Executive Summary

CVE-2026-29856 is a Regular Expression Denial of Service (ReDoS) vulnerability affecting aaPanel version 7.57.0 and earlier. It occurs in the VirtualHost configuration parser component, where certain regular expression patterns used to parse <VirtualHost ...:> blocks contain nested and ambiguous quantifiers.

Specifically, the regex pattern includes sequences like .* followed by (\d+)+, which cause catastrophic backtracking when processing crafted inputs. This leads to excessive CPU consumption and can cause slowdowns or process hangs.

The vulnerability arises when attacker-controlled or untrusted configuration inputs trigger these problematic regex patterns in files such as domain_tool.py and ssl.py, allowing attackers to cause a denial of service by exhausting server resources.

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Impact Analysis

This vulnerability can impact you by allowing an attacker to cause a denial of service on your server running aaPanel v7.57.0 or earlier.

By supplying specially crafted inputs that exploit the vulnerable regular expressions in the VirtualHost configuration parser, an attacker can trigger excessive CPU usage, leading to slowdowns or complete process hangs.

This resource exhaustion can degrade server performance, disrupt hosting services, and potentially cause downtime, affecting availability of your web hosting environment.

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Compliance Impact

I don't know

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Detection Guidance

[{'type': 'paragraph', 'content': 'This vulnerability can be detected by analyzing the VirtualHost configuration files used by aaPanel, specifically looking for the presence of the vulnerable regex patterns in the files domain_tool.py and ssl.py. The problematic regex pattern is <VirtualHost +.*:(?P<port>\\d+)+\\s*> which is prone to catastrophic backtracking when processing crafted inputs.'}, {'type': 'paragraph', 'content': 'To detect if your system is vulnerable, you can check the version of aaPanel installed (v7.57.0 or earlier are affected) and inspect the configuration parser files for the presence of the vulnerable regex patterns.'}, {'type': 'paragraph', 'content': 'While no specific detection commands are provided, you can use commands to identify the aaPanel version and search for the vulnerable regex in the source files, for example:'}, {'type': 'list_item', 'content': 'Check aaPanel version: `cat /www/server/panel/version.pl` or use the panel interface.'}, {'type': 'list_item', 'content': 'Search for vulnerable regex patterns in the source code: `grep -r "<VirtualHost +.*:(?P<port>\\d+)+\\s*>" /www/server/panel/`'}, {'type': 'list_item', 'content': 'Monitor CPU usage for processes related to aaPanel or its configuration parsing, as the ReDoS attack causes excessive CPU consumption.'}] [1]

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Mitigation Strategies

Immediate mitigation steps include avoiding the use of untrusted or attacker-controlled input in VirtualHost configuration files parsed by aaPanel, as the vulnerability is triggered by crafted inputs causing regex backtracking.

Additionally, you should upgrade aaPanel to a version later than 7.57.0 once a patch is released that fixes the vulnerable regex patterns in domain_tool.py and ssl.py.

As a temporary workaround, restrict access to configuration files and validate inputs before they are processed by the VirtualHost parser to prevent maliciously crafted configurations.

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