Service Incident - 28th of November 2025 - Less-than and greater-than symbols were wrongfully detected as HTML in the equation editor

All times in the document are recorded in UTC+1 (CET).

Incident summary

On October 29th, 2025, a user reported that one of their students noticed their equation  changing after saving it in the answer field of an open question.This would occur because LaTeX equations using the less-than (“<”) and greater-than (“>”) symbols  added via the equation editor were misinterpreted as HTML, causing a part of the equation to be removed.

The issue was first reported via a support ticket and escalated to the technical team. On October 31st, the technical team began investigating the report. After identifying the cause a hotfix was deployed on November 12th, and the technical team started working on the impact analysis. By November 19th, the support team informed the users that were impacted by this incident.

Lead-up

On October 29th, 2025, a user reported an issue that the answers of the students using the equation editor would change after saving.

Fault

It was discovered that this issue had been present since June 22nd, 2025 and would occur under the following conditions:

1. In a digital test, the student would add an equation in the answer field of an open question using the equation editor.
2. The equation included a less-than symbol (“<”) or both the less-than (“<”) and greater-than (“>”) symbols.
3. The student would click outside the answer field, which saves the answer of the student.
4. The equation would then change, where instead of parsing the equation it would show dollar signs and remove a part of the equation.

Example

• A student entering x>10 || x<y in the equation editor only sees the following in plain text: ‘$$x>10\vert\vert x’’ as <y with the ending dollar signs were removed. 
• If a student entered x>10 || x<y>k, the segment <y> would be removed, and the answer would appear as x>10 || xk.

This issue was introduced on June 22nd, 2025, when we made the change to no longer support KaTeX for rendering equations and Mathlive for building equations, but instead use Mathlive for both building and rendering equations.

Impact

The impact of this incident affected the answers of the students and could have affected the grading of the teacher. 

An impact analysis was performed on all digital tests where the answer of the student used LaTeX with the larger than sign.

The investigation resulted in 66 affected assignments, with 187 impacted results. 

Detection

The issue was first reported by a user via a support ticket. Upon review, the support team escalated the problem to the technical team for further investigation.

Response

On October 31st, 2025, the support team forwarded the ticket to the technical team and informed the user that further investigation was required. It took the technical team some time to reproduce and discover the root cause of the issue.

On November 3rd, 2025, the support team informed the user that the technical team was able to reproduce the issue, they were still investigating what the root cause was. 
On November 12th the root cause of the issue was identified and a hotfix was created and deployed the same day. While the hotfix was being deployed the technical team began their investigation determining the impact.

On November 13th, 2025, the support team informed the user that the issue has been fixed and that the technical team began their investigation to determine the impact of the issue. The impact would be shared once it’s made available.

On November 19th, 2025, the support team informed all the schools that were impacted by this issue and the specific assignments and results that were impacted.
 

Recovery

On November 12th, 2025, a hotfix was released to ensure that the issue does not persist. 

Timeline

29th of October, 2025

• 12:44 - The support team receives a ticket where the user stated that one of their students saw their answer change and that their equation included dollar signs.
• 15:23 - The support  team asks the user for more information as they could not reproduce the issue.
• 17:41 - The user responds with which questions are impacted.

31st of October, 2025

• 10:28 - The support forwards the information to the technical team and the technical team begins their investigation.

3rd of November, 2025

• 13:37 - The technical team was able to reproduce the issue, but is still investigating to determine the root cause of the issue.

11th of November, 2025

• 13:32 - The technical team confirms the root cause of the issue and created a hotfix while also starting to work on the impact analysis.

12th of November, 2025

• 20:14 - Hotfix was deployed to the production environment.

13th of November, 2025

• 15:21 - The support team informs the user that the hotfix is deployed and that the technical team is working on an impact analysis, the impact analysis will be shared once available.

19th of November, 2025

• 11:05 - All schools that were impacted have been informed and an impact analysis has been shared.

Reflection

This incident exposed several gaps in our testing and in the assumptions we made about how the equation editor handles special characters. Although our tests verify that each component processes and renders LaTeX correctly, they don't cover the specific flow where a student enters an answer in the equation editor and that answer is saved.

The root cause took longer than expected to identify because the issue wasn’t in the editor itself, the issue appeared when LaTeX was stripped from the HTML right after the student clicks outside of the answer field.

Overall, this highlighted the need for stronger end-to-end validation to ensure data integrity, and a closer examination of how user input moves through the different layers of our platform. We have expanded our digital test testing procedures, which test the overall flow of a student taking a test. The new tests ensure that the LaTeX from the equation editor is converted and sanitized correctly when the student's answer is saved.