Bugsink is vulnerable to Stored XSS via Pygments fallback in stacktrace rendering
Date: 2026-02
Affected: pip/bugsink
Attack vector: xss
Category: vulnerability-disclosure
Severity: Critical
CVE: CVE-2026-27614
CWE: CWE-79
CVSS: 9.3 (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:N)

### Summary An unauthenticated attacker who can submit events to a Bugsink project can store arbitrary JavaScript in an event. The payload executes only if a user explicitly views the affected Stacktrace in the web UI. ### Details When Pygments returns more lines than it was given (a known upstream quirk that triggers with Ruby heredoc-style input), `_pygmentize_lines()` in `theme/templatetags/issues.py:75-77` falls back to returning the raw input lines. `mark_safe()` at line 111-113 is then applied unconditionally - including to those unsanitized raw lines. Since DSN endpoints are public by Sentry protocol, no account is needed to inject. The payload sits in the database until an admin looks at the event. ```python # issues.py:75-77 - fallback path, no escaping if len(pygmented) != len(lines): return lines # raw HTML returned here # issues.py:111-113 - unconditional mark_safe return [mark_safe(line) for line in result] ``` ### Operational Signals Exploitation attempts are likely to generate the diagnostic event: ``` "Pygments line count mismatch, falling back to unformatted code" ``` Installations that monitor Bugsink with Bugsink (or otherwise alert on internal errors) may see this message as an issue. While the condition can occur benignly, unexpected occurrences, especially from unusual languages (specifically ruby), warrant review. ### PoC Send a Sentry event to `/api/<project-id>/store/` with a valid DSN: ```python import requests payload = { "exc


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