Seems that the URI length check is superfluous and fails under certain conditions. It does not make sense for the URI to have zero bytes, so just use the first seen zero byte as end marker. I have a reproducible test case where the restore inserts URI with multiple zero-bytes to wire. This happens if the original URI is smaller than the one we rewrote it to using uac_replace_from. Signed-off-by: Timo Teräs &lt;timo.teras(a)iki.fi&gt; --- modules_k/uac/from.c | 14 ++++++++------ 1 files changed, 8 insertions(+), 6 deletions(-) However, I think the delta encoding used for the RR attribute is flawed. Hostile remote server could rewrite the RR attribute and/or From/To headers in a way to forge it to something it was not in the first place. Additionally the delta-encoded RR attribute breaks if the From/To header isn't exact copy of what we sent. Would it not make more sense to just send the real original header (possibly encrypted) but with a checksum? We could then verify if someone had clobbered the RR attribute and ignore it. And we could always restore the original URI even if the URI we are swapping was modified unexpectedly. diff --git a/modules_k/uac/from.c b/modules_k/uac/from.c index 4657e11..50822b6 100644 --- a/modules_k/uac/from.c +++ b/modules_k/uac/from.c @@ -463,15 +463,17 @@ int restore_from( struct sip_msg *msg, int *is_from ) LM_ERR("new URI shorter than old URI\n"); goto failed; } - for( i=0 ; i<old_uri.len ; i++ ) + for( i=0 ; i<old_uri.len ; i++ ) { new_uri.s[i] ^= old_uri.s[i]; - if (new_uri.len==old_uri.len) { - for( ; new_uri.len && (new_uri.s[new_uri.len-1]==0) ; new_uri.len-- ); - if (new_uri.len==0) { - LM_ERR("new URI got 0 len\n"); - goto failed; + if (new_uri.s[i] == 0) { + new_uri.len = i; + break; } } + if (new_uri.len==0) { + LM_ERR("new URI got 0 len\n"); + goto failed; + } LM_DBG("decoded uris are: new=[%.*s] old=[%.*s]\n", new_uri.len, new_uri.s, old_uri.len, old_uri.s); -- 1.7.1