public exploit code is referenced; the attack requires only the ability to POST to the endpoint, so attack complexity is low.