The DigestAuthentication.apply() method in Jetty's HTTP client uses getBytes(StandardCharsets.ISO_8859_1) at three locations (lines 171, 179, 196) to compute Digest auth response hashes. ISO-8859-1 silently replaces any character above U+00FF (Chinese, Japanese, Cyrillic, Arabic, Emoji, etc.) with 0x3F (?), causing all such characters to produce identical hash contributions. An attacker who knows a victim's username can bypass Digest authentication by replacing all non-Latin-1 characters in the password with ? characters, since the collision password produces the same MD5-based Digest response hash as the original password.
Details
Root Cause
In jetty-core/jetty-client/src/main/java/org/eclipse/jetty/client/DigestAuthentication.java, the apply() method computes the three Digest auth hashes (H(A1), H(A2), and the final response) using ISO-8859-1 character encoding:
// Line 171 — H(A1)
String hashA1 = toHexString(digester.digest(a1.getBytes(StandardCharsets.ISO_8859_1)));
// Line 179 — H(A2)
String hashA2 = toHexString(digester.digest(a2.getBytes(StandardCharsets.ISO_8859_1)));
// Line 196 — Final response hash
final String hashA3 = toHexString(digester.digest(a3.getBytes(StandardCharsets.ISO_8859_1)));
ISO-8859-1 (Latin-1) can only encode characters in the range U+0000–U+00FF. Any character outside this range — including all CJK, Cyrillic, Arabic, Greek, Hangul, and emoji characters — is silently replaced with the byte 0x3F (?). String.getBytes(ISO_8859_1) in Java performs this replacement without any warning or exception.
If a service using Jetty for Digest authentication has a user with a non-Latin-1 password (e.g., Chinese, Japanese, Russian), an attacker can authenticate as that user using a collision password where all non-Latin-1 characters are replaced with ?:
Original password: 我爱Java!密码123★
Collision password: ??Java!??123?
Both produce identical MD5 hashes under ISO-8859-1 → Authentication succeeds
This affects any password containing characters > U+00FF, which covers:
Chinese (CJK): U+4E00–U+9FFF
Japanese (Hiragana/Katakana/Kanji): U+3040–U+30FF, U+4E00+
Latin Extended: U+0100–U+024F (accented European characters like ĉ, ğ, ñ when > U+00FF)
Emoji / Symbols > U+00FF
Scenario 2: Denial of Service for Non-Latin-1 Users
Most modern web applications store password hashes computed using UTF-8. When Jetty's Digest client computes a hash with ISO-8859-1, the bytes differ from what the server stored/expects. This means any user with non-ASCII (Latin-1+) characters in their password can never successfully authenticate via Digest auth — even the legitimate user. This is not just a security issue but a functional correctness bug that silently breaks authentication for most non-European-language users.