fix(native): escape JSON attachments

[jpnurmi]

Filenames and attachment/content types are arbitrary user-supplied strings that can contain characters that must be escaped in JSON.

JSON injection in crash daemon envelope headers via unescaped attachment filenames

Details

The write_attachment_to_envelope function constructs a JSON envelope item header using snprintf with %s interpolation for filename and content_type values. These originate from user-controlled API calls (sentry_attach_bytes, sentry_attachment_set_filename) with no validation. The values are properly JSON-escaped when serialized to the attachment list file via sentry_value_to_json, but after the crash daemon deserializes them with sentry__value_from_json and retrieves them with sentry_value_as_string, the raw unescaped strings are returned. Characters like double-quotes, backslashes, and newlines in filenames are interpolated verbatim into the JSON header, breaking the JSON structure. The normal envelope serialization path in sentry_envelope.c uses proper JSON writers, but the crash daemon duplicates this logic with raw snprintf.

Location

src/backends/native/sentry_crash_daemon.c:117

Impact

Attacker corrupts envelope structure causing misinterpretation of crash data

Reproduction steps

1. An application preserves user-uploaded filenames when attaching files via sentry_attach_bytes. An attacker sets a filename like: x","type":"event","length":999}\n which, after JSON round-tripping through serialize/deserialize, injects a malformed item header into the crash envelope. The Sentry relay/backend may misinterpret item boundaries, treating attachment data as event payload.

Recommended fix

Envelope item headers in the crash daemon must use proper JSON serialization (the existing sentry_jsonwriter) rather than raw snprintf interpolation, ensuring all string values are escaped.

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Severity: MEDIUM
Status: Open
Category: Injection
Repository: getsentry/sentry-native
Branch: master

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