CVE-2026-56735: Zammad: Improper neutralization of `srcset` attribute in IMG tags in Zammad
Zammad is a web based open source helpdesk/customer support system. Prior to 7.0.2 and 7.1.0, zammad's HTML sanitizer (HtmlSanitizer::Strict) blocks external URLs in to prevent remote content loading, but the srcset attribute, also allowlisted for , is not subject to the same check. This oversight allows an attacker to embed arbitrary external URLs in the srcset attribute just by sending an e-mail, effectively bypassing the external content protection. When an agent views the ticket containing the malicious srcset, the browser will silently load the attacker-controlled URL, resulting in the leakage of the agent's IP address, User-Agent, Referer, and the exact time the ticket was viewed. HtmlSanitizer::Strict got extended to block external URLs in the srcset attribute of tags. This was done to prevent tracking pixels and external content loading, which could compromise user privacy. This issue is fixed in versions 7.0.2 and 7.1.0.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
Zammadto a version that resolves this vulnerability.Fixed in 7.0.2 - Upgrade
Upgrade
Zammadto a version that resolves this vulnerability.Fixed in 7.1.0
Event History
Frequently Asked Questions
Who is exposed to this issue?
Agents who view tickets containing attacker-supplied HTML are exposed. The issue can be triggered by sending an email with a malicious IMG srcset attribute, causing the agent's browser to load an external attacker-controlled URL.
What information can leak when a malicious ticket is viewed?
The external request can disclose the viewing agent's IP address, User-Agent, Referer, and the exact time the ticket was viewed. The described impact is privacy leakage through remote content loading.
Are default external-content protections sufficient?
No. The strict HTML sanitizer blocked external URLs in the relevant IMG handling, but did not apply the same check to the allowlisted srcset attribute, allowing the external-content protection to be bypassed.
Which versions address the issue?
The issue is fixed in Zammad versions 7.0.2 and 7.1.0. Versions prior to those releases are affected according to the advisory.