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Security update
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This release includes two security fixes:
- Switch to using a cryptographically-secure random number generator for token strings, ensuring they cannot be predicted by an attacker. Thanks to @opnsec for identifying and responsibly disclosing this issue! ([\#5133](https://github.com/matrix-org/synapse/issues/5133))
- Blacklist 0.0.0.0 and :: by default for URL previews. Thanks to @opnsec for identifying and responsibly disclosing this issue too! ([\#5134](https://github.com/matrix-org/synapse/issues/5134))
