124 lines
5 KiB
Markdown
124 lines
5 KiB
Markdown
# OpenSSL — CWE-407 Disclosure Brief
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**Project:** OpenSSL
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**Disclosure date:** 2026-03-27
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**Severity:** HIGH
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**Speedup:** varies with n×m, n², C×S
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**Status:** PATCHED
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---
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## Finding
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OpenSSL contains three independent quadratic-complexity defects in cipher negotiation and SRTP profile matching within `ssl/ssl_ciph.c` and `ssl/statem/extensions_srvr.c`. Two are in cipher suite deduplication during TLS connection setup (one TLS 1.2, one TLS 1.3), and one is in SRTP profile matching for DTLS/WebRTC connections. All three execute on the hot path of TLS handshake processing.
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## The Defect(s)
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| ID | Location | Pattern | Complexity |
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|----|----------|---------|------------|
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| openssl-0001 | `ssl/ssl_ciph.c` | `SSL_get_shared_ciphers()` O(n×m) scan per connection when server stack unsorted | O(n×m) |
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| openssl-0002 | `ssl/ssl_ciph.c` | `ciphersuite_cb` TLS 1.3 dedup O(n²) during config parsing | O(n²) |
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| openssl-0003 | `ssl/statem/extensions_srvr.c` | `tls_parse_ctos_use_srtp()` outer while over C client IDs × inner for over S server profiles | O(C×S) |
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## Complexity Proof
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**openssl-0001** — Let n = size of client cipher list, m = size of server cipher list.
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`SSL_get_shared_ciphers()` finds the intersection by iterating all n client ciphers and for each scanning all m server ciphers linearly:
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```
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For client_cipher_1: scan m server ciphers
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For client_cipher_2: scan m server ciphers
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...
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For client_cipher_n: scan m server ciphers
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Total: n × m comparisons per connection
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```
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With a hash set of server cipher IDs (built once at config time), each client cipher lookup is O(1), reducing total work to O(n+m).
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**openssl-0002** — Let n = number of TLS 1.3 ciphersuites in the configured list.
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The `ciphersuite_cb` deduplication iterates the accumulating output list for each new candidate, scanning up to n entries:
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```
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Total: 0 + 1 + ... + (n-1) = n(n-1)/2 = O(n²)
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```
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A bitmask indexed by the cipher table index (bounded by the small fixed TLS 1.3 suite count) reduces each check to O(1).
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**openssl-0003** — Let C = number of client SRTP profile IDs, S = number of server SRTP profiles.
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`tls_parse_ctos_use_srtp()` iterates client IDs in an outer `while` loop and for each scans the server profile list in an inner `for` loop:
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```
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Total: C × S comparisons per DTLS handshake
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```
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A 32-slot Knuth hash set (the SRTP profile space is small) reduces this to O(C+S).
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## Impact
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**openssl-0001** affects all TLS 1.2 connections where the server cipher stack is not pre-sorted or filtered. High-connection-rate servers (HTTPS load balancers, API gateways, CDN edge nodes) pay this on every handshake.
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**openssl-0002** affects OpenSSL configuration parsing and TLS context setup when custom TLS 1.3 ciphersuite lists are configured. Applications that dynamically create SSL_CTX objects (multi-tenant servers, connection pooling) are most affected.
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**openssl-0003** affects all DTLS/WebRTC connections using SRTP (RFC 5764). Media servers, SFUs, and WebRTC gateways processing many simultaneous DTLS handshakes are most affected. C = 20 client profiles × S = 10 server profiles = 200 comparisons per handshake, reduced to 30 with the hash set.
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## The Fix
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**openssl-0001:** Pre-build a hash set of server cipher IDs at `SSL_CTX` configuration time. Use it in `SSL_get_shared_ciphers()`.
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**openssl-0002:** Use a `uint64_t` bitmask indexed by position in the fixed TLS 1.3 cipher table for O(1) dedup.
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**openssl-0003:** Build a 32-slot Knuth multiplicative hash set of server profile IDs before the outer client loop.
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## Patch
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```diff
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# openssl-0001: ssl/ssl_ciph.c
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- for (i = 0; i < client_len; i++) {
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- for (j = 0; j < server_len; j++) {
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- if (client[i]->id == server[j]->id) { ... }
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- }
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- }
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+ /* pre-build server_id_set at ctx init */
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+ for (i = 0; i < client_len; i++) {
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+ if (hash_set_contains(server_id_set, client[i]->id)) { ... }
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+ }
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# openssl-0002: ssl/ssl_ciph.c
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- /* TLS 1.3 ciphersuite dedup — O(n²) */
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- for (i = 0; i < num; i++) {
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- for (j = 0; j < i; j++) {
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- if (list[j] == list[i]) { duplicate = 1; break; }
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- }
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- }
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+ /* bitmask dedup — O(n) */
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+ uint64_t seen = 0;
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+ for (i = 0; i < num; i++) {
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+ int idx = cipher_table_index(list[i]);
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+ if (seen & (1ULL << idx)) { duplicate = 1; continue; }
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+ seen |= (1ULL << idx);
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+ }
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# openssl-0003: ssl/statem/extensions_srvr.c
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- while (PACKET_get_net_2(&profiles_list, &id)) {
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- for (i = 0; i < server->srtp_profiles->num; i++) {
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- if (server->srtp_profiles->profiles[i].id == id) { match = 1; }
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- }
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- }
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+ /* build 32-slot hash set of server profile IDs first */
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+ srtp_hash_set_t srv_set;
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+ srtp_hash_init(&srv_set, server->srtp_profiles);
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+ while (PACKET_get_net_2(&profiles_list, &id)) {
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+ if (srtp_hash_contains(&srv_set, id)) { match = 1; break; }
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+ }
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```
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## What We Ask
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Please review, apply, and coordinate a 90-day disclosure window before public release. Reply to security@undefect.com.
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---
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*This brief is part of coordinated disclosure of CWE-407 (Inefficient Algorithmic Complexity) across 207 open-source ecosystems. Full report: https://undefect.com*
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