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ari :prideified: (ar@is-a.cat)'s status on Thursday, 08-May-2025 03:42:52 JST ari :prideified:

Today in silly matrix findings: you can have a user with only a single / in the localpart. which now means i now have a @/:is-a.cat user on my instance.
The requirements say that the only characters allowed are [a-z0-9._=/-] and that there must be one or more characters, but noone made a distinction for which characters the localpart can start with. The other characters would probably work as well :blobcatjoy:
I suppose the / was added to the list of allowed characters in localpart to make mapping usernames to kerberos realms easier :blobcatshrug:
here's the spec, if anyone's interested:
https://spec.matrix.org/v1.1/appendices/#user-identifiers
In conversation about 4 days ago from is-a.cat permalink
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  Appendices
  
  Unpadded Base64 Unpadded Base64 refers to ‘standard’ Base64 encoding as defined in RFC 4648, without “=” padding. Specifically, where RFC 4648 requires that encoded data be padded to a multiple of four characters using = characters, unpadded Base64 omits this padding. For reference, RFC 4648 uses the following alphabet for Base 64: Value Encoding Value Encoding Value Encoding Value Encoding 0 A 17 R 34 i 51 z 1 B 18 S 35 j 52 0 2 C 19 T 36 k 53 1 3 D 20 U 37 l 54 2 4 E 21 V 38 m 55 3 5 F 22 W 39 n 56 4 6 G 23 X 40 o 57 5 7 H 24 Y 41 p 58 6 8 I 25 Z 42 q 59 7 9 J 26 a 43 r 60 8 10 K 27 b 44 s 61 9 11 L 28 c 45 t 62 + 12 M 29 d 46 u 63 / 13 N 30 e 47 v 14 O 31 f 48 w 15 P 32 g 49 x 16 Q 33 h 50 y Examples of strings encoded using unpadded Base64:
- Jesse 🇫🇷 repeated this.

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