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PYSEC-2026-1825
No affected components available
Impact
This issue describes a ReDOS bug found within the figure caption extension (pymdownx.blocks.caption ).
In systems that take unchecked user content, this could cause long hangs when processing the data if a malicious payload was crafted.
Patches
This issue is patched in Release 10.16.1.
Workarounds
Some possible workarounds
If users are concerned about this vulnerability and process unknown user content without timeouts or other safeguards in place to prevent really large, malicious content being aimed at systems, the use of pymdownx.blocks.caption could be avoided until the library is updated to 10.16.1+.
References
The original issue https://github.com/facelessuser/pymdown-extensions/issues/2716.
Description
The original issue came through PyMdown Extensions' normal issue tracker instead of the typical security flow: https://github.com/facelessuser/pymdown-extensions/issues/2716. Because this came through the normal issue flow, it was handled as a normal issue. In the future, PyMdown Extensions will ensure such issues, even if prematurely made public through the normal issue flow, are redirected through the typical security process.
The regular expression pattern in question is as follows:
RE_FIG_NUM = re.compile(r'^(\^)?([1-9][0-9]*(?:.[1-9][0-9]*)*)(?= |$)')
The POC was provided by @ShangzhiXu
import re
import time
regex_pattern = re.compile(r'^(\^)?([1-9][0-9]*(?:.[1-9][0-9]*)*)(?= |$)')
for i in range(50, 500, 50):
long_string = '1' * i + 'a'
start_time = time.time()
match = re.match(regex_pattern, long_string)
end_time = time.time()
print(f"long_string execution time: {end_time - start_time:.6f} s")
The issue with the above pattern is that . was used, which accepts any character when we meant to use \.. The fix was to update the pattern to:
RE_FIG_NUM = re.compile(r'^(\^)?([1-9][0-9]*(?:\.[1-9][0-9]*)*)(?= |$)')
Relevant PR with fix: https://github.com/facelessuser/pymdown-extensions/pull/2717
Version(s) & System Info
- Operating System: Any
- Python Version: Any
The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability.
Limited exploitation activity has been observed. Close monitoring and planned remediation are recommended.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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