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Fencer Vulns
medium 6.5

CVE-2026-77399 json

icalendar: Denial of service via unbounded VALARM REPEAT expansion

published 2026-09-22 47 d ago modified 2026-09-23 46 d ago also EUVD-2026-84800

cvss
6.5
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.3%
EPSS 0.3% chance of exploitation in 30 days, percentile 24%
public exploits
0
none known

Summary

icalendar is an RFC 5545 compatible parser and generator of iCalendar files for Python. From 6.1.0 until 7.2.2, vInt.from_ical accepts an attacker-controlled VALARM REPEAT value and applications that request alarm times can eagerly expand it without an application-level limit. Alarms.times and Alarms.active reach the unbounded expansion in versions starting with 6.1.0, while Alarm.triggers adds a second affected path starting with 7.0.0. Parsing alone does not trigger the issue, but accessing these properties can consume excessive CPU time and heap memory and terminate or stall a service. This issue is fixed in version 7.2.2.

Patch / fix

  • python-icalendar Debian 14 7.3.0-1

debian advisories DEBIAN-CVE-2026-77399

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
47 d ago

Exploit prediction

0.31% chance of exploitation in the next 30 days, higher than 24% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 3.1 6.5 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Affected products

  • collective icalendar

Classification

Weakness
  • CWE-400 Uncontrolled Resource Consumption
  • CWE-834 Excessive Iteration

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.