CVE-2026-27614: Bugsink is vulnerable to Stored XSS via Pygments fallback in stacktrace rendering

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
• EPSS 0.4%, percentile 37: FIRST’s model estimates a 0.4% chance of exploitation activity in the next 30 days, higher than 37% of all scored CVEs (low). EPSS is a probability, not a verdict.
• Needs a user to click or open something. Phishing makes that realistic, but it is not wormable.
• Only applies if your dependency tree includes bugsink (PyPI) <= 2.0.12, fixed in 2.0.13. Check your lockfile, including transitive dependencies.

What it is

Bugsink is a self-hosted error tracking tool. In versions prior to 2.0.13, 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. When Pygments returns more lines than it was given (a known upstream quirk that triggers with Ruby heredoc-style input), pygmentizelines() in theme/templatetags/issues.py:75-77 falls back to returning the raw input lines. marksafe() 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. Successful exploitation requires that the attacker to be able to submit events to the project (i.e. knows the DSN or can access a client that uses it), the Bugsink ingest endpoint is reachable to the attacker, and an administrator explicitly views the crafted event in the UI. Under those conditions, the attacker can execute JavaScript in the administrator’s browser and act with that user’s privileges within Bugsink. Version 2.0.13 fixes the vulnerability.

From the GitHub advisory

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), pygmentizelines() in theme/templatetags/issues.py:75-77 falls back to returning the raw input lines. marksafe() 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.

Operational Signals

Weakness type: CWE-79.

Product: bugsink (PyPI) · Assigned by: GitHub_M · Published: 2026-02-25

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: required (a user must click/open something)
• Scope: changed (can affect other components)

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation poc, automatable no, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.44% probability of exploitation activity in the next 30 days; percentile 37.

Timeline

• 2026-02-25 — CVE record last updated
• 2026-02-25 — CVE record published by GitHub_M
• 2026-02-25 — GitHub-reviewed advisory GHSA-vp6q-7m36-pq3w published (critical)

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-vp6q-7m36-pq3w github.com
2. CVE.org record CVE-2026-27614 cve.org
3. NVD entry CVE-2026-27614 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. github.com github.com

Critical PyPI advisories · Today’s Security Digest